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HEWLETT-PACKARD 

JOURNAL 



December 1993 




■J 









4 









m0 




rX«l HEWLETT 
Wf/iM PACKARD 



© Copr. 1949-1998 Hewlatt-Pd!l<ard Co. 



HEWLETT-PACKARD 

JOURNAL 



DGcember 1993 Volume 44 • Number E 



Articles 



1^ Vector Signal Analyzers for Difficult Measurements on Time-Varying and Complex Modulated 
Signals, by Kenneth J. Blue, Robert T. Cutler, Dennis P. O'Brten. Douglas H. Wagner, and 
Benjamin fl. Zar lingo 

^ Q The Resampling Process 

"I 2 Applications for Demodulation 

^ ^ A Firmware Architecture for Multiple High-Performance Measurements, by Dennis P. Q'BriEn 

2Q Run-Tlme-Configurable Hardware Drivers 

2 2 Remote Debugging 

< n Baseband Vector Signal Analyzer Hardware Design, by Manfred BarU, Keith A. Bayern, 
Joseph R. Diederichs. and David F. Kelley 

38 ^"'^ Dynamic Range 

44 What Is Dithering? 

RF Vector Signal Analyzer Hardware Design, by Robert T. Curler, William J. Ginder, 
Timothy L Hillstrom, Kevin L Johnson, Roy L Mason, and James Pietsch 



Microwave Plate Assembly 

A Versatile Tracking and Arbitrary Source 

Vector Measurements beyond 1.B GHz 



Editor, RiiliM-rlP DnI-in ■ Auocliin Eiliioi ''-'.H'l'' I li'-/'.. Publicaiion piortuciion Mamgei. SusjriL Wnslit- lllmlraBo* RaoSe Q. Pijhinr • 
Tfipogfephv/LBV'"''' ^i^ilv Hij^iin > Te^and MeaiurBmEiiE Oigamzalion Lidiaan, SviIheyC AvGV 

AdviBory Boord, Sreven Brnienfiam. Di'ai M^my 0<vt5m. Borse, Wato* WiltiamW 8mwn, InSggiafBd Cin:wl BtisfBS Owision. 5flnra Clara i^^'crmfa- Frank J 
Calvillfl. Gt^iey Siofage Oiviaioi' Gfeelsy Colataiio •tiarri CliDU, Mrcromve Techiiplogy Qtyi^in" S-inrflflDW, Uslilomis ■ Dflret T Dang, SYSIe"' Suppoil Owision, 
MBitnlBin ViEM, Calilomia't^i^^ Desai, Cijnjmatclal SySJEm^ Dmsto". Cupenmii^ Ca<i!umB* Ksvin G E^rr, Intsgialetl Sys^mi^ DivrsJm Sunnyvale, CaliUrtit^* 
Bamhsrd Fischor, BdMlneeiy Mait'cal Dtvis/on. Bfiblingen^ Gernianv Douglas SenneP^n, Srestey Haiikt^iri Dmsim, Greelsy, Colors *Q>i<\ Gorrton. UPLabo'ato'tss, 
Palo Mo CSIifamia* Man J Hiullne. Syslams Tac'inalGgy Division. Rasawlle. ■ Bryan Hoog, Late St&yeiis Insliwgni Oivtsijn, f^iel, Wasningion » G'ai^B 

Jndv, Srenotile t^elwniis Division Cupsnioo r^'rfemifl • Rogei L JuJigennan, M'cfDameracfmDiogy Drns^on Saias ffoM, Cal'fomii* Paula H k^snarBk. /n^fer 
Cotupoiient£ Dm^ian Coivallis, Dtagan* llTornas F Kraemer, Cohtado Sivirigs Dtvision. (jTtoraDfl Sivirigs. CdIo'biId' RuDy B Jes fJefiAoiliiid SysiBms fflorrp, Cupariinrt 
Cslrfernu • ffill Uoyii, HP laliaalatles Jaiiao. *'sn-iss<i. JsfBr" Al^ad Maole, HSWtfjinn Jnsfrtjra/ Oiman, kVs/jtoorm. Gwrainy^ Miihaa^ P Unore. V*(S>i£emi 
OrHsmrj, Lovalend CaJD^ajW* Slitllev I Mnine, SanDlago Prifm DiVisioi', SaitOiegn. Cal'lomia'^wia L Mo'rill, WDililwidR rusiDmBi Soppiin Divjsion. f^unlsiti 
VieWr &/^/flmj3 ■ SfMn J NarciSd, VICI Svslem Division, lovel^rnl Colomlii*G3Ti Orwlini Soltware Ipclmlogy [iivsio", flasevrWe. Ca/rrlvrrra- Rai Dza. SotMBie 
Tschnolaffy Division. Mountain View. &/r'[?mra • Han Tian Phus, Asu I'onjiliBrals Oivisicn. SrirgaiUNei KeJi PaullDT, HFialmialiiiiES. PsmAllii. C^ilooiiS' Gonmr 
Riebeiell, Beolmgen Insfutnenrs Divismn d^lingen. Geimany^ Wan: SahaTeJIa Si^llv^aio Engineei'ng Syslooii Divsion, faiCoFlm. CrpVo'sflc ■ Midraal B Saunrlers, 
frrregrdrfii^ ^J'rrjJJ ifosmpfs ifMjJOn, Cor\allis, OrEgont PhihD Slnninr, l)Pl;3iiaraionps^slol, SnsnjI. EnglamI* Bang-Hang Tav. Singspoie ^elmris Opgirnm, 
5rr(ff3j™e ■ Stephen R L/nilv, J^sre/rrs Taclimlasy Division, fan Collins. Ciilcfatia *Jm WiHila, l^lwoii amiSystamManogBinen! Division, Fan Collins Colorado • 
KoicKi Yanagawa, *rofJe /jif/fwnenr Ojvrsjfji, JapaJi* Dennta C Yurk. ClIlval^ls Division. Corvaliis. Drffljon" Basbara^immp" CoquHaistnginaeiiog. Palo Alio, 
Califmtn 



©Haiivlen-Packard Ccim(HHy 1993 Prlnttilin U SA Tha rta^lotl-Packard Joumal is [!inW tm recvtlBl paoer 



2 DEfcmber 19U3 Hewlett-I'ackard Joumnl 

©Copr. 1949-199B H ew I etl- Packard Co. 



Optical Spectrum Analyzers with High Dynamic Range and Excellent Input Sensitivtty, 

by David A. Bdiley and James R. Stimple 

^2 Oplit^sl Spectrum Analysis 



l8 



A Doubte-Pass Manochromator for Wavelength Selection in an Optical Spectrum Analyzer, 

by Kenneth R Wildnauer and Zoltan Azary 



~j Q Diffractiun Grating 
y ^ Polarization Sensitivity 



y A High-Resolution Direct-Drive DiHraclion Grating Rotation System, by Joseph N. West and 
J. Douglas Knight 



Wn ATwo-AxisMicropDsitioner for Optical Fiber Alignment, by J. Douglas Knight and 
Joseph N. West 



85 ^ ^'^"i'^"' "^'^ Format tor Instrument Data Interchange, by Michael L Hall 

North American Cellular CDMA, by David P. Whipple 
32 ''^""'^^ Technologies 



DECT Measurements with a Microwave Spectrum Analyzer, by Mark A. Eh 

Departments 



4 In this Issue 

5 Cover 

5 What's Ahead 

107 1993 Index 

116 Authors 



The Hewl^n-PaDkBrdJOEimBl n pubHshed bimonthly by rtiG HswIaD-PackQrd Campsny ia rficQgniuuchnical DDntributiana mEidsi by HbwI sin Packard 
IHPlparsDfiriBl WIiiIb Ibe inlormalioii lound in iJiis publication is bsliavsd lo be Bctuma. Ihe Hewletl-PackBrd Compamdlstloims all wanantiesot 
iTiarclLBiilalillltvBrtd fitnaEB lor D pnniculBr purpoi^e and all DbllLjatiDfii ^nd llBbilpiist lai dsnidgas. including bulnoi limiTad id indirecr, spacal. or 
cansaQuanrial dainaQas, aETornev'sand aioerti fe^s, and coun co^, arising oijr of or in connadion win jiublicalion 

Suljsciifflipn?. The Hawla El-Packard Joumal is diBtribuIad tracol charga [o HP resEorch, tiasign and manulaclunng engineering personnel, as well ai w 
gualifiBd non-HP individiiats. Iibrflrias, and aducabonal msulLrUons Plaasa addrasa aubscnptiDn cr cbanga □! addra&s raqjiasU an printed IflCtaihead (or 
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please inclEida vdurzip 0' poiral cede and a cepv olvQur old label Fma aub&cripTians may not ba available in all nouninas 

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HP'relarad research or seliitiens ro lechnical prnblemE mad? paa^iDIa by using HP vguipmenl are else considered lar pLiblicaEion Please conlact^e 
Edildr belore submimng «ucb articles. Also. Ihe Hew lert- Pa chard Joumal ancnuragas lachnicat dripUEsionb oltba tuples presenlad In recent a III cles 
and may publish letters enpEitlBd to be nhntarDft to readers Laneis should be brial. and ara subrecE la editing by HP 

Copyright ''j 19K Howl en-Pa chard Companv All tigbis rasarvad Parmisaion Ee cupy without lea all or part uflhis DublicaEinn is hereby granted provided 
UiaE 11 dip r:upiesarD net mada. used, displayed, pr disliibuEed tar coniiiierr:iat advaniaae. ?fEliB Hewlatt-Packard Company cupyrighi nptice and Fba tide 
nlEha Dublicatlon and dale appear un Iha cupias. and 31 a nolica staling that the copying Is by peimission of the HewleEI-Packard Company 

Pl9BHaiIdrBsiini]ulnes. submluioos, andrequestEtD Editor, Hnvlett-Pickard Jdumal, 3000 Hanover Straot, PiloAitD, CAW30t JS ' 



© Copr. 1949-1998 Hewlett-Packard Co. 



DwL'ititwr 19tlH Hewlett -PiK^kJirtl Juumal 



In this Issue 



HIn 19B6, shortly after I joined the staff of the HP Joumel, we published an article 
an the HP 8405A RF vector voltmeter. It was one of the first instruments that 
could measure the phase of a signal as well as its amplitude, and engineers 
were excited about it. The display consisted of two analog meters. You made 
measurements one point at a time and plotted them manually on graph paper. 
Advanced for its time, the vector voltmeter would be useless for analyzing the 
time-varving signals and complex modulation types that are common today in 
communications, video, data storage, radar, sonar, and medical and industrial 
ultrasound. However, the same technologies that make these signals possible 
have made the vector voltmeter's descendants equal to the challenge, and no 
doubt as exciting to today's engineers as the vector voltmeter was in its day. The new HP vector signal 
analyzers whose theory and applications are described tn the article on page 6 not only make the tradi- 
tional measurements of frequency, power, distortion, and noise on time-varying and complex signals, but 
also offer new analysis types based on digital signal processing, such as amplitude, frequency, and 
phase demodulation, digital modulation analysis, correlation, coherence, and vector spectrum analysis. 
(See page 12 for applications of the demodulation capabilities, ) The HP B9410A baseband vector signal 
analyzer is the foundation of the family, acting either as a standalone 10-MHz analyzer or as the user 
interface, final frequency converter, signal processor, and display for higher-frequency members of the 
family. Its design, which features a large-scale dithered analog-to-digital converter, is the subject of the 
article on page 31. The HP89410Aand the HPeSMDA RF section make up the !.B-GHz HP89440A RF vector 
signal analyzer. The design and calibration of the RF section are discussed in the article on page 47 The 
HP B9411 A down-convener (see page 5B|, with the HP89410A and a higher-frequency HP spectrum ana- 
lyzer, ertends the family's vector signal analysis capabilities to frequencies above l.B GHz. The baseband 
and l.B-GHz analyzers both have l)uilt-in sources for stimulus-response measurements. Source types 
provided are CW, random noise, periodic chirp, and arbitrary. Behind all of the measurement capabilities 
of these analyzers is a powerful three-processor architecture and a firmware system that's described in 
the article on page 17. 

Optical spectrum analysis is the measurement of the optical power in a light beam as a function of wave- 
length. It's especially important now in the telecommunications industry, where high-performance fiber- 
optic systems are prevalent. Spectral measurements are essential for characterizing the components of 
these systems, such as laser sources, fibers, optical amplifiers, and receivers, and verifying their perfor- 
mance in the system. The HP 71450A optical spectrum analyzer makes optical spectrum measurements 
over a wavelength range of 600 to 1700 nanometers, covering all of the widely used fiber-optic bands. 
The HP 71451A optical spectrum analyzer makes spectrum measurements and offers four additional mea- 
surement modes for other types of measurements. The article on page 60 introduces these analyzers, 
describes their user interface, and demonstrates the capabilities of several ot the downloadable appli- 
cation programs that are available, including programs tor light-emitting diodes, Fabry-Perot lasers, and 
distributed feedback lasers. The analyzers acquire both high dynamic range and high sensitivity from a 
double-pass monochromator design (page 68|. The monochromator, which is the wavelength-selective 
element of the analyzer, is based on a rotating diffraction grating. The grating is driven by a direct-drive 
positioning system that provides both high resolution and high speed |see page 75), At the output of the 
monochromator the light is coupled into a fiber This design provides significant advantages, but it's not 
trivial to keep the light beam accurately aligned with the output fiber as the diffraction grating rotates. 
The article on page 80 describes a two-axis micropositioner that addresses this problem. 

The HP vector signal analyzers featured in this issue are only one of many types ot digital signal analyzers 
manufactured by the Hewlett-Packard Lake Stevens Instrument Division. This division's newer analyzers, 
including the vector signal analyzers, store data in a standard data format that allows Lake Stevens ana- 
lyzers to exchange data with each other and with applications software Isee page 851. Utility programs 
shipped with all Lake Stevens analyzers make it possible to convert data between the standard format 
and other formats, and to edit, display, and plot data stored in the standard format 



4 Decembor IBSO Hewtett-Packord Joumnl 

©Copr. 1949-199B Hewlett-Packard Co. 



New digital cellular telephone technologies, developed to increase the number of users that can share a 
given frequency band, offer prime examples of the time-varying signals and complex modulations that 
HP vector signal analyzers are designed to measure. In the article on page 90, Dave Whipple of Itie 
Hewlett-Packard Spokane Division describes one of these technologies, a code division multiple access 
(CDMAI system standardized by the Telecommunications Industry Association for North American cellular 
applications CDMA is a type of modulation in which all channels in a frequency band use the entire 
band and are separated by means of specialized codes. 

Unlike North American cellular CDMA, the Digital European Cordless Telecommunications standard, or 
DECT, is not a standard implementation of a type of modulation, but a standard protocol for the radio 
portion o! cordless communication links (DECT modulation is actually time division multiple access, or 
TDMA), DECT is defined by the European Telecommunications Standards Institute. The DECT standard 
def nes channel frequencies and data packet formats and spells out the tests that cordless equipment 
must pass to be certified as conforming to the standard. On page 98, Mark Elo of the Hewlett-Packard 
Dueensferry Microwave Division describes the DECT standard and a new downloadable program for HP 
8590 E-Series spectrum analyzers that gives the analyzers measurement capabilitiesfortesting to the 
DECT stanriard. 

December is our annual index issue. The 1393 index begins on page 107. 

R.PDolan 
Editor 



Cover 

By adding a third axis Icolor) to the traditional spectrum analyzer display, the HP B9410A and 89440A 
vector signal analyzers can reveal the frequency content of a rapidly-changing signal in a particularly 
informative way. This spectrogram display represents more than 300 spectrum measurements covering 
the first 20 milliseconds of the turn-on transient of a marine-band handheld transmitter The horizontal 
axis is frequency, but each spectrum measurement has been compressed to fit m one line ol the display, 
with power levels shown as different colors. This allows a single screen to show vastly more information 
and reveals phenomena that would be difficult to spot otherwise. For example, transient distortion side- 
bands can be seen clearly in this measurement although they are present for only a few milliseconds 
just after turn-on. The sidebands gradually disappear toward the bottom of the screen. It can also he 
seen that these sidebands temporarily disappear each time the signal changes direction. 



What's Ahead 

The February issue will have twelve articles covering the design of the HP Des!(Jet 1200C color office 
printer. 



©Copr. 1949-1998 H ew I att- Packard Co. 



Decptviber 1993 HeWleCt-PockardJiiunuil 5 



Vector Signal Analyzers for Difficult 
Measurements on Time-Varying and 
Complex Modulated Signals 



Called vector analyzers for their ability to quadrature detect an input 
signal and measure its magnitude and phase, these new analyzers offer 
conventional spectrum analysis capabilities along w/ith a full set of 
measurements based on digital signal processing. The three-processor 
architecture includes a frequency selective front end and a digital IF 
section. 



by Keimeth J. Blue, Robert T. Cutler, Dennis P. O'Brien, Douglas R. Wagner, and Benjamin R. Zarlingo 



Swept spPTTrum analyzrrs are a fiiiKianienlal tool fur riesign- 
ers working in aU aspects i)relpf-ir(>iiirs at rreqiiencies Ironi 
HF (liigh frfqiiency] Ihroiigli microwave. Tiiey are poweifiil 
and accurate tools for measuring basic signal properties 
such as power, freqiit'iicy. distoition. and noise. They have 
also been pressed into seivice to measure a gi oiip of more 
complex anti dyTianiic signal propeilies tJiat Jtre oflen 
grouped togellier as modulation and sidebands (inteiilional 
amplilude, frequency, and phase modulation) ur phase noise 
(generally uiiinicntional or undesirable). 

Recent trends Iwe conspired to move many of today's sig- 
nals beyond the raeasuremeut reach of traditional .siDeclnini 
aniilyzers. Tliese difficult signals generally fall uilo two 
groups: 

"nnie-vaiyiiig. Burst, [julsed, gated, or linie-ilivision multi- 
plexed signals wlio.se measured properties change during a 
iiieasureriieTil sweep. 

rnniplex n\odtilaled. Signals with modulation that cannot 
be descriiied in terms of simple AM, FM. and PM. E\am|ile^^ 
include the multiple v arieties of Q.AJVl, (JP.SK, and PSK, To 
comphcate the measurement task fiuther, these complex 
modulated signals are often time-varying as well. 

For design engineers in niajiy dilTerenl ai)p)icaIions areas, 
dealing wilh these c<iiii[)leN and clialleiigiiig signals is now 
the nile rather tluui the exception. Kxamjiles include iiiain- 
stieam ap|Dli cations such as video, data storage, radar, sonar, 
and medical and industrial ultrasound. 

By far tlie laigest applical ion area for l.liese diTiiimie signals 
is com numi cations, A staggering jiroliferaiion of wireless 
technologies is imderway, bringing botli new uses and vast 
numbers of new users to the limited RF spectruin that we all 
nmsi share. The only way to accommodate these new de- 
mands within the exisi infj frequency spectrum is to use il 
mure efficiently, ajni diis is the primarj- force driving llie 
ijnTeiised use of complex ajid tinie-\"aryijig signals. 

A telling example is the transition from the current analog 
cellulai- telephone technology to the new digital methods in 
Europe (GSM), -Japan (PDC), and North America ( NADC). 
The capacity of the current analog system has been exceeded 



in many areas, and tJie new teclinologies will support at least 
TWO to three limes the number of users in a gi\ en frequem/y 
band. Wiile they differ in some respects, all three of these 
new stajidards involve signals that are at once digital, time- 
varying, anti complex modulated. They pose a (iistincl chal- 
lenge to traditional signal analyzers, which are optimised for 
sleady-slale and simply moduUited sigiiiils. 

Several types of newIooLs have already been developed to 
address different poitions of these application requirements: 

• Digital Oscilloscopes. Oscilloscopes are excellent, tools for 
captiuing and viewing almost any complex or time-varying 
signal. However, they are optimized for \iewing signals in 
the time tlomain ajid have ijisufficienl digilizing resolution 
and accuracy for precision frequency, power, distortion, and 
noise meiisuremenis. 

• Peak Power Analyzers, Fast-reacting power meters with 
data storage jnid displays can track the power component 
of rapidly changmg signals. They solve one pai1 of the signal 
measurenient problem wlien frequency selective mcasiu'e- 
menfs are not required. 

• Modulation Domain Analyzei-s. Also called frequency and 
rime mterval analyzers, tliey measure the I'requency behav- 
ior of dynamic signals using hust *:er<)-dead-iime counter 
technology. Their analysis is liniiled to frejinency and phase. 
They ciuinoi measure amplil Li<le or thstort.ioii, and cannot 
separate iiuiltiple signaU. 

• Specmmi .Analyzers wilh Sweep Gating. Wliere Iime-\;u7ing 
sign;ils j-ejjeal consistently ;uid wliere a trigger sjgjud Ls a\ail- 
able. some spectrum mialyzers can perform linie-fjated anal- 
ysis. Tlie analyzer sweeps selectively, synclironized with die 
trigger signal, and gradually builds up a measurement from 
many sweep segnienls. 

Vector Signal .\nalyzers 

The anal og-t 0-1 iigital eon\ eision and digital signal pi ncessing 
technologies that ha\-e maik- these "problem" sifiiuds jjossible 
ha\ e also ntade possible a new generarioij of measurement 
solutions. HP's new \-ector signal mialyzers (see Fig. 1 ) rep- 
resent a two-pronged approach to dealing v^ith today's linie- 
varying and complex modtilated signals and the systems 



6 Pi'i-i'iiilipr lOKJ Hewlptl-Pni-karrl .Idimiiil 

©Copr. 1949-1998 Hewlett-Packard Co. 




lhai us* llieni. Traditional measure (iienis — precision niea- 
surenienls of Frequency, power, dislortion. and noise — am 
be made as simply on time-\-arying signals ;is tliey are on 
steacly-siate signals. Tliis allows desigiierH lo use Ihe insijihl 
and design skills tliey have developed for simpler signals un 
more complex signals. Going beyond the iradilional mfa- 
siiremenls, digital signal processing of precision sampled 
signals makes possible a variety of new analysis types in- 
cliidmg vector AM. FM. and PM demodulation, digital modu- 
lation analysis, correlation, cohoicnce, and vector spectnim 
analysis. These new measurements are ideally suited for 
testing throuRliout the block diagrEuns of today's aiivanced 
comnnmication and measurement systems. 

The HP 894xxA vector signal analyzers ai'e all based on a 
common measm-emcnl engine, the HP 80410A in-MHz base- 
band iuialyzer. The IIP 891 KIA acts as the user iiUerface. 
final tiigital IF (intennediatc frequency) section, signal pro- 
cessor, and display section for the entire family of analyzers. 
The HP Sti'WdA and HPSfJllIA use aiuUog RF (radio fre- 
((iiency) harciwaii- lo down-convert hiahci-frciiiiency hiuids 
iitto the infonnalion bandwidlli of llic IIP ffil410A. The IIP 
8!I4-1()A RF section extends the vccloi' signal analysis capa- 
bilities up lo 1.8G[lz. Tlie HP 89-1 1 lA is used in conjunction 
wilh a liigiier-frtHiUency ;uial,vzor to extend die me;Lsurement 
coverage even higher in frcfjueney. This design approach has 
made il imssiblc l<:i develop a core set of mcasurcmenl fea- 
tures in Ihc HP89II0A and make them immediately avaihible 
for use in tliffeR'nt fmiuency rfuiges. This jjrovides measiu-c- 
nient capabilities al RF employing digital signal pj ocessing 
(DSP) leclmiques lhai were previously only possible al 
Iiaseiiand frequency ranges. 

The HP S94iOA and HP S.444nA include a standard dc-to-10- 
Mllz signal source. The HP 8f)44tJA also offers an optional 



Fig. 1. The HP 8iJ4inAvMlOE 

sigiul arialj^zer (cenler) is a 
H)-MHz liasclwdfl aiial.\-Zf r. It acis 
as tfif usf-r liiUTfate. rtnal 'ligilal 
IF. signal proi'rasor, and display 
sprliod for ihp HP 894ss.A family 
of analyzer?. Tii-.- HP mUDA RF 
5e<iiciii (Imttdiii Ipfl ) pxietids llu- 
analj-zers rapabllilies up lo 1 .8 
Gllz. The HP894ilA (righl) is 
used a liLgher-frmiiwicy ana- 
lyzer i<j I'sieiid nieasurcnieni 
(.-overage to frpQuency riuifjes 
above 1-8 GHz. 

RF source. Both the b;ise!)ai\d ami the RF sources feature 
multiple source lypes for fiexibilily in circuif stimulation. 
Besides CW, random noise, and periodic chirp sources, the 
anal.yzers uiclude an arhili*ary source. I'sers can create Uieir 
own signals or capl.ine signals for later playback. 

Signal Flow 

The high-level measurement processing in the vector signal 
analyzei-s is shown in the signal flow diagram, Fig. 2. The HP 
8r)41().A. has an ahas-protected analog ijiput bandwidth of dc 
to 10 MHz. The analog input signal flows into a hybrid dith- 
ered ADC (analog-to-digital converter), which sfunples the 
signal at a riile of 2G.(i MHz, Tfie digitiil daia slremn coming 
oul of die .-MK' is rouied lo die digital local oscillator 1 LO). 
Tlie digital LO [jerfomis frequeuc.v trmislalion ajid qiiadraliire 
detects the di^tized input signal, resulling in a complex data 
stream consisting of real and iniagiuaiy pails. Tliis liill-rate 
data is Iheii inpiii to the digital decinialing llllerK. These 
filters ijeriVirin hiiiaiy decimaiion (divide-liy-2 sample rate 
reduction) in addiiion to providing image rejection. The 
output of these digital filters represents a bandlimited digi- 
tal version of the analog input signal in the lime domain. 
Tliis digilal dala si ream is then ca|jlured in a sample RAM 
(random-access memory). The sample RAM is a ciicular 
FIFO (lirsl in, first out) liiiffer thai collects uidividuid dala 
samples into blocks to be nianipiUatetl by the DSP {digital 
signal processor). 

The AD(^'. digital W). and filter sectioiLS are key contributions 
In the I IP 89 lxxi\ analyzers. These blocks allow the use of 
powt'iful D.SP leclmiques liii syslems and signals that liave 
wider infomiatioii baiidwidths than previous fasi F'oiuier 
I ransform (FFT) analyzers have allowed, as explained later. 



HP 8944DA RF Sectiofl 
otHPSHIIA 



HP B94II1A or HP S9440A IF Seclian 



o- 



ttEituuncii 
translalion 



-o 



Analog 
Fiant End 





Digital 
Oecimdling 
Tillert 




DSP 


■1 

IB 


GSP 


1 
■ 


DnplBy 




CPU 


■ 





Fig. 2. .Sigiiiil fluwarcliitei^iun- uf ilic 111' H!)4xxA VPPlor slanal aiialyztrs. (ADC = fliiaii«-Ui-diKli.;jl umiverter. LO - local oseillwuir 
DSP = dig'tHi signal processor. liSI' = graphics system proeiissiir CPU = eentral processing unit,) 



©Copr. 1949-1998 H sw I att- Packard Co. 



UfH'Piiiliur U(!*i lliiwlftl-Pai'karilJoiinirii 7 



The fimdanientaJ data used to produce all nwiasurenietit 
residt.s, whcdier in llie riiiiiiency dumaiii nr the liiiii- rlfinifiin, 
is Uie linie dala ii'oJiefied iii llip siuiiplf liAM, Tills cjm lie 
conirasted with swcpl spefliuiii anaiyzcrs, ivhicli prodiK-e 
rpsulls direcLiy in the fritiutncy [Joniain iLsing swept tiller ainl 
deiection Techniques. Since the inpiil signal is prot-pssed aiid 
captured in iJiis fundamental tinie-donuiin furni, all aspects 
of the signal at tliat niornent in time are eapHu ed — -aiiiplii uik', 
frefiuenry, and phase. The application of DSP algorilhins tm 
this time data can chiu acterizc and allow the user insigiil 
into all of these different iiews of the signal. 

Multiple Processors 

The HP 894xxA uses a set of three processors, each opti- 
mized for specific computadonjil and control tasks, to per- 
fomi the measurement poslproc essing on the saniple RAM 
time (lata. The.se processors are imder the control of a real- 
time rnuitilasking operating sysl em (pS(!lS'") (o pro\1(!e over- 
lapped operation and maximize Ihrouglipul.. The main CPU, 
which haJidie.s system managenieiii and processes u.ser 
mpiii . is a Motorola fiSECOSO. 

The difiilai signal processor used in the HP 894xxA family is 
the Motorola DSP96002 floating-point dual-port processor. 
Tliis processor jierfonns all the block-oriented mathematical 
operations required by the DSP alsorithmH. A typical se- 
quence of DSP operations to produce a spcctnun result 
from the time data might incluile liie Tollowing: input, scal- 
ing, resampling, linif-douiain correcl ioas, wiudowiug, KFP, 
frequency-domain corrections, and display sealing. The HP 
894kxA DSP architecture provides many more features thati 
just the computation of FF1\. It has a full set of prcflefined 
measurement results meluding time, powei" spectrum, 
powei' spectral density, autocorrelation, crosseorrelation, 
cross spectrum, coherence, frequency response, and others. 
The IIP 894xxA also supports a user-defined math capabihty, 
which allows the user to specify custom algorithms to be 
executed by the DSP. 

The GSP (graphics system processor) used in the HP 8!14xxA 
family is the Texas Instnmients TMS;3102O processor. The 
GSP and color display provide display capabilities that ex- 
ploit the measurement features of the HP 894xxA, These 
include multiple trace displays for simultaneous insigiit into 
different result domains, such as time and freiiiieru'y. Oihei' 
featiues include the use of waterfall and specirograni dis- 
plays to project measurement results into a third dlmeiisiim 
on the display — history' over time. This is useful for data 
presentation since the measurement and display tliroughput 
of the IIP S94xxA can often reach or exceed 60 updates per 
second. 

A flexible firmware architecture was required lo realize (he 
large featme set of the HP 89-lxxA, A core set of fuudamental 
tow-level DSP routines was de\'cloped to be used by all the 
various lueasureLnenl modes and features within ihe aitalj"^- 
ers. At a higlier level, an exteasilile measurement architecture 
was designed 10 use these low-level DSP routuies 10 pro- 
duce tlie riiffereiit measuremeul results. This ardiilecture is 
described in detail in the article on page 17. 

FFT Use in Previous Analyzers 

Most spectmm analyzers do not use FFT proce.ssing to 
produce frequency-domain measurements. The majority of 



spectrum analyzers employ swept inlennediate fn^iiiency 
ilV) hardware filter techniques lo measure signal antpli'udp 
chai"act eristics in the frequency domain. These aiiiilyzers 
have analog resolution bandwidth filters with electrical 
characteristics (hat limit their sweep rate to approximately 
one-half the s(]uare of the resolution bjindwiddi. .Since tiiese 
filters require a finite settling time before they can acciualely 
repi esenl ihe amplitude of a signal passing dirough the IF. 
the signiU of interest must be static (nonvaiianl) over time. 
This produces two constraints for the user: the measure- 
ment speed for narrow resolution bajidwidths is shiw and 
the signals must not vary over lime. TIie.se bmitalions can be 
overcome willi ihe apijlication of digital signal pioeessing. 

The HP 894){xA vector signal analyzers represent the second 
generation of HPsign:il ;mal,vzers ihal employ digilal signal 
processing at RF. Tlie HP ;35S8A' sperlrum analyzer and HP 
:15a.<)A network analyzer were pioneei^; in using digital lilier- 
ing and FFT processing tedinitiues in eoi\jimt t.icm with tradi- 
tional swept analyzer hai'dware. These [iredecessors used an 
ali-digilal final IF section lo .support higher-speed swept 
nieasui-ements while also suppoiUng an FTT mode for faster 
naiTowbaiid measurements. These ituslriLments laid the 
groundwork for the measiLrement arcliileeture and approaeii 
of Ihe HP S!14xxA family of vector signal analyzers. 

The application of the FFT in signal analyzers has been 
restricted In the pasi because of one of its fuLidmiieiital algo- 
rithmic relalionshljis— die frecjuency handwiddi of mi FFT 
result Is direeily relat.eil to Uie .sample mte of the mpiit data. 
This gives rise to two design challenges in incoq>orating the 
FFT into higher-Crequeuey analyzers. Because of the sam- 
pling rate constraints of high -dynmnic -range ADt's, the fie- 
C|uency coverage (information bandwidth ) of previous FFT 
analyzere has been hmiled, usually to less than 100 kHz. 
This has restricted their application lo measur ements of 
phenomena with a low information bandwidth (e.g., roiaiiiig 
machuiei7, servo loops, acoustics, ami mecbtmical vibra- 
tions). .\iiother prevaleni limitation in these previous analyz- 
ers is also a result of 1 he FIT fundamentals and hardwai-e 
limitations. If Ihe .selections of saniphng rates for time- 
doinaiii data are limited going into the FFT algorithm, the 
user's abihty to select an aifaitnuy trequency sp;m for analy- 
sis is also limited to predefined and fixed analysis spans 
(e.g., 100 kHz, 50 kib, 20 kHz, 10 kHz). A user who requires 
a span between one of these fixed values is forced lo select 
either a larger biuidwidlh. thereby sacrificing signal isola- 
tion, or 3 smaller biuulwidlh. thereby sacrificing insight into 
adjacent speeiral aeii^ily. 

FFT Ad\'antages 

So why bollier designing an FFT-bascd analyzer.' The an- 
swers are speed and infomial ion. In a swept analyzer, the 
filter miLst be swept to a freiiueney and settle<i before a result 
is oblained at that suigie (requeney'. The filter is then swept to 
the next frequency, and so on. The PIT algoiithm emulates a 
parallel bank of filters thai can settle and measure simulta- 
neously. For compai-able-resolulion fillers, the FFT nieasiu^ 
nient can be much faster than the swepi filler technique. 
The second pari of the answer is inl'onnaiion. Since the Ume 
data is the fundamental data type in die HP 8tl4xxA signal 
flow, all chaiacteristics of the signal are [irescrved for sub- 
sequent analysis. Tlie FFT algorithm is a particutarly useful 



8 npi-fmbor laa^ Hfv.Ii'11-rii.-k.irii rtiiima] 

©Copr. 1949-1998 Hewlett-Packard Co. 



way to cliarat terize ainpiilude aiid phase ai all frequencies 
simuhanedusly. This makes the FFT technique useful for 
measiiriiLg lijiie-variaiu ()r (raiisieni isignids. 

Another ad^'antage of the FFT is that il provides true-m\s 
detection. This is useful in making band power measure- 
meiils. TrndilioiiaJ aiial.vners usually employ a peak rietec- 
lioH merhanism to ensure thai signals are "measured" when- 
ever the resohition hin i^dih is less than a display bin nidih. 
However, the result of this operation is thai Ihe noise floor 
is biased. The FFT algoritlini has ii') such effect and thus 
allows accurate siniultaneous measurenieiils on signals and 
noise. 

FFT Design Challenges 

The fixed ftequency span constraint was a significant issue 
m the design and de\'elo])meni of the IIP 894xxA family. 
Traditioiial swept sppctnun analyzer users are not accus- 
tomed to beuig limited to predefined analysis spans. With a 
swept IF analyzer, the user lias almost iidinite control over 
seltina the measurement span. Another significant issue was 
the Bxcd relationsiiip of resolutiof I Ij^uitiwidlh to the span and 
whidow filter function |jreseni in FPf aii;il.\'Kers- SwepI spec- 
Inmi analyzers have tlie freedom to seleel a span ai liitrarily. 
and then to change the resolution bandwidth, changing the 
frequency resohition. 

These challenges are overcome in Ihe HP S94xx.\ while still 
using the FFT algoridmi as the basis for all frequency-domain 
results. Tlie fixed frequency span limilalion is overcome by 
using the DSP to alter the sample rale of the lime data input 
to the FPT alf;orilhin. Gy changing the sample rale in soft- 
ware, ibe user is given back the ability to control llie sr)an 
setting arbitrarily without sacrificing the a(}v;intages oC FFT 
processing [see "Frequency Selective Analysis" helow], The 
frted t'esolniioii bandwidtli/span/window limitation is over- 
come by another i)Sl' I echniqne. Using fretiuency-domain 
inlt'ipolation Izeni paddinj!) in the lime domain before the 
FFT, the effective filler liandwidth of a window can he 
changed whhouf changing Ihe span of Ihe meiisnremeni. 
This restores Ihe abilily nfllie user to control resolution 
bandwidth iiidependenily ol'.'span IwUtiin hmits). 

These cJiaracleristics combine to provide a I.radilional spec- 
trum analyzer look and feel in what is limdamenlally an Fl-T 
aiLaJyzer. The HI' K94xxA's scalai' measuremeni mode is a 
prime example of Ibis transparency. This measitremeiil 
mode provides (he jjrealesl independence of span and reso- 
lution bandwidlh of all Ihe measurement modes. When .sel 
,^as|)an mid a resoltiliun biniilwidth that allow coverage 
by a single FFT, litis measurement mode will sel ail of Ibe 
fre([ueney translating I,()s (both analog and digilall 1o fixed 
values and process Uie inconimg time data wiili a single FhT 
to provide the entire mejisiuemcnl result al once. If Ihe user 
widens the span or reduces the resolution harulwidib so ibal 
the measurement eaiiiuit be realized by a single FtT, the 
scalar measurement mode will revert to a stepped mode of 
operation. In thLsmode, Ihe frequency translaling Lflsare 
stepiied lo llie beginning l)and within die swee|). f)[ice 
settled, the inatnunenl collms a time record and Iraiisforms 
il lo produce Ihespecli-ai result for thai segnieni of Ibe 
sweej), Tills segment result can be nin Ihroiigh a software 
peak detector nmniiig in ibe DSP if the re.sohilion band- 
widtli ur bui widDi of the FFT is naiTower lhaii Ibe display 



bin width. This detected se^ient of the sweep is then pro- 
cessed for a partial update of ihe display. The measurement 
ihen steps the LOs to the nest frequency span segmeni and 
repeats the process until the entire measuremeni span is 
covered (end of sweep). Since the display is updated as Ihe 
I/)s and FI'Ts are (racked across the span, the appearance 
is the same as lliat of a traililional swepi spectnmi analyzer. 

Frequency Selective Analysis 

FVequcnc\' selecd^ e analysis is a term used to describe the 
leclor signal aniilyzer's abihty to apply a bandlimiting filter 
to Ihe measured signal to limit the signal's information band- 
width. In one sense this capaliiliiy is not unique. Many osciUo- 
scopes liave a bandwidth limiting filler Also, most traditional 
spectnmi analyzers ha\'e a set of resolmion ban<1widlh filters 
trom which to choose, These filters Imiii llie infomialion 
bandwidth of the signal applied to the detector, Wlial makes 
the vector signal analyzer unique is Ihe combination of infi- 
nite atljustability of the filter bandwidth with nearly ideal 
filter characteristics, ttliat ai e ideal characteristics? In this 
application an ideal filter would tia\'e a frequency response 
t onespondhig to a BECT flmcrion. That is, il would ha\ e hnear 
phase, zero group delay, no amphtude ripple, infinite slop 
band uttenuatimi, and a 1;1 shape factor. Oh^usly Ihe filter 
described caimot be realized. However, using digital filters, 
resampling techniques, and time-domain corrections, a fil- 
tered signal path cim be created that very iieaily meets these 
goals. The I'omposite filler and time-domain corrections 
tyjiically result in a filler with a 1.3: 1 shape factor, passband 
rip|ile less lhan (1,1 (IB, zero gn.iup delay, phiise linearity of 
±1 dcgiee, and stop-band attemialiun of 111 dB, This filter is 
optimized for isolating signals, and not for lime-domain 
chaiacteristjcs such as overshoot atul ringing. 

Why does one need infinitely aeljustable filters? There are 
two answers r.o this question. The first answer is based on 
the types of measurements that can be made with a vector 
signal analyzer, such as modulation iinalysis, Tiie vector 
.signal aiuilyzer lias buill-in deniodnlation capability, and like 
any receiver, die fidelity of ibe demodulated .signal will be 
degiaded if signals other lhan llie one !,o be demodulalcd are 
present. In most receiver .system.^ the IF bandwidlh is made 
as narrow us possible lo provifle an opiiiimm amount of se- 
lecli\ity. Since Ihe \'(Tli:r signal anal.vzer is a general-ptiniose 
tool and not a receiver optimized lor a particular type of 
signal, the optimum batiilwidlh for the filter cannot be deter- 
mined beforehand. IVIany traditional speclnim analyzers also 
have demodulators, lii these instruments the resolution 
buiidwidih filteis serve to limit Ihe information biuidwidth. 
However, with only a finile number of resolution bandwidlh 
fillers to choose from, it's possible thai the user is left with a 
choice between a filter that is either too narrow lo pass Ibe 
signal or loo wide t<i reject anollier signal in close proximity. 
With the vector signal aiial>7.er's infinitely adjustable hand- 
widlli, an optimal inf'ormati<in bandwidlh ciui be set for any 
class of signal or measurement, in the vector signal analyzer 
the lenns span and inf'onnatiiin bandwidth mean roughly the 
same thing. The only tlistiri<lioii between Ihe twit is that the 
infomialion bandwidth eorres|ioiids to the :i-rl(i batidwiddi. 
wliicli is 12 to 17 percent wider than the span. 

The second reason for wanting variable information band- 
width has to do with selecting a span, or equivalenUy, a 



© Copr. 1949-1998 Hewlett-Packard Co. 



betfiiititT ISlUa HewleU-pHckard Jouniat 8 



sample rate. With swppt analyzers tiiere aren't any liniils iiti 
the selection of s|iiUT. The user is frei? to ehoose any stari- 
sfop tipiiuency jiair TIds has nol liefii true for instnmienlH 
using FFTs lo fomimte ihe spertmin. In tliese ijistnuiieniji 
only a rixe<l nuinljer of siiaiis c:in tie selccl oil. This limi(atiijii 
is baseil on llie ijislnmienCs abilily to change iho rate al 
which rhf (lala is samjileil Iti most FFT imalyzcrs, such as 
ilie Hi* .'}.i6liriA (lyruimic signal analyiier. ihe saniple ratf is 
f'haiigeil al'ter Ihe sigTial has lii^cn digilizcd liy ihe AI.K". This 
is (lone llirough the use of digital (icciniatinf; fillers. These 
filters, which are i[ii])leniented in hardware, halve the sam- 
ple rale by first hiuidlimiting the liala aiid then discaniiiig 
every uther s!iniplc of ihe resulting oversaiiipled data. Fur 
example, the ADC iji the HP 8i>l lOA operates al a 25-(i-MH/ 
rale to produce a lO-Mliz miixinuim spaji. To reduce Ihe 
span to 5 MHz, the signal is passed llirough a digital llller 
whicli rednces Ihe liandwidtlior Ihe signal froiii HI MH/ lo 5 
MH;c. Then, every oDier sample is discarded lu produce a 
12.8-Ml Iz sajnple rate. By cascading severai tleeimaling ni- 
ters, tile sample rale and span crni be changed by 1/(2^) 
where N is Ihe number of deeiinalijig filters used. For more 
delail im how these fillers aie implemented iji the vector 
signal iuialyzer, refer to t he article on page 31. 

The derimat.iiig fillers allow Ihe sianiple rate and span to be 
changed by powei's of two. To obtain an arbin ary span, llie 
sample rate must be made infinitely ad.iuslable. This is done 
by means of a resampling or inTerjjolatioii filter, which fol- 
lows the decimation fillers. A brief descri[ it ion of the resam- 
pling algorithm and liuw it can lie used to change tlie sample 
rate is given in "The Rcsampiing Pi'oce.ss"" at right. 

Urn e- Domain Corrections 

Wlien il cnmes to c;dibratioii.s and con-ections, pre\ious 
generations of FFT ajialyzers have mostly ignored the time 
data. This has occiuTed because it is much easier to correct 
the frequency sped mm data using muhiplicalion lhan it is 
lo correct the lime dala Ibnjvigli convoliilioii. In Ihe veclor 
signal aiialy/er ihe accuracy of Ihe lime dala is very impor- 
tant, Nol only is il the basis for all of die demodulation mea- 
surements, but it is also used directly for measurements 
such iis instantaneous power as a function of lime, Correct- 
uig tlie time data is the last step in creating a nemly ideal 
bandlimiting signal path. 

Wliile the digital filters and ipsampling algorifJnns are re- 
sponsible for esialilislnng the :irbilraiy bandwidth (sami'le 
rate and span). Ihe lime-doraain conections determine Ihe 
final iiassbmid ( haracierisfics of the signal paih. Time-domain 
corrections w ould be unneressaiy if the analog and digit;d 
signal paths could be made ideal, rnforfunately, achieving 
nearly ideal cliaraci eristics in Ihe analog and digital fillers is 
either impossible or impracliciil. For example, tlie bardwai e 
decimatioiL filters aie iniplenienteil as inllnlie inipidse re- 
sponse (JIR) filters rather than as linear-phase finile impulse 
resjDonse (FIR) filters. The IIR filters were chosen o\er the 
FIR filters because Ihey ai e more eeononiical lo implement 
given the requiremems for speed, shape factor, stop-band 
rejection, and tlie nmnber of slages of filtering requii ed. 

nme-domain conections work as an equalization filter to 
compensate for passliand imperfections. These imperfec- 
tions come from many sources. Tlie IF niters in the HP 
89440ARF .section, tiie analog aiili-aJiaaing filler m front of 



The Resampling Process 

In the HP 894xxA vecioi signal analyzers, ihe ADC and djgiiBl liliers prnduCB a 
digilal Eequance "i[n), which isoblained hy sampling ihe lillerad input signal 
at an effective sampling inierval of T[ The resampling algunthm prorfuees a dil 
fHieni sequence *2[ml lhai is irfeniical In Ihe sequence Itiai would have been 
oMained had xHj tieen sampled at a peuodic irrterval T2 ^ T, In mhsi words. 
fBsamplmg clianyea ilie sample rata from 1/T) to l/T; aflei sampling has already 
occurred. 

TliB basic i:oncepl behind i^sampling corties from standard sampling itieory, which 
stales iliai a signal can ba iH[:onstnii:te(l fram (is samples pcnuiriKri Hiai the sam- 
ples are spaced so as in pieveni aliasing Using ihis concepi. ihe musi direcl 
approach to changing ihe sample rale would lie to lecotisiiuui the oiiginal signal 
x(i| from a sampled version of the signal and than sample the reconstructed signal 
at the new sample rate 

Huwevar, it's not necessary lo cuni/erl the signal back into its cnnimuuus form id 
create K^jml In ilie following [lenvaiior. the sequeni:ei|(n| is Heated as a contin- 
uous signal and is represented as a series nf weighted Dirac delta funclions The 
sequence <iln] is described by 



= y 



!i|i)6(T-nT, 



Tlie original signal Is reconstructed by passing xiIt| through a recnnstiuclion filter 
The filler is desmbad by iis impulse lespnnse h(t| Assuming that hill is an appro- 
priate Tiliai, than IS obtained by performing the convolution Si[tl*h|il To distin- 
guish ihis resuli from ilie original xli|, the tecDnsiiucled signal will tis refensd to 
as x|i] Performing the convolution. 



m-fj 



x,|T|h(t-T|dT, 
Using equations 1 and 2. 



121 



illl = 



xlTl6|T-nTi| h(i-i)dT 



= Y 



x(T|h(t'-Tl6(t-nT,)dT 



With the order of summaiinn and integraiiun teversad, the sifting ptaperly of the 
Diiac delta lunciion can be used to evaluate Ihe inIHgral. 



ill) = y x(nfi)h(T-nTtl, 



131 



Given that Ulz) is known (or any value of t, egualion 3 can be used to calculate x(i) 
for any value of t from the samples >;(nT|| Limiting the values of t to the sample 
points foi Ihe seqiiencB i^jm] produces ihe desired result, which can be wniten tn 
sequence form es: 



"iN = V xiln|h(mT7-nT]) 



I'll 



Robert T Cutler 

Development Engineer 

Lake Stevens Insirument Division 



Ihe AI.K', and the decimarion anil resam|iling filtere all con- 
Iribule to passbanci tipple and phase nonlineaiities within 
Ihe selected span. Tlte time-domain coireclion or eqiialina- 
tion filler imisl comiiensate for these imperfections wilhoul 
ailding any of its own. 



10 ni,ci'iiihfr lW;i ik-wlcd-Paefcuil-Iijiinial 

©Copr. 1949-1998 Hewlett-Packard Co. 



The design of the equalization filter has many constraints: 

• The filter nuisl compensaip for amplitude imflatiiess ajul 
phase noniinearitj' wiihin the sperifipd infumiaiion hatul- 
«idth I span). 

• The filler must not eoinpensate beyond the spaii or Ihe 
desirable effects of the |>re\ious filters, siirh as stop-hand 
atteniiatii III. will Im? diminished. 

• The filter miist have a miiiimum-lengih impulse response, 

• The niter nuist be designed on-ihe-fly for the current insfni- 
iiieiil setup (e.g., span) using the ralibration data stored in 
nonvolatile RAM. 

Tile design of the equalization filter begins by extracting tlie 
appropriate information about the analog signal path fiom 
the r;dibi-alion data. This calibration data, wliirli is generated 
by llie instniinenl during seLl-calibration and stoivd in noji- 
volatile nieniory. contains data for all jiossihle instrument 
configurations. The data extracted for the filter design will 
be a function of the selected center frequency, span, input 
range (attenuator sellings), coupling, and input impedance. 
Once pxCracted, the data is used to create 3 Ereqtiency-do- 
main conection vector (curve). 

Once the analog coiTPction vector has been computed, it is 
modified to include the effects of the decimation fillers and 
tlie resampling filfer. W!ii!e the frequency response of eadi 
indi\idua! filter is known by design, the comliineri lespojise 
camiol be l ompiiteil until jitter tJie user has selected a span 
because die span detemiiiies the number of stages of decima- 
tion as weU as the resampling ratio. The composite correction 
vector selves as the basis for (he design of the equalization 
Tiller Ihiil will be applied lo the time data. 

Fig. 3 shows a typical plot of tiie composite analog/digiial 
correction \Tctor for a fl.!)-MHz span. Tlie ujiper trace shows 
that the aniplifude of the correction vaiios over ti.2 dB. The 
lower ti'ace shows the amount of comiieiLsation needed to 
correct phase nonlinearily. Over the (l.il-MHz span, this 
corresponds to group delay distortion of over iil)(( ns. 

Band limited AM, PM, and FM Demiidulatitm 
Advaticed lloaling-puinl DSF power in the IIP894xxA ha-i 
enabled tlH' developiiieiil of high speed AM, PM, and FM 
den lui I Illation aignrilliiiis<ai>ul)le of uji to 111) disjilay upilales 
per second. The hardware digital local oscilluior ami deci- 
maiiiiji digital fillers allow hilly alias-prolected, bandlhiiiled 
deniotliilation with spans as small as 3 Hz. and as large as 
7 MH;^ for (lie RF receiver mode and 10 Mll^t for baseband 
receiver modes. Addiliiinally, ilie HI' S(I-1IIA exieuds die liF 
Ireqiiencies that can lie ileniodiilated lo well aliove \.H CH'/.. 
Tlie ivide frequency coverage, baiidlimiled analysis, liigh 
accuracy through time-domain calibration filters, and typical 
dynamic nmge above 70 dB offer new iusif;hl into many de- 
iiiodulalion applicatiotis (see ".\piilicalioiLS for Demodula- 
tion" on page 12). 

Tile aiiiilog deniodiilaliiin signal processing l)locK' in Fig. 1 
on page 17 coiilahis die AM, PM, and FM demodulation 
algoi idims. This hlfick precedes time averaging, the only 
difference between the analog demodulaliou ;iiid vector 
nieasiiremenl niodes. Thus, most of the tinie-wavefonn .signjil 
procMsing capabilities in vector niciLsuremcnts, such ;ls 
sf>eclninis, liiue gating, and averaging, iue also available for 
demodulated time waveforms. For example, a PM spectnini 



TRACE A 01 Cal 
AOtfsel 

9 

itB 

LogMag 



SMSOODHz 



6.781 ItB 



1 



-1 

dB 



CenlBrSMIb 



Span: S.9 MHz 



TRACES: Dt Cai 



900 

too 

/div 



-in 
ifeg 

























































































E 


I 






























1 





























































Canter 5 MHi 



Span: 9.9 MHz 



Fig. 3, Tlie vBKwr signal analyzers correct the basic titiie data for 
siMiiiil |i;ith iiiipnrfpt'lions. .Shciwn here is a typical plot of the ninfitu- 
njfie [Trsre A J fUitl phase C&ace B.) of the composile aivilti^digilal 
i-iirrectifiit verUir for a 9,i)-MHz fipiin. 

cmi be averaged to generate a phase noise display. Tlie mea- 
surement processes time data in block fiisliion with the res- 
olution bantlwidtli determining the length of the I iiiie record. 

Low-ft-equency information can be carried on a high- 
frequency sinusoidal signal, or carrier, by varying the 
carrier's ampliriidc and iihiLse iuigle. Tlie instantaneous fre- 
quency of a sinusoidal signal is given by (lie lime derivative 
of its phase. Thus, the frequency niodulalion of a carrier can 
be computed by taking Uie tinie ck'rivative of the phase niotl- 
ulation. These are the basic princi]iles applied in the IIP 
8y ixxA ileinodulation algorithms, originally developed lor 
die ilP :i5G2A dynamic signal analyzer.^ 



A carrier without niodulalkin can be expressed as: 

CeJiWft+it'd 



(1) 



Willi aniiiiiiiKie C. frequency nv, and phase offset f]),.. Con- 
sider a complex modulating wavel'onu: 

m(t) = [l +a(t)lei''"" 

where a(l ) and ^{t) are real amplitude and phase modulat- 
ing waveforms, respectively. Tlius. a real modulatetl carrier 
can he wrillen im: 

x(tj - limdjej""'-'-'*'! + CmnOe"'"'"'''**"' 
= 2C|1 -<- a(t)] C0H[ci),;t -I- <ti,. + (|)(t)l. 

■ffitOc is large enough to prevent sidebands in the positive 
and negative frequency images of m(l) from overlapping 
when summed in x(l ], the amplitude ;mrt phiLse iiiodidating 
componcnls in m(t 1 Ciui be iiiialliliiguously ohiaiiu'd liy 
shifting die posit i\ e Ireijueiiry image in x(l ] down lo a fre- 
quency band near tic and filtering die complex time wavt*- 
tiinii with a digital low-pass filter to reject tlie other image. 

(cominiied an ii^ge 131 



©Copr. 1949-1998 H ew I att- Packard Co. 



liwr'iiilHi liiw;i llewleir-Patluir'l.liiiiirial 11 



Applications for Demodulation 



The HP BMnA veciur signal analyzers provide digilai signai proiiesfling IDSPI 
tJemodulaiioir alfjisnll-iriis lu esiraci AM, PM, aril FM moriulanrg signals Wirli ilie 
demodulaiot plated upsrream in the signal ptocessing How, nary ul [Ire vetWr 
siyiial anaiyzei's powerlul arialysrs tealjies can be used on rterncuiulaied wave- 
farms A few ol itie many possible applications of veclor signal analy?er 
demodulation are described here 

Phase Noise Measurements 

Iliecliara[:leiizatiunDi piiasenaise rs an increasingly impcrtani rBquiremflni m 
mndEtri [□mmiiricalinns sysiems Tfadilionally tins has beena ueiy difficull and 
iime-tnrisuming measurement The HP B^itiA vectui signal analyzers greatly 
lacilitate this measurement for all systeitrs but tliose with Ihe most demanding 
phase noisa repuiremenls The powei of these analyzers is in then ability In make 
very fast direct phase noise measurements in any domain relevant to ihe user For 
instance, transmitter designers are most likely interested in measuring the noise 
spectral density erounrt the earner or the iniegraied band puwer in adjaceni chan- 
nels Users iniereslEd in the recovery nl digitally modulated intormaimn may bo 
mnsi cnncerned wilh the peak or rms phase deyiatron ol the recovered vectois, 
which can be direclly measured using ihe vector signal analy/ers' PM aemadula- 
Iiun capsbilily The HPa94uA produces results quickly and easily in any of these 
domains, A typical measurBmein Is shown in Fig. 1. 

The vector signal analyzers a/e capable of mathematically locking to unlocked or 
drifting carriers using the AulnCartier features las long as the earner is not digi- 
tally mndulalEdl. allowing fast and accurate averaged measuremenis even under 
these uondiiioris How lasi is 'Tasi"7 For many measuremeni situations, users will 
make accurate measuiemenis in seconds thai had previously taken mmuiBS or 
tens of mmutesio complete Measurement speed improvemenis of 10 to IDOO 
limes can be eipecled For more inlonnation refer to HP Product Note B9440A-2 

VCO Turn-on and LO settling 

The vector signal anelyier's demodulailon capatnlilies are powerful fur a variety of 
VCO or local oscillator iiansiem measurements For example, the frequency traiec- 
tory ol a VCO at turn-on can be evaluated using the FM demodulaiinn feature. 
Similarly, an LO or phase-locked loop transieni, such as that following a frequency 
change, can be directly measured lor frequency or phase settling lra|HClories 
using Ihe FM or PM demodulation features For amplitude or power vartalidns 
during the transient, the equivalent of a jero-sfian measuremeni is used, rather 
than the AM demodulation feature (see "Zero-Span Measurements" belowl This 
IS because the AM demodulator measures percent AM and the carrier power 
estimation is biased by the transient event itself. 

When a transient isacquired in time -capture mode, it can be played track into the 
measurement as many times as is desired Thus, one playback can be done with 

TRACE Ch1 PUPSD 



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Start 125 Hi Slop: 10D kHz 



Hg- 1. A phase noise measuiBmeni made by an HP S94xxA vector si^l analyiei 



THACE A: Ch1 FM Main lime 

AMfkei I,5IXHMms 10.188 Hz 



Real 



Hi 



-50 I I I , i ' 

SMrtDi Slop: nn 

Fifl.Z. An m ItsQuency settling mesBuremeni 

the analyzer set up to make the equivaleni of a zero-span measurement, and 
another into the Pfvl or FM demodulator Displaying both results together provides 
a side-by-side compadson of insianianeous earner turn-on power and phase or 
frequency during the transient 

Because of the vector signal analyzer's internal quadrature down-conversiun. 
phase and frequency modulation are measured essentially continuously through- 
out the time record, without itie cycle quantization limitalions of counter-based 
modulation analyzers This resulls in an extremely good combination of lime reso- 
lution (of Ihe transient event) and frequency or phase resolution fnr those rtemodu- 
lalion types An example ul an LO frequency settling measurement is shown in 
Fig 2 For mora information reter to HP Produci Mote 89440A-5 

Zero- Span Measurements 

Zero-span measurements with swept spectrum analyzers measure the AM enve- 
lope as a function ol time The vector signal analyzer also measures AM envelope, 
but with a differeni approach thai offers distinct advaniages over traditional 
swept technology 

In a swept spectrum analyzer, the center frequency is set near the can'ier Tlie 
frequency span is sei to ^ero to prevent the LO from sweeping The sweep lime is 
sei to some nonzero value, limited by the resolution bandwidih hiters and detec- 
lur In a vector signal analyzer, the RF signal is dnwn-convertefl tu IF and sampled 
The signal is then down-convened in single-sideband fasbion to dc anti handlimited. 
The bandhmiiing is performed by digital fillers, which perform the (unclional 
equivalent of resolution bandwidth filters for swept spednjm analyzers in zero-span 
mode. Thus, in a vecior signal analyzer, the user sets ;he measurement span, not 
the resolution bandwidth Setting the width of the mam ume record is equivalent 
to setting the sweep time The complex lime waveform is converted into an AM 
envelope by selecting either the linear or Ihe loganihmic magnitude data format 

For clarification, Ihe veaor signal analyzer implements iBSolution bandwidth 
liters by way of the fast Fourier transform algorithm The input signal bandwidth 
is hist limited by digital hiters set a little wider than the measurement span In 
generating a spectrum display, the FF can be thought of as a parallel bank of 
narrow resolution bandwidth hiters that form a comb across Ihe measuremeni 
span However, no FFT is used if the lime domain is displayed Thus, Ihe band- 
width for lime displays IS limited only by digital hiters The only efleci of changing 
tlie resolution bandwidth an a time-domain display isio r^hangeits length 

Communication systems are requiring measurements of increasmgly faster canier 
ramps with ;Bro-span measuremenis The rate ai which a zero-span measurement 
can handle a earner ramp is hmited by the rise time of the selected resolution 
bandwidth filiet As the resolution baixtwidth is increassl, the rise lime decreases, 
allowing finer lime resolution. 



12 LVfpmtipr um IIen-|eii-PBckarrl JoumBl 

©Copr. 1949-199B Hewlett-Packard Co. 



Troically, swBpl SBecitwn analyzers have s maiimum 'esolution bandwidtfi of 
3 MHl Ifi connasl. Bie maiimum span o! Ihe HP 8W><fl vectoi sigral anatmi is 
7MHiinmeW(st6ive'(iiodBand lBUH;inttiebasgba(«receivermode Thus, a 
key attvaniage in ihe veciut signal analv^i rs ihai finei time tesa'ution i% possible 

The accufacy of swepi spectrum an3l»7era is limited by tne tesolunon bsndwuflti 
(ilieis. loga'ithmii; smphfiet and deiEtlOf. all o( vAwtti are implemented m analog 
eircu'n^ toi wide msaiWion bandwmUis Tfie viKiaf signal analyzei sampies the 
sjgnal Jrreclly at IF rjltetnig. logafdlimi: [wniefsion, ard deteclion a'e all fS'- 
tormed digitally in the vsctai signal snaly^i. cioviding unmatched accuracy tai 
AM eftvelope measu'emenis 

Addilionallv, Ilieveclui Jrgnal analyzer offers signal processing that displays the 
AW moditlating signal in unns of AM modulation depth for nortransieni amplitiiOe- 
iTwdulateri carriers This is done with signal processin? algonihms That auiomaticallv 
remDve UiGcarriBr amplitude oifsel and nDmtalize to the tamer amplitude 

intTBSoand 7ransdui;Br Ana lysis 

DemnriiilaiiDn car proiride useful insights imo the behavior and perfomance of 
ultrasound transducers The voltage on the transducer can be measured by the 
vector signal analyzer and F!i^ demodulated lo prouide a frequency profile ouer 
time of a transmit pulse and echo T?ie demodulated FM time waveform will gen- 
erally show fouf components First, large FM noise deviations will dominalethe 
time display where there is no signal, such as before eicilamn and before the 
echo IS received When the exr;itation arriues, the noise fluctuations torn into a 
flat signal vyith an FM tteviaiion corresponding to the differance bsiwaen the 
encitation frenuency and the measurement cenisr ffequHncy Upon removal of the 
esciiaiion, the transducer begins to vibrate at its natural resonant frequency, 
dissipating stored energy This induces an elecincai signal, which the uecior signal 
analyzer can measure and demodulate Thus, the natural resonant frequency of 
the transducer can be trteasured. 

Finally, the echn will produce transducer uibraiion, which can be detected in the 
demodulated FM time waveform This is generally uery weak and noisy in appear- 
ance If a trigger is available, this measurement can be averaged to improye the 
signal-io-naise ratio. 

The envalopesol the transmit and receive pulses can also lie obsen/ed by select- 
ing the linear or loganthmic display fnniiat in vector measurement mode. Band- 
width IS controllBd by (he measurement span The analyzer's high sensitivity and 



wife dynamic range a^iov; surprisingly weak edioe^ *: "- - .eo on s loga'rtiv 
mpc lime disolsv 

AuiDtnaiic Canier Frequency Oetenninaiion for Broadband Transmissions 

Autonoiic caniet frequency estimatiiin is svatlable for PM and FM demoduleiiroi 
Thecames frequency iseslimatsd inderendenlly over each time record Anaddi- 
lional teaiure in FM allows delermining the carrier frejijency more acnuiately 
when measuring signals wdh hroadbaia rrwdulation 

With averaginfl turned on. the FM demoduiatoi iwll perfonn a wetgtiied average 
on each of Ihe earner freQuency estimates This average can be displayed on the 
demodulated trace as a maiker function It is also used 10 compensate the FM 
result fo' Ihe frequency offset arising fiam the d if feience between the carrier 
frequency and the measurement canter froQuency 

Best tesolls are obtained by setting the measurement span slightly larger than the 
bandwidth of the modulated signal Ttie digital filters can easily reieci aliernaie 
channels, but overlapping adjacBni r:hannels will degrade performance One should 
select the minimum lesoluiior bandwidth possible and select 32ni frequency 
paints tn increase the number of samples in the lime record 1o the maximum 
allowable More time samples will reduce the variance on each carrier frequency 
estimate 

Although not obvious, the earner frequency of digitally modulated carriers as well 
as FM carriers can be determined using this technique. Data on a digitally modu- 
lated carrier must be random for die estimated earner frequency to converge on 
the true carrier frequency. This cofidiltnn is Qenerally met in channels carrying 
normal data. 

A i^ical application is to find the canier frequency of a signal on a satellite chan- 
nel Satellites often carry a mmiure of FM and digitally modulated carriers The 
vector signal analyzer's digital filters can be chosen to select one channel of inter- 
est and reject others The estimated carrier frequency can be used Id venfy thai 
the proper transmitter frequency is being used forthat assigned channel space 

Timothy L Hillsiram 
Douglas Wagner 
Dave I op men I Engineers 
Lake Stevens Instrument Division 



The digital receiver architecture of the HP 894xxA performs 
1hr> pqiiivaJenf of this singlc-sitleri shift and filter with a 
L|LiatlraiT.ire kical oscillator aiid sepaiatc low-pass decimatiiig 
fillers to reject higher-order mixing components. The in-phase 
and quadrature-phase components art' re combined iti DSP 
memtiry and treated as a single-sided complex waveform. 

The local oscillator can be represented as a complex 
sinusoid: 

e-j(">i,t-i-il'Ll (21 

where ftiL must be close eiinugh to dv that after miiltipi.ying 
x(l ) by equation 2. the positive frequency image in x(ll is 
shifted near enough to dc to keep all sidebands r:if m(l) within 
the passhanci of the low-pass dechiialins filters, f'nder this 
assumption, the negative freqiienc.v image of x{t) iscom- 
plelely re.jected after fillering. and the resulting complex 
time waveform is expressed as: 

= C|l + a(ti)ejt(<"r-i'i|J'-f 11>c-4iLl+*f)l_ 

The amiilit.ude modtiiating waveform, a(lj, is recovered by 
Hi's! laJ(iiig the magnitude of y(t): 



|y(t)| =C[l-l-aft)l, (3) 

estimating the carrier amplitude with a weighted average on 
eqiiatirm -i, removing the carrier amplitude from equation 3, 
and normalizing by ilie canier amplittide: 

a(l) = I y(t)| - C|/C. 

The result, a(l), is in iitiits of aoipliltnle nu>dulalion depth. 
For example, a maximum zero-to-peak amplitude of 0,5 for 
sinu.s(iid;il a(l) corresponds to a modulation index of 50% on 
t!ie carrier. 

Equation 3 represents the AM envelope and can be obtained 
in the vector niea.surement mode by selecting the linear- 
magnitude data format for the time wavefonn. This is useful 
for capturing transient events for which an average on equa- 
tion 3 does not give a true estimate of the carrier aniplilude. 

The phase of y(t} includes the desired phase modulating 
waveform. (|>(l ), as well as a phase offMel anil ramp; 

Ly{l) = - fl)!,) -I- ((11,. - wijt + <t>(l). (4J 

Ideally, the local oscillator is equivalent to a coherent car- 
rier, providing carrier-locked demodulalion. The condilion 



©Copr. 1949-1998 Hewlett-Packard Co. 



Iim<iiib(!r imilcwIett-PackaiitJniimal 13 



of carrier ]ovk is met whon v>i - m,. and <p], = ipt, in wliicli 
case the phase ory(I) yields tiie desireil jifiasp tnndiilating 
wavelbrm; 

The IIP 8.94xxApro\'idPs a rear-panel 1,2, 5, or ll)-MHzref- 
pri'iire iiipLLi, The lIPSiMxxA's digital incid uscilliitur I'lm \iv 
sef l.o I.iif .siifiie ireijui'iicy ;ls a carrier syiilliPSiKfd tVoui l.lie 
refereiire. In this case, tlie tligilai local osciiiator Ls fr«|iien- 
cy-!orl(od and phase stable with rcsi;eL'l to the c aiTier. The 
phiise ti( (he earlier is iiol available becaiiye ide digiliil liK'al 
oscillator is derived fri)]n Ireijiicncy divisiiiji aJid iiuiliiplica- 
lioii wilh respecl to tlie t-arrier. This leaves a phase otTset in 
llie phase of y(l): 

^yih){t) = I*.' - *!.! + (5) 

To obtain the desired phase moduiating waveform for fre- 
qiii'iiry-lorked raeasuienienls, tlie phiLsc offset (qi,. - 
removed by computijig a weighted average on ei)iiatioii 5. 

A second PM demodulation mode is a\'ailable when a fre- 
guency reference cannot fie obtained. In ihis case, Ihe car- 
rier phase ramp, Itii,. - ij|[JI, reniihes different compensation 
fii retrieve (he desired phaHe iiiodulalinf; vvaveform. 'lime 
dilTerenlialing equation 4 eiimhiates tlie phase offset, BivinH: 

[!(t) = ((-I,. - m,J + d^/iit. (ti) 

(.'alculalinf! a weighled average on eqiialion (i gives an esti- 
niale 111' (lie lieqlieiii y ulfsel. (ii,. - I'l^. Keniavin;; Ihe esli- 
niiited IVeqiiency nfl'set Frnni einiation ti leaves Ihe frequency 
niDdiiliiiing wavefimn. This is imegrnieil wilh respect to 
time, providing t.tie de.shed phase morkilating waveform, 
(lj(t ), wilh all carrier ooniponents reiiiDved. 

i'requeiicy demodiilalion follows essentially the same steps 
iLs pluLse demodiilalion. A higli-<niahty differential or i.s ap- 
lilied n> equation n to relrievelhe frequency modulating 
waveform. 

All carrier offset compensation can be turned off In Ihis 
case, I'M deiiiod illation is equivalenl to selecting the pliase 
data fonnat on a lime wavefonii in tlie vector measurement 
miide. Ueaeiivaling carrier offset cnuLpensaiion is iweliil (<>v 
caplnring transiem events for which an average on equation 
5 or I) wUI not provide a true estimate of phase oflsei or 
frequency offset, 

A frwiiiency offset is indejienderUly eslimaled for each data 
record sent to (he demodiilalion signal processing block, 
Theesiimaieil frwiiiency olfsel siminied with ll:e HI' SWxxA's 
Ini al n.scillalnr IVeqiiericy (Ihe center lieqiiency of llie niea- 
siireiiieui) isavailalile for display on the demodulaled data 
trace as the estimated cariler frequency. 

The accuracy of the cmrier l ompensation algorithms de- 
pends on how dose a(l) and 4)(t) approjiimaie Kcz-o-mean 
fimctions over eacli data record sent to the demodulation 
signal ])rocessing block. Biases on the esllmated carrier fre- 
quency become significant if a sideb;m(! amphtude is wiliihi 
several dB of the caixier and close enough to the carrier to 
result in fewer than approximately ten cycles over a dalii 
record. 

With complex modulation, there can he unacceptable \'ari- 
ance in ihe estimated carrier frequency. Iliis vaiiance c;ui lie 



reduced sul.islantially by averaging the estinialed carrier Ire- 
queni'y, Fur FM demodulation only, averaging will activate an 
exponenliii! average of each esliniated frequency offset, ami 
this avemgi^ eMlimate will he iLsed lo com[>eris;ile (^jimlion 0. 
.'Ui equivalent exponenlial average is iierfonned uu esliiiiiites 
of the (-arrier amplitude for averaging in AM demod illation. 

Tlie HP y!l-J4l)A provides one channel of PF information for 
demodulation. Two clianneLs nan bo mdependenlly demodu- 
laled ill llie liiiseband receiver mode. When the receiver is in 
mode, I be second IF chmmel I'an measiiri' hiisebaiul .sig- 
nals fur cimiparison wilh a demodulated signal in the UF 
channel. 

Simuilaneniis RF and Baseband Measurements 

One of the more iLsefiil lealures iifilie IiPS!l-14l)A vector 
signal analyzer is its ability To demodulate ;ui RF signal on 
one chmmel wliile simultaneously measuring a baseband 
signal on the second. This featuie can he usetl to isolate ihc 
sign;il causing dislurtiances in an L{ ), to mi'asure the fre- 
([iiency re.sponse of a modulator, to study 1 he loo)) charai'- 
teristics of a phase-lockeil loop, or simjily lo nieasme tlie 
time delays between baseband and modulaleil RF signals. 
.\II of the two-channel time and frequency measurements 
ihat can be |ieiformed with Ihe iiaseband ajialj'zer. such iis 
frequency response, correlation, coherence, or cioss spec- 
tiiim, can he performed for a (icmodulaled RF signal and a 
hasebanil signal. 

The concept behind these measurements is quite simple. 
Imagine comjiaring a signal that was used to modulate a 
cai'rier w ilh tlie our|nit of a demodulator o|jeraIing on the 
modulated carrier. If tlic modulaior and demodulator ai*e 
ideal, Iheii Ihe two signals will be identical. If only the de- 
modulator is ideal, then the two signals can be used lo study 
the chara(.l eristics of the impcrfeci modulator. The vector 
signal analj-zer can direcily meiLsure ihe Tii-st signal (on Ihe 
baseband channel ) mid accurately demodulate llie RF carrier 

10 measure ihe second signal. 

To measure the response of a modulator, Ihe baseband 
source would be connected to the input of tJie modulator, 
l-'or fri'quency response uie;isurements, the source would he 
used lo general e broadbmid sign;ils such as a |ieiiodic chiip 
or random noise. For linie-dnniain measurements, the arbi- 
liai'y i<i|ialiiliiy of ibe source might be used lo gi'iierale a 
ramp or step. The source signal applied to the circuit under 
lest would also he comiected to llie ba.seband input so thai 

11 could he measurer I as a reference signal. The RF channel 
would he used to demodulate ihe carrier at ihe modulator 
output, llsing data from iioih channels, ihe response of the 
modulator to the source signal can be determined. The stim- 
ulus and response can he compared ui the lime domain or 
coml.iined lo compiile a frequency response. 

For detennining the source of disturbances on an oscillator. 
Ihe RF channel would heconnecied lo ihe o.scillator oiilpitl 
and a demodulator seleded. 1>f pically a phase or frequency 
demodulalur would he used. .\ probe would tie attached lo 
the baseband cbaiuiel so llial various .signals in the circuit 
could be easily mea.sured and compared lo ihe demodulator 
outijul. For ex;unple. die jirohe might fu^st be coimecteil lo 
the power supply line, mid then moved to a nearby logic line. 
Depending on the type of clishubance, tJie user would eitlier 
compare ilie signals directly in the time domain or use Ihe 



14 IVTi-rnliiT V.m Howlou-r.iikanf.lnviniiil 

©Copr. 1949-199B Hewlett-Packard Co. 



two sisals to compute the crosscorrelation or coherence 
functions. 

Iniemal to tlie instrument, both input chaimels are identi- 
cally configured for RFAiaseband measurements except that 
the RF chanaiel has a dt'Tnodutalor added to the signal parh 
and die distal LU fur liie liasebaiul cliaiiiii'l is coiirigiire<l for 
a center frequency of 0 Hz. "nus rcsiilLs in both channeb 
haraig the same sample rate, wliich is nwessary lo allow 
cross^■hiUlnel measurenienLs sucfi as I'reiiiiency rpsporise. In 
the process of converting a complex signal into a real signal 
(with the same sample rate ), tlie demodulator reduces tlie 
infonnation bandwidth of ihe RF channel by half. The data 
for tlie second channel is also I'oniplex, bul since the imagi- 
nary jjarl of liie wa^-efnrtii is Kern (iiecause of tlie frequencj' 
and phase of [he digital LO), itie signal is ireaicd as a real 
signal and only half of the specinim is cUsplayetl. 

Hme Selective Frequency Analysis 

Often, today's siiecirum analyzere are t-allec! upon lo analyze 
signals that are not continuous. To avoid coniaminaiion liy 
unwanted signals, pulsed or transient signal.s must l)e Isolated 
in time before being conveiled lo the frequency domain. The 
foUowing few examples show how pervasive time-vaiiant 
signals ha\ e become: 

In the Uriiled States, a frame of tele\ision video is broadcast 
as 52-5 lines in two interlaced fields. Lines IT Ihroiigli 21 of 
eau'h fieltl are resen-ed forlesi sii;nals. which may coexist 
with noniiiil iiicKirt' infontialion, 
Tinie-ilivisiun nmlliple access (TDMA) signals require 
pulsed nil id Illation of an RF carrier. For the North American 
Digital Cellular' INADC) .system, a -KI-jus franie is coniposed 
of stK (i.iili-ms sluts. A frame can carry lliree conversations, 
each iLsing two slots, Ba.se station Iran.smissifin Is not 
pulsetl since power consiuiiption is not a m^jor concern. 
Mobile station transmission is pulsed to conaeive power. 
Code-rlivi.sion multiple access (CDMA) communication sys- 
tems combine digital modulation and s]jecinijn .spreading 
teciuiiqlies In cn*;ili' lnoadband signals Ilial ;iit' irmniine to 
noise, Pcjwcr in a mobile ('DMA phone is gated on and off in 
1.25-ms bursts when the tiala rate is less than the full [ifiOO 
hits |)er second. 

Ill addition In isolaliiig valid inlormalion within a pulsed 
signal, limeseleclivily is criliciil ki analyning trarisieiil.s siicli 
as tirnvsniitier iiini-<m. 

Dflen the user must thhik about a measiin'meni i)roi)lem in 
the time anil frequency doniaitis at the s;mie time. As dis- 
ciiEKiil em her, die IIP 8!*4xxA vector signal aiiidyKcrs' acljiist- 
able mformatioii bandwifllh proviiles the nsei' with control 
over the frequency-domain ;is|iecls of the measiuemenl. 
With accurate hardware triggering and flexible lime-record 
processing tlie user can address many of the time-domain 
issues. However, tlie.se two asjiecls of the measurement are 
not indepentlcnl. Because of the nature of the FPT the ex- 
tents anil resthliitions (jf the time-dijmain and fr-equency- 
doniain data arc inlenela^ed. This is shown Fig. 4. Most FFT 
,sign;i] aiialy/ers force liie user to interai'i primarily in ilie 
frequency ilomain. The time record sb.e is a fixed number of 
points {generally a powf'r of two) mid span is fhe only param- 
eter under the user's conlnd. This means that tune recoril 
length (in seconds] is deterniined directly by llie span. Ttie 
user has no control over the instrument's resolution band- 
widlli aside from cluuiging the span. Some of the newer FfT 





Tone Fiequenty 


Heuilulion 


Saopltble ■ — • flesoliUHm Btnilwiilk 



Fig. 4. S'liTi-'- iiicfisiiri'nieiiis tuive liDiti (i;iie--..l<.nia!ii ami frtTjufjiiTf- 
(lomiiin aspects. Itet-afise of ihe nalure uf Ihe fast f'ouncr irausform, 
tliese (ivoaspeclsare niil indepeiitieiil. TIie exieiils ;iri'i rrMlulloiu 
(•f Ihe liiiif .-.ivl frpqueiKy data are interrelaliMl as shown here 



signal analyzers give the user several options for the number 
of time points, but the time record length is still directly 
dependent on the spmi. While the HP S!14xxA supports this 
traditiona! FFT mode it also allows the user to select the 
time record iengdi or resolution bandKidth independently of 
the span. Time record length and resolution bandwidth are 
still related by: 

RBW = WBWn", 

^v liere RBW is the resolution bandwidth in lieitz, WBW is 
Ihe noise bandwidth of the lime window in bins, and T is the 
lime record length in .seconds. The numlier of time points 
can no longer be directly .set. Instead, the maximum nimiber 
of time points ihe analyzer r;ui store imposes a limit on Ihe 
choice of T (and on the miiiinium allowable res<iliilion band- 
width] for ihe particular sample rate, which is determined 
by tiie span. 

ll Is- easy to see that lo support variaiile resoliilion baiiri- 
widths, the HP S94kxA measiiremenl areliiieetiire must be 
able to handle v aii able-sized time re<"oi'dB. This affects all 
nioasiuement and disjilay- related signal jirocessing, espe- 
cially conversion between the time and freijuency domains. 
Since Ihe FFT is peifomied only on time records that aie 2'^' 
samples in size, the [IP 8!i4xxA's arbitraiily sized time re- 
conis must be zero padded before [lerforniing the FFL Zero 
padding merely arlds zeros to the ends of the time R'cord 
and changes the resolution of the frequency record, h floes 
not alter Ihe information content of the data. Zero padiliiig is 
iniplemenled ihroiigli the application of the lime window. 
The width of Ihe rime window is not constrained lo be inte- 
ger multiples of the sample time. Tlierefore, all hough tlie HP 
SMkxA verlor signal analyzer operates on discrete time 
data, this imposes no quantizing effects on the resolution 
bamlwidth. Based on Hs phase characteristics, the time win- 
dow selection dei.ennines if zero padding is done at the end 
of the time record or if the time record is centered and zero 
jiadded at both ends. Ttie effects of zero padding must be 
undone following the inverse FFT of the fr equency data. In 
this case tlie time record must be resized and jjossibly 
shifled lo remove all artificial zeros at the ends. 

Often during a measiiremenl, fhe user is no! sure of what to 
look for or where Ihe signal of interest lies. In this case Ihe 
user may want to see a signal over a large sediment of lime 
to help isolate the poilion to be luialyzed. The HP H!l4!(xA 
vector signal analyzer supports this ihroiigh iis lime gating 
feature. The u.ser seLs a main data lenglh. which defines the 
extent of the time ilata lo be acquired. Once the main data 
has been acquired the user can gale out a region of Ihe main 
data to be analyzed fun her. All sul)sequeiit time and fre- 
quency analysis will be performed on the gale region. Gener- 
aliy, Ihe size :ui(i position ofthe gale region ciui be ihanged 



© Copr. 1949-1998 Hewlett-Packard Co. 



DiTi-mlHT IHtKi ll.'nli'ii-Pwlimrl .liMtniiil 15 



TRACE A: Cht Main Time 



AMatker 9.IZ1«|is 179.31 mV 




inW 



Slart: 0 5 Slap: 37.94G42SS714 



TRACE B: Chi Gate Spectrum 

BMarker SOIil^OOOHi 54.Z14dB[n 




Center: m.5 MHz Span: J UHz 

Fig. 5. Time scleclive frequency analysfs fJiii dispTiiy [lit' sfieftniiii 
of Ihc spcriiifl piilsf iif tliis iwo-pulac wavi'foirii. 

ati(i llie (iafa n-jimilywd wifbout ri'iakirii! Hie main data. By 
sLiiiiillaiieoiisly (ilwerving llif main limi', gale lime, and gale 
sperlruni the user gets a coniplele picture of liie signal of 
inlpresT. including where it fitfi wilhin a larger sequence of 
events. Fig. "i sliows liow the HP \'e<-tc)r signal analyzers 
interact with the user to perform time selective frequency 
analysis. The upper trace shows a two-pulse linie wavefonn. 
A pair of vertical gate markers are placed over the right 
pulse. The freqiiency-iloniain representation of that gated 
pulse is shown in Ihe lower trace of the display. 

Certain measuremenl situations require thai a large amoimt 
of time data be analyzed. If the signal is transient in nature, 
if it must be aniilyzeil without any time gups, oi" if il must 
reanal.yzed several times Ihe Hi* 8!MsxA's time caplure fca- 
(iire ran be used. In lime capUire iiuxle. up to 10'" samples 
(or half a.s many for each of two chmmels) ean he acquired 
and stored in the sample RAM. Tliis data ean then be playeil 
back into the measurement as many times as necessary. 
During playback all of the measurement's featm-es aix' avail- 
able. For instance, the user can reanalyze the data with 
several resolution bandwidlhs or time windows. 

Conclusions 

Making the complex and powerful measurements associated 
with today's signals and systems is inevitably more difneult. 



Aprimaiy goal for Hie vector signal analyzer jiroj eel leaiua 
wa.s to create aiialyzei-s lhat ciui handle (he comiilex imerac- 
tions between iJie frequency, time, and niiifUilalioji domauis 
by iheniselves. freeing the user lo nmcentrale on the de- 
sired measuremenl results. This upproaeh avoids alienating 
the users of traditional analyzers while providing the fools 
required for the demanding ineiisuremenl needs ofloday 
and lomotrow. 

Acknowledgments 

A large population from the Lake Stevens Instnimeni Divi- 
sion ciitilnbiiled to the vector signal analyzer program, .lerry 
Daniels (nriiiwarc manager) focu.sed the products' definition 
with help from Charlie Poller and Bill Spaiilding (RAD lab 
seci.iou mauagersj, .Jim Rouiuls |H&D lab manger), anil Fred 
Cruger (marketing manager). The inarketing team provided 
cracial input lo aid in easuring the produces' success. The 
lead cusHimer [>roRrani developed by the uiarketing team 
provided laiique iiwigb! for (and the ilivisioni inio cils- 
tomer mea.snremeni neeils and requirements before Ihe 
products were released. This allowed RAD the opporttmity 
to res])ond to this timely feedback, providing beHer solu- 
tions for meeting customer needs. The RAD linnware leam's 
coutribiilions are detailed in ihe article on page 17. The R&D 
bardware contributors aie acknowledged m ihe articles on 
pages 3] and 47. Bon Potter (now retired from HP) deserves 
special ihanks. His theoretical ;uid algoritluuii' re.sean'h are 
the backbone of the vector signal analyzer. His contributions 
include algorithms for demodulaiion, resampling, time- 
domain winci(>wing. and corrections, Don Ililler is noted for 
having the foresiglit to recognize the importance of the cor- 
rected iime-domain capabilities now present in the vector 
signal analyzer family. The documentation team — Ron 
Anderson, Lucie Johns, ^uld t^eiie Taylor — produced a com- 
prehensive set of manuals and online help lhat fully ileiai! a 
complex family of analysers. Thanks go to the manufac turing 
and leehnie;i] siippoil people who put in extraorclinary efforts 
to bring the HP a94xxA mto prociucUon. 

References 

1, K.r. Carlson, et al, "A 10-Hz-to 150-MHz Spectrum Analyzer with a 
Digiial IF Seelion," Hrnlcil-Pnrkiml Jnuriml. Vol. A2, no, 3„ .Iiitie 
lOni, -H-W). 

1. !!,(.'. Blai-kiiaiii, I'l ai, "Mfa-siiretiieiil Modes ami Difiital Demodula- 
liim for a U^w-Fri-qiieni-y Aoaiyzer," HftiiKll-Fnrkuiil ■htiniai. Vol, 
:ia, no. 1, -luniiary 1987, pp. 17-25. 

pSOS It alrademarhof SoftwaiB CnrrpcnentE (iroup. Inc 



16 llpi'pnihpr lPfl;U1pwtpn-Pfli-kaiTl Journal 

©Copr. 1949-1998 Hewlett-Packard Co. 



A Firmware Architecture for Multiple 
High-Performance Measurements 

The HP 894xxA vector signal analyzers perform fast, sophisticated 
measurements on complex waveforms. The firmware architecture 
provides access to multiple processors to meet the high-performance 
requirements while allowing individual measurements to share common 
features and protocol. 

by Dennis P. O'Brien 



The HP S94XXA vector signal analyzers offer a diverse set of 
advanced nieasiiienieiil.s. The nieasLirement firmware arclii- 
leciiire was designed witli Ihe Iwo preeminent retiiiirements 
of performance (i.e., speed of operation) and functional 
leverage across multiple measurement modes. 

Measiucmcnt performance is measurrri in terms of I lol h 
loop lime and user conuiiaiul response time. Loop time Ls 



expressed as either display updates per second or real-tinie 
liiindwidih. iliai is. ni.iximum frenuency spaji ai which 
rlie inpui signal can lie processed without missing data. l.)f 
these two performance measures, loop time generally look 
precedence when design trade-offs were necessai>'. In Fig. I 
which shows the flow of signal processing for tiiree of the 
IIP S94xxA's four measurement modes, the loop time is the 



SDmple 


Arliiuary 


flAM 


Span 



Time- Domain 
CortBOlon 



MHflBuremenl- 
Specilic 
Signal 
Process ing 



□atsand 
Uisr Math 



CaordinaTB 
TfanslarniBlion 
and UniK 
ConueriroB 





I 


Diiplay, 


I 


Un/log 




ConverslM. 


■i 


anil Scale 


1 



¥ Marker 



^ Display 



Vacur Maraurenienl Mods 



Time 


1 


Time 


1 


ConvDiT Dnia 


Avars ga 


1 


Gala 


1 


10 Fiequancy 



Frequency 
fliid Iilgger 
Correcllon : 




Analog DanodalaiiDR Messurametii Mods 





Analog 


1 


Time 


1 


Time 




DemoitulBliDn 


1 


Avorage 


1 


HilB 



Convort Tims 
To FrBqjiDncy 



Frequflficy 
and Tnggnr 

CoirfFtTiDn 




Scalar MeasuremGnt Mode 



CanvBrtTima 
to Frequencf 



ffoquancy 
Correclion 
migilBll 




Ffeqiioncy 
Car re Clio II 
(Anelofll 




FiR, 1. HP 894xxA vector signal analjaermeasorcnii'iii dalii ritw. 

© Copr. 1949-1998 Hewlett-Packard Co. 



December lUHa Hewli-LL-Hiickiird .loiinuil 



17 



tiinc TTqiiirpd to process the data from the sample RAM to 
the display. 

Each measure men 1 niiide offers the user a variety of signal 
processing ciislDitiizalion.s in adililion (o numerous suuiitard 
feanirPB. Tlie slandajxl fealui es, l omnion to all uieiusure- 
menl modes, prpsf'nt a cousislent look and feel to Ihe user 
ami in;ike each niKLSuremem operate as a "uood ciii^en" 
williin die overall instrument finnwrnx' ardii lecture. Fig, 1 
shows that each of the HP 8i)4xxA's mcasm'cments can use 
arhitrai-y span imd lime-domain rorrections to conrlition Ihe 
incoming lime dala. ;md dial trace (iis|ilay conditioning op- 
eralioiis such as user math, coordinate Iransrorinalioii, uniis 
conversion, sciiling and pixelation ai'e also iised in common 
by all rneasureinenl.s. 

Numerous addiliuns lo Ihe standard features aie renuired In 
meet the signal processing demands of individual measure- 
ments. In many ca-ses this special signal processing Ls lever- 
aged aiToss se\'ei al measui-enienis — averaging, lime gating, 
and time-to-l'reiiiienc-y conveisioii jue good examples. Bow- 
ever, some addilional signal processing is so speriaiized lliai 
it meets the needs of only a single iiieasurenieni — analog 
and liigitai (lemodulalion. for example. 

A far bigger part of the measurement task dian signal pro- 
ces.sing is I'onlrol, It is by oliser\inR common conventions at 
il.s flnnwaie iiticrfaee thai a mcasm-emenl maintains ils 
"good citizen" sliuuling. Also, il is in observing common in- 
lernal conventions that all ineasiiremcms iire.seni a consis- 
tent interface lo the user. Most of die control feal tires of the 
HP ft94xyA's nu»a.surenieni sysleni are shared liy all of its 
measurements. Some of Ihe stamlard coiUrol features 
designed to provide consi.siency to Ihe u.serme: 
Measmenient pause/conlmue 
Mciisuremenl restait 
Single/contimious swce|i modt^ 
S(.'P1''' operation complete 
SCPl status register control 

Recompulalion of results following jjarameler clianges. 

Some of tiie standard control features designed lo provide a 
uniform interface lo die rest of the firmware are; 
Parameter change h;m<lling 

Fonnalized interaction willi Ihe calibration process 
Formalized interaction wilh Ihe autoratige process 
Measurement aclivalion and deactivation 
Measurement nu^mory reallocation 
Prenieasuremeni trace results informal ion tracking 
Run-dme measuiemeni trace resuli.s infonnalioii Irackiiig. 

lb support a diverse set of measurements that share many 
common control an'i processing Icaliircs iuid whose piimiuy 
i"ei|uirement is high peifomiance, the nieiisurenient finnwaie 
mchitecture was shapetl by these goal.s: 
Fidly utiUze tJie DSP9li(l(l2 signal processor I o jirovidc Llie 
highest possible perfomiauce. 

Provide default control :uid signal processing iiiiplemenla- 
tions of basic featiu^es iJnat m^e easily inherited by individual 
measurements. 

Make il easy Iti customize the default conirol and processing 
features for individual nu'iisuremenls. 
Implerneiil the measurcnieni architect ii if as a platfonn 
upon which futiu"c measurement designs can b*^ huilt. 

SCPl iSWndard Cciminsndstaf Pfogrdintiiatile Insuunienls 



Data Structure 

All of llie MP Hi)4.\xA measurements jue block-oriented, lhai 
is, lliey operate on blocks or arrays of data dial represent 
Ihe signal ovei- a given range rather tlian at Jusi one point. 
Tiie fundaznental daU structure is a vector, which embodies 
both a dala anay and a header .stiiicture. The dala porlion ol' 
du- vector is generally represented as ;i2-bit floating-point 
numbers Ihroiighoul the nieasiirement and may he eitlier 
real or coin])lex. h is conveded to integer formal as pail of 
the display processing. The header sinicluie contains infor- 
mation aliout both the measurement setup that pniduces Ihe 
data (cenler frequency, sjjan, input range, etc.) and the dala 
il.self (such as whether or nol it contains overloarls ). 

Each measurement uses a eoUectinn of vectors. Fig, 2shows 
tJie veclor allocation for the HP 8fi4\x.A vector measunmient 
mode. The limeDala, IreqDaiB, anci avgBulDala \eciors contain 
the measui'emert's raw (iaia. These are Ihe measuri'menl's 
fLiudamemal resuli.s. All iLser-seleeiable measurement re- 
.sulls must be derived from these. The measDaia, dispDaia. and 
pixBufData vectors are trace-orienled, .so there im- fom' of 
eacli, or <ine per trace. The mEasData vectors are veclor 
pointers dial poinl to one of llie malhlVleaEOala vectors, wirich 
contain ihe final v ersion of each user-selected measurement 
data ifsult. Tln're may be many more than four of diese, 
sinci' user math uses them as input operands and it is not 
con.strainod lo use only displayed dala. The mathMeasData, 
measDaia, dispDaia, and piuBjfData vccloi-s are collectively re- 
fened lo as tile trace vi'clors. As will be seen, several pro- 
cesses have access lo the trace vectors. Therefore, a iirolec- 
titm niechani,sm is retiuired In rcsirici access lo the measDala 
vecloi-s and all others that are farther downsireani. Finally, 
there are a numlicr of suijjjort vectors, Some of Ihese contain 
vector constants that will nol change during a ineasurement. 
Other supiioil vectors are used for lemimrary storage (not 
shown in Hg. 2), 

The timeData vector is the imtput of Ihe time-domain correc- 
tion liller block in Fig. 1 iuid is the input to the nieasorenient- 
specific .signal processing block. At the other end of ihe 
meiisureinent loop, the matliMeasData veclot^ are the output 
of the measurement data and user math block. 

The amomil of memory for all required vectors vaiies from 
measmement to measurement. Nol only does it vary be- 
tween meiiHiiremenl modes, il may change with Ihe setup of 
a given measiireinenl. For cxaini)h'. l iianging llie number of 
fr«iuency points, Ihe n'sohuion bajulwidtb, or die average 
slate can have considerable impact on the mmiher or size of 
Ihe vecloi-s required. To make Ihe besi use of tJie available 
system RA.M wiihoul im|i!einenting a sophisticated inenioiy 
iiiaiiagement .scheme, all of the veclors are reallocated when 
a new measurement mode is selected. Tlieir sizes are deter- 
ntuied by user configuration of key parameter Umits such as 
niaximimi freiniency points. They m*e maile l.'irge enough !o 
siip|]ort all pennutations of Ihe selected measnremeru mode 
within die linuts set by the user, .^s menlioned earlier, some 
of tlie signal processing is shared by all nieasuremems. 
Therefoi-e. nnicit of the veclor allocation scheme Is leveraged 
across all measurement modes, 

Tlie DSI^lilH):; signal processor Ihereafler referi'cd to as the 
DSP) has a limiied muouni of liiglv.speed HAM available. 
Operations on ilaia blocks dial reside in ibis lii\M me up to 
seven times faster tlum if the dala block is resident iji iJic 



18 DecenitMT 1W(3 HewlBU-Pai-kHrdJoiinift) 

©Copr. 1949-199B Hewlett-Packard Co. 



Shared Headers 



limcDruHul 



IxIxH 





daldHegistiri 
6xIxBK 



^ Number dI Channels 
= Number ol Tiaces 
r Unpadiled lime Record Length 
= Zero Padited Time Hecord LengHi 
= Number ot Ffequenej Points 
s Number of AMas-Proiecred Frequency PoJni^ 
VD = Number of Possible niEdsDjird types 
mb = mcasHcadei 
cAh = channel 'n' header 
d4i = data Header 
vh = veclHeedcr 

' = HeadersFDundin Each Voctotin GroupllnadditionrDdalal 



Fig. 2. lil'SiWxKA (lata vptioriLrcluli'.-liirf. Tlip iHnngaloil lilooks mpmiiCTil. t.lwdnta [lortlons of ttie VCftors. 



syslpni RAM, Td take advanlagp of this improved perfor- 
TTi;mce I he musl {■rilirai vecliifb are pljimi in the DSP RAM. 
Tiilike iheit synleiii RAM coiinU-Tpans, iheso vectors arc 
real!iicaJe<i li) lie exaclly flie reqiiiivil sm- xvhi'iu'vera pa- 
rameter cliatige is iiiaile I hat affcel.-s llie veilor makeup of 
the meaKurpnient. Reallncatiiig and relocaiing veetors in 
response to parameter cliaiifies such a-s user sclecliun iifa 
new riiefLNiirenienl result makes dixy iJSI'-residenI veeliirs 
Miliieralile In hi'iiiH lost. This is aeceptaiile for veeliu-s from 
mathMeasDaia ilowTistreaiii beeause t.iiey eaii l)c reeuiiipuierl 
I'nm die measurpment's raw data. However, die loss of raw 
data would lie a serious iiroliIeTii. To i-irciiiiivenf tliis prob- 
lem Uie iiieiLsureiiuml sysleiii imiilemeitt.s a sclieme Hi pre- 
serve raw ilala aero.ss parameter i luuiges, Tlii' scheme in- 
volves copyinfl DSI'-resident veetni's to their system RAM 
counierpiuts before memory reaiiocalion anti dien copying 
diem hack ( if they conlimie to reside in l!ie I ISP memory) 
follijwing reallocaliim. Much ejigineerttig elTori WiiS spent in 



ensuring thai the measurements make the beat use of the 
fast I!1SP memory. Tliis is a major eontributor to the IIP 
89'lxxA's high T.S-kllz real-tiiiie rate. 

Process Structure 

To achieve tlie highest possible peifomiaiiee, the measiire- 
nieiil loop is spht between the DSP and the main CPU While 
many emliedded systems h;ivi> ninth processors that o|icriUe 
iis slaves of I he central processor, in the IIP 8lilxx;\ the 
mi'iLsnrenieiit loop nuis primarily ui the DSP. The DSP uses 
the main CPU as a slave lo; 

Perrorm certain housekeeping hmctions via remoli' proce- 
iliire calls sncii as nni-lime trace vector heailer information 
trackijig 

Perform integer math on pixi'lateil trace results 

Inierfaee with the graphics system pnicessor (GSP), which 

is the (lifiplay processor. 

IronnniieEl on (iaBo7i| 



©Copr. 1949-1998 H sw I att- Packard Co. 



[iwi'iubtii nWi Ilc'wlrll-I'uc liard .lijuniiil li) 



Run -Time -Configurable Hardware Drivers 



One of ihB difficull cfiallenges facmg any firmware learn duiing a several-yeai 
project is Ijimoil in The software hecaiise of evolurion of Ilie fiardware scBcifrca- 
tion upon wfiicli the software rnusl run This hardware evolution is 3 natural pru- 
cess giVHo iIie comple«ity of our systems arti the (Jetnanding environmeoisl and 
RFI testing ihev musi pass. All tnn often the simplest of ftanfivere changes can 
wiBak haw on the softt^iara mlertace to the hardware 

The high cost oi prato types and ihe evoluiinnaty nature of cumples hardware 
systems also can lead to the requiramert thai safiware he flexible entiugh tn work 
with a mix uf various hardwars versiuns Some uf the laclots caotribuiing lo soft- 
ware tuimoil in hardware drivers are 

• Increai^ed markei understanding causing new exoedations 

• Technology changes 

• Defective or marginal parts causing redesrgn 

• Design flaws 

• Interface reguiremenis to oiher hardware in flux 

• Software nui available for tomplele lurn-on 

• Standards 

• Printed circuit hoard layout constraints 

• lylanufaciuring processes. 

Eaily in the developmeni of ihe HP894xsA analyrers we lealwd that itiis would 
he a considerable prohlem since we were developing a new hardware platform. 
Given that the software would be required id deal with multiple reuistons of each 
hardware board as well as yarious combinations of hoards (Fig 1 ], the following 
lie sign criteria developed 

• Must he easy to accommodate multiple levisipns, 

• Must he easy to share code common to all revisions, Tfiis ptEvents having N 
versions of common algorithms 

• Musi be easy to remove obsolete versions if code forali versions is intermixed, 
pulling out □f)soletB code may be riiffjculi and prone to introducing new problems 

• fvlust offer a clear, consislent interface from an extemal perspective. Clients of a 
hardwara driver should not need to Imow or care which revision of the hardware is 
present 

C+t Hardware Drivers 

Our solution was to deyeiop aciass hierarchy based on C++ classes. By providing 
a base class for a particular set of hardware and generalmg derived classes for 
particular hardware revisions, it became an easy task to support mulnple hanjware 
revisions. 

A C++ base class is used id define the external interface of the hardware and to 
define what capabilities are required by derived classes of hie base hardware 
class The base class will often serve only as a defmilion and may have nu useful 
code In other cases ihe base class may have considerable code encompassing 
shared algorithms and advanced interface functions. 

An example of a class structure is shown in Rg 2 T)ie base class AdcHw provides 
the exlemal interface for the ADC hardware and Ihe default implemsntaltDr of much 



Base 
Class 




DerlvBil 



Fig. I EKainple itl a class structure 

nf Ihe interlace The derived classes AdcHwW. AdcHwfia. and ABcHwR4 provide 
overrides for unique aspects of revisions two, three, and tour of ilie liardware. A 
portion of the Ad cHw class is shown below 

class AdcHw 



protectaeti 

int adclndBK, 

HwCliannel channel; 

ml revision =0; 
gublic. 

AdcHwIvDidl, 

uiruial vQld gstAdcGal char " 
vinual void gslRFAdcGafchar 



dataPii, int ■ siiel; 
■dalaPlr, int 'size); 



virtual void gBtBBAdcGa(char " datsPlr, int'siiBl; 

virtual void dnharCalSBtlingtini valuel. 

virtual uoid r e turn Control Bits I vmd " bilsPtr, ml ' countj; 

virtual void saiAdclndEX(inl indexl, 

virtual void setCoritralWordlunsignsd int conlrDl); 

virtual void sotPreLoadlunsigned char newValue}; 

virtual UDid setMuxConTrollvoidl, 

virtual void clear MuxControKvaidl; 

virtual intadcRevlvoid), 

); 

extarn A(^[^Hw 'adcHwChi, 
ExlBin AdcHw " 3dcHtvCh2; 

Member functions declaiBd virtual can Ijb oplionallv overridden by a derived class 
written for a different revision. Tlie two yanables adcHwChi and adcHwCh! repre- 
sent the external interface handle through which all oiher sottwara interfaces 
with the ADC hardware. 

Revision i of the ADC hardware requited a new gale array program as well as a 
change in haw coniroi bits are configured The implementation of a driver for 
this revision consisted o1 implementing several member functions to handle the 
incremental differences 

class AdoHwRA ; pulilic AdcHw 



Board 1 



8Danl2 



RsviKlon 1 Rnvisiofl 3 



Revision t I nevjs*on2 



Time 



Fig. 1. Ilie firmware must Seal with rnuluple rwiionsglsadi hiiilJware tuard and with 
drfferanl combi nations of Dcards. 



AdcHwRdI; II class constructor 
virtual void ret urnConlrolBitsl void " bitsPtr, int 'countl; 
virtual void setConliDlWcrdl unsigned inl contrail, 
virtual void gclAdcGalcliar"*dataPti, int'siiel. 

1, 

The two fuoclions retum[:DntrolBitE(l and setControlWcrdll deal with Ihe tact that 
the contra I hit interface changed and Ihe funciion getAdcGal) deals with providing 
a unique gate array program The software is now able la use version '1 ADC 
hardware without changing any other souice code m Ihe insliument When a 
reference is made to adcHmCh!->setCQniiolWord(l. C+- will automatically arrange 
to invoke the version defined in the adcHwMciass 

How do we determine whicfi version ot the class to use^ Since multiple hardware 
drivers exist in ifie software, soma mechanism needs to dstetmme which driver to 



20 DpTPniher IfKin Hewlett -Packard ,Tt.ii!iia] 

©Copr. 1949-1998 Hewlett-Packard Co. 



UK ivher- !r* iTisirument powers up In Sie WBMxnA, ihis delenninstiDn is done 
bv reading ar ID regrsiei or ihe fiaiflwatB Bnard o' miefESt Tnis lO legisler is 
(wsi ofifi" a fiaid-wirgd legrsla that lies itiree M% of veision infomaiion When 
a new QoarO is mads wiih g ditfe'Hni nsrtlwaie inifitiace nr capaliiliiy. ihis vsrsion 
nwniiK IS dwngat The coOs tc do Uiis miglii be as simple as 

iWTi^ ladcflevnlogt 
( 

dBlsgIt 

case I. 
3tf(HwChl = new AdcHw, 
at)cHwCTi! = nBiv A[tcttw, 
hreaK 

adcHwCtil = new AdcHwflJ; 
adcHivCW = new AdcHiwRi 
break: 

esse 3: 

stlcHwChl = nB* fldcHwRJ; 
adcHwCh; = new AdcHwRl 
break," 
case 4: 

aflcHwChl = new AdcHwM; 



adcHwCh! ^ new AdcHwB4: 
(Hesh; 

» 

Sunnnary 

B. ei- b-iiir\g our hardware Siveis as C'-^ Classes, ivs ware able to maintam a 
consisiert software mterface to ihe hardware while ptoviding suppoil fo' muliifiie 
versions oi Qie lardwate Common code sJiarsd among gli flnrers ls encapsulated 
in s base class, whicn ilselr knows nothing of Itie particular vErsion m use. but 
raeaO reiies on vimial funchons lo allow (Evuion-specirc drivers to ovenide 
functional ity seamlessly Ths code is much cleaner Oecause the software wrrter 
floes not rieet) to spiintle conditional statements in trie source code m accijumo- 
dale iiBiiBus versions In addition, it becomes an easy matter lo remove obsolete 
versions of llie hardware driver Fmallv, the Ct-r compiler takes care of the mun- 
dane (ask of making suie the ri^l hinction overrides are invoked for the insialled 
version 

Glenn H. Engel 

Development Engineer 

Lake Stevens Insirurnent Division 



For the measurement lixip li> niii in llie DSP ivitli iiu run- 
lime intervention or conirol Trom some higher am horiiy in 
the main CPU, that, loop has to be completely deJlnpfl before 
being executed- Several opliim.s wtre open lo the rie.sign 
team toatUress Ihis problem. The appioarh chosen was to 
provide a minimum set of DfiP-residetil measurement loops, 
hereafter refeiTed to as a measSequencer. which can he cus- 
foniized by uidiviriual mpasiiremenl.s beftirp run time. The 
cu.sl.omization is done through a compile process which will 
he described in a subsequent section. The default meas- 
Sequencers provide eveiy measurement with all of the control 
required to interact in a uniform way with all other entities 
in Ihe insuiimcnt llrmware architecture, 

WitJi tills approach, a single deaigiier was able to implement 
Ihe measSaquBncers for all mea.surement modes. In addition to 
jiromoting a uniform interface for measurement loops of all 
measuremetit modes, Ihis approach allowed other designers 
freedom to concentrate on Ihe specific ciistomizations re- 
iliiired by llieir particular nieasurenienl mode. II is true that 
(liis Is one trade-off thai sacrificed inn-time perfomiarice for 
ease of ilevelojiment. However, the overhead to siippori a 
generic measSBquencer state machine is only about 300 |is, 
which is less than 1.5% of the loop time required for 78-kIIz 
leal-time operation at ItiOl frequency points. 

Fig. 3 shows the HP 894xxA meastirenieiil rinnware archi- 
tecture. The solid bulihles represent .software modules. A 
module depicied vs'jth concentric rings is a jirocess. Bubbles 
that are stacked indicate thai jiiuliijiie versions of that mod- 
ule exist to supporl the miilliple mettsuremcni niodes. Data 
stores are represented as piiirs of parallel lines enclosing Ihc 
slores' names. 

Measurement Feature Leverage 

There several ways to leverage sf;uidard features across 
measurement modes. Code can be co|iieil as a means of re- 
use. The volalihiy of measuremenl finuware modules makes 
code develnpcd m this maimer very diflicull to maintain. 
Oispatch tables, customized by Ihe activation of a nu^asure- 
nieni mode, aliow a foundational architecture to [irovide 



default functionality and yet flex to meet the custom needs 
of individual measurements, DLspatch tables, however, must 
be kept cuiTent manually. The HP S94xxA measurement 
firmware ari'liitecture uses C++ classes and inheritance to 
allow Indiv idual measurement modes to build on a fouiida- 
tional nieasurement. Several of the modules in Fif;. 3 are 
implemented as C++ classes. A partial cliiss hierai'chy is: 

MeasSeqCilr 
HP894tOMessSeqCtlr 
VectorMeasSeqCtlr 
ScalarMeasSeoCllr 
Analog DemodMessSeqCdr 
□igiialDemodMeaaSeqCtlr 
tVleasSeqGen 
HPeailOMeasSeqGen 
BluckMeasSeqGen 
VectorMeasSeqGen 
ScalarMeasSeqGen 
AnalogOemodMeasSeqGen 
DigitalDemodMeasSeqGen 

In the ca.se of the MeasSeqCdr class, a complete set of default 
actions is provided, Mi>st of ihese actions are usable by all 
measurement.^. Wlierc the defaiiit action is not what is rc- 
(|nired. Ihe XXMeasSeqCllr class (one of the classes derived 
from Ihe MeasSeqCdr chiss— "XX" stands for I lie tuime of Ihe 
mea.snremem mode) can overwrite Ihe virtual function, 
replacing thai default action with one of ils own. 

In the MeasSeqGen class, control of a complete compile se- 
quence has been implemented. However, all of the measure- 
ment mode-specific cust omizations are the responsibilily of 
a deriveri clas.s. 

Both the HP8941[IMeasSeqCllr ami HP694IOMeasSeqGen classe.s 
provide customizations requiivd by Ihe HP 994kxA measiire- 
nieiit system. Neither they, Ihe MeasSeqCdr class, nor the 
MaasSaqGon class provide sufficient functionality to imple- 
ment a fully functional measurement. As such, they are ab- 
stract classes. Tlial i.s, there are no instances of these 
classes in the system; Uiere are only instances of their 
derived classes. 



©Copr. 1949-1998 H sw I att- Packard Co. 



I)pi'ember!lS3HewleH-Packarii Journal 21 



CPU 



SDH SCAN DONE 
SDH SCAN DDNF SEQ CDMPLTTE 
SDH HEPOnr SCM DONE 
SDH SEQ COMPUTE 



DSP 



C_M_D 





T = TDkanlsl 



HPC = Remote 
Procedure 
Call 



! Dala Flow ► Control Flow 



Fig. 3. Wi ior sianixl analyzer rnpiiBuri'miuit rii'iiiwiirp BmifiDri'. 



Custdfiiizations 

Tilt' measSequencer is lln' sole pcofoss mnning in llie DHR li 
proviiii's the (ii-'fLiiii! nicasurfnifnt loop control while the 
cusiomMeasStates mtxiiik' cxefiil l'h CTiskimizations miii|ue to 
Ihf (.'iiTTi.'nI activf nR'ii.siii rnu'nt, The oxacl set ol'i'iislomiza- 
litins iiKlalleil is [Iclenitinetl by iht' ciirri'nt meusiircment 
mode's liilen>r''lation of the cjrreritMeasDefiriiiioti ilata store 
(C_M_D in Fig, 'i), CurrentMeasDefinltion reprcspnts that portion 
of Uie iiisiiiiiiieni stale (iSlate (lata store) that l oinpleleiy 
defines the measmement state. The sBqGEnaralor eonlruls the 
compile operation, which prodiiees tJie eomposhe measure- 
nieni loop |i.e.. measSequencer ;inci cusinniMEasSiatEs). The 
XXMeasSeqGen modules ("XX" stands lor the name of the 
inea.su reiiient mode) interpret curreniMeasDelirition aeconiing 
III ilie reiiitiremenls of the current measurement mode. 
prndiiriiLg the measSeqjencer nisiomizations execatecl by 
custom MaasStales. 

At the end of liie measurement looj) the mEasSEquencer has, 
for each trace, produced pixelaied data and Irarked rrilical 
ruii-time infomiation into boih measStatE and iraceStata, Al 



Ibis [loinl. before be^innint! to process Die next scan, tlie 
measSeqjencer signals the scaiiDoneHandler process by a remote 
procedure call. The scanDcreHardlEr then completes tracking 
header infonnution from measSiaie and iraceState into the 
I race vectoi's' headers. Wlien the vector headers are coiTect 
the scanDonBHandlEr computes ;my required maJ'ker values 
and futlher scales and formats tlie pixelated ilata. Finally, 
the data is transferred lo the GSP for display, A-s the scan- 
DorBHandler operates on its scan, ihe measSequEncer is ;i] ready 
processing the next scan. Since the measSequericer and ihe 
scanDoieHandler both access the trace vectors, a semapliore, 
VECTOR_RESOURCE. has been established lo conirol access to 
these veO.ors. 

In addition to the modules lhal generate acustomized mea- 
surement loop and those that execute that loop, there are 
those thai esl.ubiish ilie overall interface to the measure- 
meni. Ttie MeasSeqCtIr class prov ides defaull rcsjioiises to 
meastiremeiil requesLs. It may handle a request itself or it 
may piis.s il ;ilong to the measSequencer. Examples of requests 
that it handles itself are those thai cau.se the generation and 



22 Ili-ceirbcT 11H113 Hewlptl-Pa.'kar.l InnniHl 

©Copr. 1949-199B Hewlett-Packard Co. 



ISTflTt .CHANGE 
MUS COMUAPtD 
SEO.UPDATE 
MEAS MESSAGE 



iwds II illCniliHurilfd 




^^^^ '"«T.H»«. 



SEO UPDASE " 
WEAE MESSAGE 



chgloHanillt MCHIiIleSS cmdToHgndlf ««Hentfled 

chqHandlBd 



li I IncliiStale UA Kit 

ul = unlDckiSlate REM 

KA ± liBDpAlrveTlllCamplBlell 

HSM ^ lunSubWacliinesll 

I = Logical NDi 

MCHXX ; McBsChangc Handler Stale Macdine Slate XX 
SCHXX = SeqChangeHandlSF Stale Machine Stale XX 

Fig. 4, EuBnlHdndler state jnachine. 

downloading of a new composite ineasuretneiil loop. Stan, 
[jause, and roiirinLip aj-e examples of retiuests t.hal. it directs 
to die measSequencer. 

The XXMeasSeqCtlr module ( "XX" sl.iintis ibr the name fif the 
measiirenieni mode) provideB nii'fisuremenl -specific control 
when the <lerHiill ;ic?i(iiisiir:he MeasSeqCilr class are nol snl- 
fifjeni. IJltewise, the XXMBaslStaleOuerrides inixhile pfuvidcs 
n leasii re riiei II -specific rilleruigor iisi^rinpiil when Ihe default 
coniitiajid inlerprelaiioii is iioi siifficicm. 

Each speciiic measurement is defineil by the defaiili sysieni 
operation plus the modifiralioiis made hy the measiiremenl- 
sppcitie modules. To im]iU'nienl a new iiieasiirenieni mode, 
a measurement designer generates die Ihree modules • 
XXMeasSeqCtIf, XXMeasSeqGen, and XXMesslStaieOverndes, • 

■ 

Measwement Control 

flach of llic HP 8!)4x.xA measurenient modes is implemented 
hy single instances of XXMeasSeqCtIr and XXMeasSeqGeri ob- 
jects. Allhinigli lliere are four measurement modes, only a 
single mode is active al any lime. This is referred lo as die 
BciiusMeas. 

The activeMeas must respond to many different lyjjes of re- 
(juesls. Tlie mosi oljvioiis are lliose dial aic the results of 
iisi'r actions. A change in the selecleil measurenienl resiills 
ot i viurdinatcs musl be hinidied iiimiedialely wilhuul affen- 
iiig die raw data. A pause mils ( he coordinaled with the 
measSaquencer lo ensure a consistent set of valid trace vectors 
when tlie pause goes into effeci, Tliere are literally himdreds 
of softkeys that have an effei'l on a miuiiiig measurement, 
Wliilc ihis is llie source of the nuiioi ity of iiu'asiiremeiit re- 
ilitests by far, there are inlerfially geiieraled reijuests as well, 
Foi exiunple, the caliliraliiiii iiHuiajier reiniesls permission 
of the activeMeas before performing Ihe calibralioii. The 



SEQ.UPOATT 





SCHSIopMli 



activeMeas grants pentiission based on the its state and the 
stale of the measSequencer. 

Tlie MeasSeqCilr class musi provide defaiill actions lo each 
request made of the activeMeas. To meel I lie goal of providing 
control of basic features whicli are easily inheriled by indi- 
vidual measutemeiil.s, diese defaiill aclions must be useful 
for die ni;^ority of llie XXMeasSeqCtIr ilefived classes. 

At (he core of the MeasSeqCtir class are several state ma- 
chines, which impiement sets of basic ai. dons, Tliese state 
macliines aie nol independenl. Often their traiisil ions rely 
on cnrreni stales or slate transitions of nthi-r machines. 
Three of [he more central state machines are: 
I EvertHandler 
' MeasChangeHancfler 
> SeqCharigeHardler. 

The activeMeas is nomially a blocked process, li wakes iij) 
when one or more pSOS operating system events are re- 
ceived. Kacli event iiidicalcs tiial a different type of re(]iiesl 
has been made or dial die measSequencer is inlbniiiiig ihe 
activeMeas of some iniportanl action. The EvertHandler .state 
machine (Fig. A ) addi'esses each event in a specified order. It 
makes sure l.hai all requests represented by an event have 
bei-n fully liatiilled beiore addressing the nexl event. The 
EveriHaridlBi itself rarely fully iiandles a reqiiesl, Insteiid, h 
generally ask.s one or ntore of the acliveMeas's other stale 
machines to handle il, A single evenl may represent many 
requests. This oflen happens when many iState changiis are 
grouped logeiher by thi- commandExsc process, hi an effort to 
improve user cniiiiiiaiid icspotLSe lime, die EuenlHandler and 
die odier activeMeas stale niaciiines are di-signed io handle 
muhiph' requests al liie stuiie imie. Mosi requests require 
inleracUori willi oilier processes before 1 heir haiidluig is 
complete. If nolhing else, the measSequEncer generally must 



© Copr. 1949-1998 Hewlett-Packard Co. 



riPi-niiilitif lOfl:) HpwIelil-PaKkanl JoiuiiqI 2;J 



IfCDHcBpluCEllIleSailriglSa I ICDHcBiltiirgldlB IMMglU 
IPS nmPauied II IPS nmPauieil II 

PS nolPlusoPetldingl PS nntpBusnPsndrng) 



^^^^^ ln>dv< ^^^^ 




MSS seqRuDEIing 



PS pauBed II 
PS pBusBPendlng 



SBBpBndfleqd 




IMSE ieqEBBpBBded AS. 
IMSS wqRBCfmipuIB 



M5SXX = Stale XX ol MeasSBqi^EncEiStBluE SiBle Machine 
SCH» = State XX ol SBaChaflgeXandlei State Machine 
DCH\X = Stale XX of DisplByChBrgBHandlBF Stale Machine 
FCm ^ Stale XX dI FrDiitEndCliBr^geHandlEr Slate Mechlnc 
CDHXX - Stale XX of CapiurnDsiaHsBdici Slate Machine 
PSXI ^ State XX Dt Pau^eStatBs SiBte Machine 
I = Logical NOT 

II liallEd) 



TEadY 



^ IMSS ^uspsndBd I 
SI:h idle && 
DCH idle S& 
FCH lEadv 



be suspo tided as part of the request tiantiliiifi. In this case 
Ihe EveriHantller will block in a state other lhaii WaitForEvert. 

All rif the activeMeas's other state machines run luider the 
control of the EveniHandler. The normal steps m hiuidling an 
event are; 1 1 ) For the lii st request copy ciilicjil infiDrmation 
from iStaiB lo currenlMeasDeliniiion, (2) Request appropriale 
aclionsiirollipf.sliiie luachiiies. (3) Alltiw all slate iiififhines 
Id niii iirilil ihey ivafh a qiiiesceiil slali^ (i.e.. iin luilher stale 
iransitlDU.s). This is donp in rurSutJMachiresd. (-1) 11" the re- 
qtiesl luLs Hfii hppri cr)m])iplply h;ituiled then lilnck. Thi-s is 
done in kaepAlii/eTillComplBieO. Wlien the request has been 
completely handled go to (he next request. (6) When all n^- 
i]iiesls for lliis event have been coiiiplelely h;uifllpci go to Ibe 
next event. (7) Wiien all events have been complelely 
handled wait for more events in the WaitForEvent stale. 

Once slarted, the EveniHandler will nin as long as any other 
state machine is making state li-ansi!ions. Onre no machines 
can make any further advancement the EvenlHaniller hliicks. 
knowing that it will take an outside influence m the form of 
an event lo allow any of the niachines to adi'ance further. 

The MeasChangBHandler slate machine I Fi^. ^) is responsible 
for Slopping Ihe measSequencer, waiting until llie current re- 
quest has been handled and then restarting it. There are two 
options for this service: the measSepiiencBr can be either siis- 
jiended and resumed or aborted and reslarled. The differ- 
ence is that a siLspension Ls coordinated with measSequencer 



Fig. 5. MeasChangeiiandlBi stslp 
iiiacliint'. 

signal processing so that no data is lost, nor iloes raw mea- 
surement data become inconsistent with data in the trace 
vectors, wlitle an al)ort happens immediately without regard 
lor Ihe valiility afilie vector data. 

Finally, the SeqChangeHandler slate inachinc (Fig. (5) is charged 
with (he regeneration of a fully I'linctioHEil composite mea- 
surement loop. This may include reqne.siing selection nl a 
[lew measSequencEr ;uid a recompile of the cuslomMeasStates by 
Ihe XXtvlGasSeqGen, It must also compute many proces.sing 
paiameiers such as the FFT window, Ihe correction vectors, 
and the acquisition time record length. If any front-end hiird- 
ware changes are required, then the SeqChangBHandler, in its 
DnSvnchronizeilChanges state, will courtlinate with juiother 
state machine to make sure that, ihe tiardware setup is 
accomphshed. 

The SeqChangeHaniller plays a critical role in tneasiiremenl 
mode changes. In the StopSequenceCtlr slate the activeMeas 
perl'onns housekeeping required to deactivate itself. It then 
advances to Siopldle where control tviiims to the mea'iure- 
ment manager, where a new measurement object is selected 
as Ihe activeMeas. (The measuremeitl manager is merely a 
repository for measnrement objects and is not shown in Fig. 
3). The now activeMeas then wakes tip in the OveilayXXMeas- 
SeqCUr state where it initializes itself to operate as a fully 
functional, fully active measurement. It is iiere that available 



24 IVcembpr IBtn HewlPtt-PnrkBrtl ,J(nimal 

© Copr. 1949-199B Hewlett-Pactiard Co. 



■wCnulabli U tndiCgnvlIt 




EOMPtU DDNEU 
fnrwEiecdiBlili 



COMPOE.DONE && 



CaMPIlE DONE 



nnrEiKualtli II 
trachCcMnpiIe 



.^^^HPI^^K^^ mea^Chgd ^^^^^^^^^ 



nqSiopfiBd 



<nawSeqGllr 



jsvqSiopped 



li = Lock isiutr^FtcaaurcB 

= Unloch lEiBieResDurce 
Iv = Loch weDltn-RssaL^rce 
av = Unlock vociorREipuretf 
measChgd = MSTaleChg W 
recompule II 
nowSoqCUr II 
ndwEiieculabLe II 
crTracliRBqd II 
, liachComplle II 

mBaBPBrmChg W 
EV"<^^ECIig 

r = Logical ^DT 
Fig. 6k StqChsngeHaruljer slate m^c^uiie. 




CONTINUE.HALIED.MUS 



s.vslt'm RAM is rcalioratpd acf ording (n l.lip \'ecl()r iipffls of 
this new aciiveMeas. 

Each of the four individual mejisuremcnt iibjectB fmodes) in 
till- HP894xxA iiu^aHiinMiifnt syslciii lias itsimii scl of staip 
iiiacliines. On a riK'itsiiromL'nt mode charge each slatp ma- 
chine of the incoming nieasuremenl (jbjecf must maintain 
continuity of its active state with its counlerjjarl in thp nul- 
gfiing mcasiircnieiti ohject. With t'-i-+ it is easy I'or each 
meiLsiiiemcnl iihjpcl Id contain customized state macliines 
ajid yet liave ctiuivaletit machines in separate measurenienl. 
objects share stale variablps. Consider tliis partial impie- 
meiitation of two siafe machine classes: 

class SlaieMachine 
{ 

lirivate: 
mt crnlState, // cuitbiH state 

protected: 
infcrnt State Ptn 

UQid cufrentStalefint newCjrrenl) 
I "crmSlatePtr - newCjrrent; ) 
virtiial void B«ecLteStatB(): 

II add default SlateMachine functionality hera 



// Eorstructor 
StateMachinBO; 
(tit curremStaied 



icrntSla[ePtr = &crntState,) 
(relurrCcrntStalePtrU 



// possible states fDrttisiances dI Custum State Ma chine 

typedef enum 

i 

CUSTOM. STATE_0, 
CUSTOM STATEJ, 
CUSTOM_STATEJ, 
I Custom States; 

cisss Custem State Ma chine ; public StaleUacMne 

{ 

protected; 

// Stale variable - comrnon to ALL instances of 
// CjsIomStflteMachme 
static int cutrentState j, 

public: 
// consiruclor 

CustomSlateMacliinefcrniStatePtr = ScurreniStatej 

curremSlatelCUSTDM_STATE_D);} 

// add defauli CustomStaieMachine functionality iiere 
virtual UDid cuslomStateOII; 
virtual void cuslomSlatell): 
virtual void cusiomSiate2(): 

I; 

By accessing tlie current stale Ihrongh a pointer in the 
currentStatell meniber i'uncli'm, CustomStale Machine can declare 
its ()wii static current stale variable ajid lorc-e its instances 
to use it by ijiitializinj> Ihc crrlSlatePlr to &current3tate_ in its 
constmclor. Since static menilier variahles are shared by all 
iiislances of a class, classes derived from CustomStateMacbine 



©Copr. 1949-1998 H ew I att- Packard Co. 



Dpcpiiibcr l!IH;i H<'«r|c'it-I'uckiuilJr»minl 25 



always ntainlain synchrunizaiioii of the tiiiTenl slalc in all or 
their iaslJUK c.s. Thry may. Iiowpvpr. overwritp the viitiiaJ 
slale member fiiiirtiods and thus provide state machines 
eusfoinized lo Die needs of their measure nient object, ownci-, 

XXMeasSeqCtlf 

hi licsi.Tihiiig the MeasSeqCtIr class and some of the sljile ma- 
chiiips that it owns, actions siJi-eific lo a mpasTireiucTit mode 
were nipntioned. Computation of acqiiisil ion time rorord 
li'iifith Ls a Hoorl exanijile. Wiile the MeasSeqCtIr class j)ro\'ides 
a place for this to ha|i|ien. il rpU'Ratcfs I he ucliia! work lu a 
dfTivi-il class. While most actions of l)u' MeasSeqCtIr class cfUi 
Ijc ovcrvMitlcn, vcr>' l^cw are. Mo.sl of I he defaull actions are 
well-Huiled for aJI of t he HPHSMxxA measiiR'ment tnodes. 
Most of the ciislonii/a1i(Hi is liiiiiled to I wo areiis: clcterniin- 
\ng wiiich iState cluuidcs affect the meii-surenieiil and how 
they affect it, and computing acqiiisi lion-specific pEiramelers 
aiid coiislaiiL vectors. 

XXMeBslSlaleOverrides 

Wheii (he comirandExec receives a chan^ie retiuesl it tlrsl vali- 
dates the re(|ue.sl, Tlien il inil.s (he \'alidateil i'haiij!e in iState 
and informs the acliveMeas of the change through a pSOK 
operating system event. The pi'ocess of paraineier validaiion 
must often be customized for a parlieiilar measnrenieiu. For 
example, the vector anil scalar niiMsiirement iiiixles itii])i.ise 
different limits on the allowalile resoliiiion liaiidwiilili. t"us- 
tom p;u'aineler valitlation is done hy the XXMEaslStaieOverrides 
module. Wiile conceptually sepLuate, tlie XXMeaslSiateOverrides 
moilule is actually implemented as part of tile XXMeasSeqCilr 
module. 

Measurement Loop Customization 

As mentioned eiu'lier. there Lire two aspects to cusloniiziiig 
the iTjn-time measurement loop: mBasSeqijencEr selection 
and generation of CLSlom Me as States. MeasSequencer selection 
is simple, since there are only two measSequEncBrs, one for 
averaged !md ojre for nonaveraged measurements, (ieneia- 
tion of customMeasStaies is the majority of what the measSeq- 
Gen process does. Each measurejuent ol.iject liiis its own 
XXMEasSeqGen olijecl. The XXMeasSeqGen instance iissociated 
with the acliueMeas is known as tJie activeMeasSeqGer. The 
aciivBMeasSeqGen comiiile is broken down into live sesmeiils: 
Raw measiLreinenl (Indiules acquisition up to linie gating) 
Hme gate {includes FFT. frecjueiicy correction, aiid spectral 
average) 

Measmemenl data and user math calritiation 
Coordmate tran.sform and units conversion 
Scale, offset, and pixelalion. 

If is easy lo see tlie correlation between compile segments 
and the flow of .signal processing hy cuinparing this list with 
Fig I. 

To minimize the ii.ser inlerfaee response time Ihe compile 
process may be enlere<l al ajiy of the .segments. However, 
once tiie compile process is started, jill downstream seg- 
ments must be recompiled. Generally, user change requests 
result in recompiling the minimum possible number of seg- 
ments. For this to work the activeMeasSeqGer nuist obseive 
two rules. First, the source 0])eraiul vectors Tor any given 
segment must be comjiletely defnied by tlie prexious seg- 
ment and caiuiot be modilled by this segment's compile. 



Complete tietuiilion inchiries length and data memory a<l- 
dress as weU as header infonnation. This is critical smee Ihe 
compiler relies heavily tm the defiiiition of source vector 
operands to ijroduce a coned executable and lo Iriick and 
transform header information llirough math operations. For 
the very first segment, raw measurement, the activeMeas is 
responsible for' initializing the timeData vector heailers before 
stalling Die compile. 

The second rule thai must be observed is that aveclor's data 
space musi be allocated liy tlie first segment thai uses it. 
This is critical for \'ecIoi s 1 lial reside in DSP RAM since 
their adtire.sses are ujiknown until they are allocated at com- 
pile time. For system-RAM-resident vectors this opeialion 
does iKJihing since they iire allocated when the nieasure- 
luetil iTiode is switched to the current acliveMeas. 

All of the HP SMxx.A XXMeasSeqGen objects iniierit code gen- 
Piai.ion for the user math step through t.lie pixelalion step, 
{Note that, while the user malh and nieasmenient data com- 
pulations are in the same segment, they are implemented 
sepiirately ;u»l can be overwritten ;uid customized sepa- 
fiitely ) The measure II let II daia code general ion is inherited 
by all XXMeasSeqGens hut the DigilslDemodMeasSeqEen class 
completely redefines it lo supjiort its unique set of measurp- 
meiil dal;i resiill types. .As migin be expected, the raw mea- 
sureiiieiil segmeiil is overw ritlen and implemented dilfer- 
enlly fur each of the measurements. The time gate segment 
is shajcd i)y the vector and analog demodularion measure- 
ments wliile it is overwritten and (hsabled for the scalar mid 
digital demodulation measurements. 

The end resuK of a compile is a set of A*C.'w/c subroutines, 
each of whii'h is composed of a group of kCode frames. Each 
I'rjune rcpre.sents lui operation that can eilher pert'onn block- 
orienied math oti a vectoi' or execute a remote procedure 
call. Those that perforin vector niath cause a DSP-resident 
mterpreter to invoke highly tuned DSP-resident math rou- 
tines. A compile .segment can produce one or more of Ihese 
subroutines. Al run time the measSequencer invokes Ihe 
kCode inteiprcter lo execute a kCode .subroutine al specillc 
points ui the mejisurement loop. In tliis way. nisloniizations 
to ibe measurement loop signal processing are carried out, 

kCode Compiler 

The kCude cuniiiiler provides the XXMeasSeqGen designer 
access to all vector-oriented malh operations as well as sev- 
eral olher ser\ices that proved critical to efficient firmware 
development: 

• Flexible kCode generation for vecloi' math operations 

• Complex/real vector type coercion (see below) 

• DSP veclor memory management 

• Veclor header infonnation tracking 

• Veclor units tracking. 

Only a fixed set of DSP-resident math routines are provided. 
Add, subtract, mulliply, divide, coiyugale. niagnitnde. |)hase, 
real, imaguiyiy, square root, FFT, IFFT. nat.ur;il logai iilim, 
and px-ponenlial are provided, along with several routines 
tuned for specific measurement sigiiiil processing needs. 
For any given math operation the compiler generates one or 
more kCode frames. Tlie kCode generated depends not only 
on Ihe math operation but also on the operands. For exam- 
ple, computing the phase of a real vector requires filling the 



26 DiTPmlU't Hi'wlpM-Pac'kan! .Imim.il 

© Copr. 1949-199B Hewlett-Packard Co. 



destinalion vector with zeros while the phase of a complex 
vector is rompiiied with an arc tangent operalion. 

Often ihe XXMeasSeqGen developer needs to lake more control 
over liie nialln njieraiinn, |)erfiirmiiig it only on select c<l ele- 
ments of a vector. Tlie compiler fac iliiaies iliis with se\ eral 
levels of interface, each providing the user with a ilifferait 
level of control The interface for the add operalion is 



void v_a(l ill Vector 


'srcVI. 


Veclor 


•srcVt 


Vector 


■resV. 


long 


'cnuntl 


void V sddl Vector 


*srcVl. 


Offset 


'src Off sett 


Vecto 


*srcV2. 


Offset 




Vector 


'resV. 


Offset 


'resOffsai, 


long 


'cDuntl 


voitf v_ad[ilVector 


■src VI, 


long 


•srcIlncP, 


Offset 


•srclDffsei, 


Veclor 


•srcV2. 


lorg 


*src2lncP, 


Offset 


'src20fiset. 


Vector 


'resV 


long 


"raslrcP, 


Offset 


'rEsOffset, 


long 


■ count) 



V^adiiO performs addition over the count elements of the vec- 
tor. Tile user has no rontroi rithoi than coiini. _v_ad[j() allows 
(Jie user to begin Hit' operation at an offset (potentially flif- 
ferent for each upefaiKl]. _v_add() allows the user to control 
the increment as well as tlie offset 

Tiie compiler must often "coerce" the operands VieLwccn 
real and connilex mmiliors. The si]ii;ii"e tixil is ;i S'luii exam- 
ple. The squan' root of a complex veclor reqnires conversiun 
to polai' format, taking Ihe .srjiiaie root ill' I he magnitude, 
division of tlie phase by 2. !uid finally conversion back to 
recljmgular coordinates. Since a real veclor may I'lmlain 
negative values it ninsi lie coeri-ed lo a eoniple.''; vector 
befiue Lindcigoing a complex square root. The result, of 
course, is complex. 

Tlie compiler is responsible for allocating DSP-resident 
vectors. Since veclor placement has a significant impact, on 
measurement loop peifnrmance. the comjiikT musi allempl 
to place as niimy veiiors h.s possilile in I he liigli-speeil DSP 
liAM. More impn!l;uil is lhal the most heavily used vector's 
are DSH-residcnl. The besi job could be done fay a twu-jiass 
comjiiler, but this would degrade user response time loo 
much as well as add complexity lo the design. A first come, 
first seivcd approaiii wasadopled with tlii' enlimicemeni of 
allowing the XXMeasSeqGen designer to force little-used vec- 
tors into the slower system RAM. Tlie compiler gioiips Ihe 
allocated vectors by segment, so that at theoiLset of a coin- 
pile, DSP memoi^ belonging to all segments lo be recompiled 
ean be i-eclaiiiied. 

A vector's headers contain information that is dependent on 
bolli Hie data and the math nperations fidni wliiiii 11 is de- 
rived, ror example, chamiel-specilic inrormalion nf a des- 
tination operand of a multiply iaacombiiialioiiof the channel 



iiifonnaijott of both source operands. Another good example 
Ls the FFT. which requires thai Ihe data dontain (wliich is 
pan of die header infomialion i be ciiaiigetl, For many of the 
trace vectors the XXMeasSeqGen designer can pre<bcl the cor- 
reci header contents since the designer has complete con- 
trol over all signal processing operations. However, for user 
nialh this is not true. The HP SflsxA kCode com|>iler lakes 
a much safer approach, transformuig the headers of destina- 
tion operands based on Ihe som ce operands and tJie ojiera- 
tion. .As part of each requested math operalion the kCode 
compiler modifies the header of the destination operaml 
according to a set of header tracking iransfonnation rules. 

Tile units of the veclor data are critical to its interpretatiiin, 
inctu(Ung units con^-ei'sion and scalhig for thsplay. Like other 
veclor header information, units are tracked hy the compiler 

Measurement Loop Execution 

Tlie measSeqLencer state madiiiie is shown in Fig. 7. The moa- 
siuement loop if.self begins in ilie WaJtForDsia state and ends 
in LoDpDore. Many of the slates in this loop can be custom- 
ized througii kCode subroutines. The maasSequencer remains 
in WaitforOata until a complete scan has been acquired in the 
sample liAM. By the time CLstom2 is exited the raw uieasiire- 
menl data for Ibis scan is available. The GeiDsia, EnableDMA, 
and Cusloml stales are designeil to allow data for the first 
ehiitmel lo be processed while data for the second chamiel 
is being tiansferred inio timeDmaBuf (Fig. 2). Likewise, first- 
channel data Tuf the next scim can be transfeiTcd during 
Custonia and states that follow it. Tiic measSequencer is not 
allowed to operate on trace vectors until it. locks the VECTDR_ 
RESOURCE semaphore in LnckResnurces. The Ibllowing two 
states, Ciislom3 and PosiProcBSsMeasRas, jieiTiirrn all signal pro- 
cessing from me;Lsiirenienl dala on Isee Fig. 1 ), Finally, in 
the Dtsplav state the maasSeqjencBr gives VECTOR.RESOURCE lo 
Ihe acanOona process via a remote procedure cill. As de- 
siTibeil earlier, the scanDore process performs further pro- 
cessing lo ready Ihe trace data for ili.splay. To roniiilete the 
syiicbroni/.iiiion with the measSBquencer. scanDare will not 
iinluck VECTOR.HESOURCE until trace ilata has been displayed. 

One of Ihe main differences between the averaged and non- 
averaged measSequencers is a direct tnuisilion from Custom2 to 
LoDpOone. allowing Irace-related pro<'essing to be l)yt)as.sed in 
a fast average mode. Anolticr is the transition fi'oni LoopOone 
1(1 Halietl lo end an averaged measmement. 

Tlie main loop supports both free-nm and triggered opera- 
lion. An allemate |iatli has been provided for single-sweep 
operalion. By going lo AuvainngAiin from LoopDone Ibe meas- 
Saqueiicer is idled imlil Hie user arms Hie sweep. 

The PreOataCltnl slate is provided to set up bai'dware before 
starting dala acquisition. Generally the hardware has been 
set up pro|)erly by the acliveMeas. but the scalai' measure- 
ment uiusl iiiodiry Ihe i.< I frequency tlironglioul ihe sweeji. 
Before starting acquisition ilie StartOalaClinl .state waits for 
die hardwai'e lo selde. 

Assault Handling 

Due nC the most cliallenging aspects of meiisiirentent design 
is handling changes to iJie measurement .seluj) in a gracefnl 
manner. It is not suffieient lo recom|)ile and reslari a new 
measiiremcnl while ileslroyiiig the existing tneiisnrenient 



llwi'inliiT KIlKi Ili'wIi'tl-I'Mi k^il Ji.Liniiil 37 

)Copr. 1949-1998 Hsw I att- Packard Co. 




rianihig 



«inglaSwBHp ' cnnEiDDnlBSump 

flInglBSweep 



STWir_aa * CONTINUE_SEQ 

#!r<>r 



BugCDmplcle 



II = LuiiicBlaR 
tA = Logical AND 
I = Logical not 



liuapBntfRtvd Sft 

BuippnaBa -4 



ncMiHiURciil 




Fig. 7. MeasSBQijencet stat.p 
niachme. Tlic heavier liiies 
the main loop. 



28 rirrcnihcr \\m Hr^wk'tl-Packartl .Tuiimal 

© Copr. 1949-199B Hewlett-Packard Co. 



Remote Debugging 



One of the ilffficutnes olten encountered vm&\ developing software ftw embeddea 
EKlene IS 3 limited ability to debug Ilie software Fiequemtv. ifris is iKcause ol 
Ixi af physicai aress lo the CPU tm nadrtitjnsl anoroacfiBs sucti as logs analysis 
or procBSSO' emutalion Witfioul iuH [SDcessor emulation ivrth souice-level (!et>i(9 
gets and other prodiiciiviiy lewis, the emhedfled system flesrgnei is often left Aiih 
inefficient methods such as inMrtion of print sistemems, hardware tracing nf 
memory accesses, di simDly gestaltto isolate and fin complei software defecrs 

Tfia software iJevelopmail team tor The HPSMioA analyjers used a technique 
called reniDtB ifstKiggmg la aid in the develooment and rietujoging ot the soli- 
waie Hemnle dehugging is tfie use ol aOuanced deOiiggers tunning on a hosi 
compulei ID debug snftware Ita! is running m an emtiedded sysiam Icr other 
compinerl rairtet ihaii nn ihe has! cnmputei f^g, 1 shows The cnncept 

Usir^g the GNU C wmp'ler Igccl on a host wmfcslailon, an obiect code tile is gen- 
eraied, which is then read by ttie GNU debugger (gdli) Using a communication line 
Id the taigei hardware under deveiopmenl, gdb provides the w/orhtation user 
debugging capabilities and insighl mio the runnrng system. 

The GNU debugger gdb prrjvirtes capabilities such as slack backtraces, software 
breakpoinls. j)rintmg C structures, disassembly, anil other high-tavel featurBS l[ 
cnmmunicatts with the targei hardware by means of a simple ASCII protocal 
which IS inlerpreted by a small kerne) in the instrament itself. 



Connnunlcslian 




Inslrumsnl 



Bool ROM 



Flasli ROM 


gilli 
InleilacG 




Otivei 





Remote Debugging Protocol 

Ttieter ■ . pnjtKOi belweei jdbandltieTargeltnsnurrKniciinsistsot 

linens 

Command Function Response 

g Herurn The value of ihe CPU regisiers 

S Ssi the value of the CPU registers 

mAA.aAillL Read LLLL bytes at address AA AA 

MAA.AA.iiU. Write UJl bytes a! address AA.AA 

c flesume at curreni address 

cAAAA Continue al address AA AA 

5 Slap nne machine msiruerion 

sAA. AA SiBp one inslnjclion from AA AA 

' Return the cun-enl eiBCUtion status 

Each command Is preceded by a header and has a checksum ai the end to ensure 
data mtegntv Since Ihe detJiiggei has detailed knowlertge of the ctide running In 
the instrument, it can use the primitiVBS aboue to acquire irformaticn lor back' 
irates, priming vanahles, and change of execution comrol For example, instead of 
pioyiding a command in the prniocnl Inr setting a breakpoint, ihe dehugger simply 
reads the value m memorv wfiera the breakpoint is to be placed and replaces ihe 
value with a trap inslructton When tfte breakpoint is teacherJ, Ihe debugger uses 
the write memory command to restore the onginal CPU instruction. 

The use nf Ihis remote debugging capability in gdb allowed us to develop our 
firmware withoui the use of amulators There are however, several classes of 
problems toi which emulator or logic analysis siill prnye invaluable For eiample. 
when a variable is being ouerwrilten unexpectedly, hardware lools can be the 
quicfcesl way of hunting down the problem We've found lhal llie use of a SDUrce- 
level debugger can signdicamly reduce the need foi hardware solutions because 
the programmer is giuen a much belter pictuie of the state of the system and how 
it got there. 

In our environment we used for the cnmmiinication line We wanted to be 
ablntn use this line lor debugger communication as well as general mpul/output 
from printfll statements in the soflwaie Thrswas solved by using 8-biI ASCII on 
the BS 7.32 pnit and having all debugger traffic assert the mosi-signilicani data 
bit proved to he too slow for large data transfers such as the executable 
image so the HP-IB |IEEE 438, lEC 6251 was used for high-speed data iransfers 

In summary, remnta debugging gave us the following advantages 

• Source-level debug The use ot source-level debugging contrlbuied la a significani 
product I yity gain 

• Lower dBvelnpmeni cost Ihe suurce level debugger reduced our riead for capital- 
miens ive hardware debugging aids 

• Ability to debug during enyirnnmBntal testing By simply plugging in an nS-23? 
t^onnectoi, Temperati) re-sens i live calibration problems could be quickly undaislood 

I Abilily [0 debug an itisimmeni wilhoui removing couers or boards Hardware 
prDlniypBS used by marketing or for documentahon development could be iriler- 
changed with units used tor software deuelopmenl with no changes 

Glenn R Engei 

Develnpmenl Engmeer 

Lake Stevens Insfrument Division 



Fig. 1, ^F^iiip lor 'cinoie delnigging of emtieddeit syMeiTi ioftware 



(laljL In niDSl rases, a change musl be reflected in Ihi' ciii- 
rent measurenifiii reNiilts. Lit the HP 8!.Mxx.\ this is jtaiticu- 
larly true ■iiiice the tiieasuiemenT jtirii) iiicliides tnice-rriaterl 
processing. These chanfies to t.lie measSequencer arc tennerl 
assdiills. ajnl Ihe measSequencer anil thi' aciiyeMeas must both 
work clo.soly liiRelluT lo liiuitile Ibfiii. 

An tjflsaiill Id the measSequencer begijis as a change reqm^sl t.ri 
the activeMeas. As pai1 of ila hiuiclliiig the activeMaas will 



likely modify ineasSiate and traceStata setthigs tisc-d by ihp 
measSequencer. It may also recompute coastimt vectors such 
as those ii.sed in windowing and coiTcrling Ihe data. A I'om- 
pile often jiccnmpanies this change wilh a reiletlnitioii nf Ihe 
kCoile siihroiitines and ri-alloeatiiin i if DSP vectors. The 
measSequencer eaiiiiol lie miming when data critical to its 
proper operation is being chiinged. Tlierefore, one of the 
first steps {it chmigc handliiif! is lo sus[)eml Ihe measSequencer, 



© Copr. 1949-1998 Hewlett-Packard Co. 



ili'ti'nlbcr Um Ik-nli'll-Patliaril ,li)iiiiijil 29 



This is dune by lin- activeMeas's MeasChangeHandler stale ma- 
chiiic (see Fig. n). The measSequerter will liiild iifi' lliih assaiill 
imtil il hiLS ;i f (insistent set of data in the raw and trace vci'- 
lors. This means lhal it will not clierk for assaults from the 
time it leaves WaitForData until it enters LoopDctiE, The meas- 
SeqLBncer begins 1o liandle tlie assaiill by enleiiiig the sus- 
jieniled slate aiiil sHvinj; .uiy raw fJSP measuremenl veclois 
lo ilii'ir sy.steui HAM eoujireqians. It Ihen signals llie aciiue- 
Meas llmi il is HiisiJt'nili'd. Ii will lejMaiii in tliis stale iimil the 
activeMaas releases 11 iifler eonijiletely lianilling all changes 
lo the nieasiirenieiit deluiition. Ifihe rhajige reniiires that 
existing data lie inteipreled under the new measure men! 
setiiii [he aciiveMeas will reijuest the measSequencet lo reeoni- 
[jiite ihe data. The re('oni])iitaIi<jn uses the same kt'ode siihi- 
roulines as the Cjsio(n3 and PoslProcessMeasRes stales. The 
extent of the reconipulalion is delermined liy the level of the 
reconi])ik'. Like the ciiinjjile, as liilie reeompulatiun as nec- 
essai'y is <lone. The measSeqjencer Ihen signals the activeMeas 
ihal tln^ i-eeoin I ) Illation has been completed, At this point, 
the activeMeas Ibices the new data to be redisplayed and then 
tells (he measSequencer to resume processing. Tlie meas- 
Sequencer Ihen returns to the stale from which il w;is sus- 
pended iuid resumes operation. Finally, Ihe measSequencer 
niity, if directed by Ihe activeMeas. be forced to restart arqui- 
silion (if hardware was chjuiged, for example] or restart Ihe 
cnlire iiieasiiieiiieiil (for avei'iigeil measurement only). 

Conclusions 

Measurement design is a complicated process during which 
many ilecisions Iraduig off perfonnance for ease of develop- 
ment must be made. Many of the complications faced by the 



designer are compounded by Ihe evolutionary naliire of a 
measuremenl's feature definilion. Willi careful design, a 
measuremenl architecture can be pui in place iJial jirovides 
a foundation upon which rnulliple jneasiirements can he 
implenieiiied. I sing a common measSequencer. the kfijile 
compiler allows designers to customize the measuremenl 
loop quickly and easily wiih minimum aiiention to ciimmnn 
features and jiruiocol. l.ikewise. by using the oljjecl-orienled 
features of tlie {' + 4- ci impiler, designers can develop classes 
that support the measurement loop by eoneenlraling only on 
incremental changes lo a t'oundalion;d feature set. Although 
performance dues suffer slight ly, this approach yields rich 
dividends in development lime by allowing multi|ile mea- 
surement designers to inherit a complete set of foundational 
features. 

Acknowledgments 

The author wishe.s lo ihaiik ilie iu;iiiy people who contributed 
to the developnieiil of Ihe HP 894xxA measurement system. 
Dirk l liibregs developed Ihe kCoile compiler and Ihe user 
nialli scijueiiee generalor. Glen I'urdy jirovided the DSP 
maih rnuiiiies, Doug Wagner and Boh Cutler develojied 
many ol'the signal proees.sing algorithms required for 
demodulation measurements and time and frequency cor- 
rer lions. Ken !Jlue desigiierl Ihe iiistnimi'nt's scalar' mea- 
surement mode. Mike Hall and .lerry Weibel developed the 
harilwar e driv ers. Don lliller developed sjiecial marker mea- 
surements, (jlemi Engel lielped with many design delitils 
including memory management and C++ coding. Many 
thanks go to ,Jeny Daiiiets. Bill Spmilding. and Mike Aken, 
who m;uiaged tlie rirmware developnieiil, tor iheii' support. 



.30 I'r. lITOIWli'll.Pack.irr! Iniima] 

©Copr. 1949-1998 Hewlett-Packard Co. 



Baseband Vector Signal Analyzer 
Hardware Design 

The HP 8941 OA combines superior front-end linearity and high-speed data 
conversion with powerful digital signal processing to provide advanced 
measurement capabilities- Extensive calibration, flexible triggering, and 
arbitrary source types provide the accuracy and versatility needed to make 
the sophisticated measurements required for complex signal analysis at 
RF information bandwidths. 

by Manfred Bartz, Keith A. Bayem, Joseph R. Dieilerichs. and David F. Kelley 



The IIP 894iOA vector signal analyzer prtjvide.s aii array of 
new capabilities ro meet ll:e emerging nieasiireniPUl reqiiire- 
mciUs of coinijlox signals that rft]iiire simidraiieons ajialysis 
in llie ihnc. frcqueniy, aiiri modulation domahis. It makes 
!n ea-i II rente nls with resohnii.in baiidwidtlis as low as one 
ttiilliliertz antl frequency spajis as wide as 1(J MHz to ac-coni- 
modale the wide information baiiclwidllis of complex and 
freqiLsncy-agile connnuni cations signals. ItK user interface is 
familiar to users of iraditional swept s|iec1i-iiin analyzers, 
makhig it. easy to iLse, Willi an optional spcoikJ cliaimcl, a 
versatile source with arbirrary source types, Hexible tiigger- 
ing, a built-in disk drive, and a variety of software and liiird- 
ware options, the HP SiWlOA provides the upgrade paths 
necessary lo accommodate the most sopliisticated user 
needs. 

The HP 8!)41(IA meiusmemcnl engine is based upon powerful 

digital signal pnjfessir\g lecluiologies that provide its speed 

;uk1 nesibilily. The key elements nf the haJiiwaie thai sii]jporl 

I lie liPfirt410A's high performance include: 

An t'xcepiionally Unear liniu end wiLli high Input impi'dimcc 

capability, input jiroleclion, auloranging, and atiti-alias 

protection. 

A lii^h-.speed, 2G.f>MSa/s (milhon samples per secoiul], 
wkie-dyn;uuic-range anali.ij>-ti.>-digital conveiter (ADC) llial 
emplo.vs a propriefar.' iru[>k'menlalion of lai'ge-scale dither- 
in); lo provide .■superior lineaiily 

A custom 25,(i-M.Sa/s digital local oscillator and decimating 
filter chipset thai provide up lo '£i bits of effectivp ipsoiulifin 
Powerful noai ing-point .signal processing u.singdie Mi)torola 
nSP9fiO():i. which delivers up to 18-MFI.OP f million floating- 
point operations per second) peak perfonu^mce 
Dedicated display proce.ssii\g using the Texas instnimenis 
TMS;j40:iO graphii H syslem pi oeessor, pro\iding up to (id 
display updates per secund tu a color display 
A versatile signal source thai provides sine, chirj'. i"a!idom 
noi.se, and arbilraiy .signal lypes 

I'nll calibration to provide superior accuracy and signal 
fidelity 

Hexible triggering with both prptrigger and post-trigger 
delay and arm ilelay 

A backplane with sloLs lo arcoiut nodule hardware upgrade 
oplions. 



The block diagram, Fig. 1. depicts the data flow path of the 
HP SPJlll.'i liaseband vector signal analyser (.Note thai the 
IIP 89410A and Ibe HP 8!J44(IA IF section are identicid. M 
fiulher discussion will refer lo the IIP SH410A hul applies 
equally to tlie HP Sfl44(IA IF section. ) The signal inpul at the 
mialog front end ls amplinet:! .ind aUas protected. Following 
the analog signal conditioning, ihe signal undergoes ;malog- 
lo-iiigiial conversion. The digital data, which is sampled al 
25.(i M.Sa/s, is routed to a fligital swilch Jisscmbiy anti on to a 
proprielaiy digital LO and rligit:il decimating filter chipset. 
The digital I/) and decimating tillers perform frequency se- 
lective band iraiislalion. The trmi.slated data is I hen stored 
in a sample RAiVI with a capacity of .'J2K. samples per chan- 
nel, which c;m be ojil tonally increased to .'jI^K, From the 
siunple RAM, ihe eapiured data is bii.sed lo the D,SItlblKI2 
processor ;md .suhjecled lo corrertions. windows. FfT algo- 
rithms, and other malh ojieralions. The DSP aLso foniiats 
the data for viewing and wriles the data lo the display sys- 
lem. hi iIlc final step of tlu' dala path, Ihe tUspltiy processor, 
Ihe TMS.^1(I2I) (iSP ronslnicis a Irace from Ihe formatted 
(lata and presents it lo the user on jin internal T.'i-inch color 
CUT. By using the higli-s|ieed l oinponcnis in (hi- rlata path, 
the IIP 8!lll(.)A's signr.il pi'oi essing lifuiiwiu'e can proce.ss 
approximately ;!l)(l iil2-poinl complex spectra per second 
with di.splay update laies reaching (ib persectmtl. 

Tlie data nt)w paih for the source follows a similar sequence 
leading from Ihe digital processing I o the output of the ;uia- 
logsomi'e. !n conliasi to Ihe receiver pal h. Ihe digilal LO 
mid fillei-s operate in Ihe reverse mode in the source path, 
performmg the necessary operations of data intcrjjolalion 
and frequency iranslal ion, Tlie inlpqiolaied dala iscon- 
verieil by the sourr'c digital-to-analog converter fDAC] al 
25, () MSa/s. A recoii.slruclion filler and hul her conditioning 
circiiitiy pre])are Ihe luialog .signal lo he <>ul[ml on die IIP 
Sf.llllJA's front panel. The digital switch assembly between 
Ihe front-end ADC and the digital local oscillator (LO) and 
decimating fillers allows theiligital source I o be coniiecied 
lo the receiver diila palh for diagno.stic puriioses. P'ull ana- 
log calibration is ticcornjihshed by placmg ihe uitenial cali- 
braiion source path between the analog source and the two 
iniiul chmuiels. 



© Copr. 1949-1998 Hewlett-Packard Co. 



tai.TiibiT ltN«H.-wl.'l(-)'iH'kyuii.lrniriiiil 31 



Channel 1 1npuT 

jIdenlicBl Channel 2 OpTlDnalt 





Filiar 



Saurcs 



Gain/ 
Rsnfling 



10-MHi 
AnIj'AliBaina 
iDW-Pau 
Filter 



Aiiiociinging 




Rest 



Local 
Oscillator 



Csli brail oil 
Source 



Source 
Output 




Buflet 



9S.B 
MSa/s 



tD-MHz 
RBcanslructiDn 
law Pass HJter 




low Pan 
Fllldt 



Imaginaiy 



DecimBling 
FiltErs 



From 
CPU 



DC Oflsel 



low- Pail l^^^^l 




4 Sr QscillatDi 
^^^^ 


3ac-Sainplc 

SlUICD 




Waveloiin 




HAM 


^■■jjjl^f Imsginarv 




Interpnlatinn 
Fillets 



Artil'eu nnd 




Pseutforandam 




Nunihar 




Genera tof 









Systeni Mentorv 
HM Bvte RAM 
3M-ByloR0M 



SyslEm Bus/ 
Bach plane 



External Reterence in: 1,Z.!>,1DUHi 




Fig, I. Blnck (ii,iKr;iiii t)[ Uie HP 894UIA vector signal analyzer. 
Analog Input 

Tlif aiialtig inijut provides thp inter&ce between the signal 
[(I lie niPJi-siiretl aa«l Ihc ins[i-iimeiirsaiiaUig-lii-[li(;it;i] con- 
verier. Inipedaiue rnaUhiiijj. r.mgins, and wili-iilias lillering 
mast he acertnipIiKlied wttlitiut comiiromisitig signal fidelity, 
Tlie input must he rolnisf in the tat'e of real-world signals 



and inadvertent iibiLse, One inptit channei is slaril:ml, and a 
sec-ond is optional, 

Tlie input is single-etided with the low side al chassis ground 
poieniial. n(l-i>iim, T.Wihm, and l-meguhni lenninations are 
provided. The specifii!!:! reiiini loss l.s 2ri iliJ ibr the 50-ohin 



32 llcri-mliiT lM93lk'wlptt-I^'karH.tm]TTial 

©Copr. 1949-199B Hewlett-Packard Co. 



terminsiion and 20 dB for the 75^)hm, The 1-megohni inptrt 
is specified ai 2"*i accuracy and less than 80 pF of shiiiii 
capaciiance. 

Input ranges are provided in 2-dB steps. For ihe oO-ohm 
termination, inptit ranges cxTend from -30 dBiii ( 10.0 tiiV 
peak) to +24 dBm ( "i.OlV peak). Tlie 7i>-ojini ranges pxiend 
o\'er the sanie set of voltages as the .50-ohni ranges, but are 
nuinerically smaller by 1.761 dB. The l-megohni ranges also 
start at -^V) dBm 1 10.0 mV peak), ljut extend lo +28 dBm 
(7.MV peak), dBm here implying a .TO-ohn) system refemice. 

A selectable anti-alias Filter is pro\icied. Alias proiection is 
sppcincii ai SO dB atid is iM>ically better than 95 dB, 

Input Cable and RFI Suppression 

The input BNC connector shell is dc i.solaieti at the Eront 
pane! l see Fig, 2). This is done to reduce spiirif us inputs 
caused by Ihe instnmienf.'s switching power sujiply, display, 
aji<i power line circuiliy, preventing induced ciiiTents from 
llowing across the input cable shield. Dc isolating Ihe con- 
nector shell necpssitaics ac byijassing it to cha.ssis ground 
at the front panel lo reduce radio frequency mtptference 
(RFI) emissions, which othei-wise use the cable shield and 
connector shell as an exit path. 

Ac byjia-ssing is ilonc with a small priiueil circuit board be- 
hind the front paiiei. Isolated areas aromid the connectors 
make coutacl with the coimcctor shells \1a (he connector 



mounting hardware. These isolated areas are then bridged 
by surface mount c^arilors to the ground areas of the 
board in contact with the front subpanel. 

In the prototype, these isolated areas were byjiassed lo 
^imd by a group of three surface mount capacitors, ail on 
one side of ilie connectors. Tliis desi^ inadequately sup- 
pressed RFI in the f 00-MHz range. These RFI emissions 
originated from the fast data biLses in tlie iiistnimeiit, which 
are sfrong soiuces of liigh-frcquency energi'. Network ana- 
lyzer sn measurements showed that rhLs srniciure was in- 
deed resonant al around 900 .MHz and therefore was failing 
lo suppress RFJ there. The production revision connector 
bypass hoani iias four surface mount capacitors placed 
evenly around ami as closi' as possilile lu each connector. 
0.047-iiF capacitors are used, allhoiigli the particular value 
doesn't seem to be very important. ;\11 emissions itp to the 
test Umit frequency of I GHz are effectiveiy eUminated from 
the input connectors. Network analyzer mcasiu-ements on 
the new design show that the resonance has been pushed 
out to beyond 2 GHz. 

Each inpiii cable is passefl llu-ough five high-permeahi)ily 
ferrite toroids at the froni-paiiel end. These serve multiple 
purposes, iiiclufhiig fonning a low-pass structure with the 
coimector capacitors lo minimize RFI, reducing spurious 
inputs, and minimizitig the effects of ineasuremeni groimd 
loops. 



Calibration Inpul 
I From Analog Souice) 




FiR. 2. Uluck dlagrain of llic iiiiuliiH iiiiml fniiK ptkI, 



© Copr. 1949-1998 Hewlett-Packard Co. 



Di-i'SiiiliLT IIWaHewlfit-l'aiJtanlJijiiniiil 33 



Input Termination 

Till' iiApiil is tiiii(lanii'n1ally a l-megflini slnicture. Truiluii 
ami 5(l-olim li'niiiiialiuiisarc rrealcd liy shiLnling Ihr' 
l-nie^^tiliiii iiipiil viiih 7ri uhms oi 7r, olims in pjuallc] wiili 
150 ohms lo make "lO ulims. Ilowf vtT. Ijy il-si'If. ihis. ilcsign 
Ijrodiicns uiiacfcplablc return loss, cspeciiiliy Tor Tri olmis. 
ht'fause of ihi- T'l-ijF" lyjiical sliimt i-apac-ilani'f hcT'jsh IIip 
l-nn'gohm inpiil. At 111 Mllz, 75 olims in psiralle] Willi 7;) ph' 
gives only about a Ifj-dB retmn loss. 

This ti'luni liiss pi(il)lt'ni is snlvefi with a small indiiftancL- in 
series with Ilie lermiimtion rt'sLstoi; whicii scnes u> limo 
out some of Ihe effect of the shunt capacitance wif.iiiji the 
instrument's 10-MIIz frcijiiency ranfje. 0.22 nil is UJieci in 
series witli Ihc 7'i-<i)iin sluml Icniiination. Al 1(1 Mil/,, inpul 
rcluni loss is improvcil to lyjiically 2\i dU for 75 ohms and 
Ijeller than :J0 tlB for 50 oluiis. 

NoUiing is frep of courae, and one di.sadvanlage of iliis nmiiig 
leclinifjue is woree relimi loss iierronnaiire lieyoml Hi MHx. 
well heyoncl Ihe inslniniem's frwiiiency rjuige. The oliier 
disadvanlage, if II lis lechniiiiu- is piislied loo far, is exagger- 
aled treiini'iii y res|innse dineteiices aniong Ihe lin-nhiii, 
7''"i-ohm, and 1-inegohm I.erniirialiuns, Since calibralinn doe.s 
not account for Ihe different, lerniiiialioiis indepemiently, 
these differeiii'es miisl he weil-conlained. Wilh 75 j)F of 
inpul shiini caijacitancp, 2H dB of 7ri-olun retiini loss is 
achieved wilh only minimal ( on die order of U.Ol dii) 
dil'ferencfs between leroiinauon frequency responses. 

Input 'H'ip 

AcHve iii[nit irip circuitry defects excwisive voltage on the 
50-ohni and 7iVnhni input leniiinatinns. IJiode-capacil or 
peai4 delecioni capiure peals pusidve and negative voltages. 
Comparators llien I'eact, caiLsing the logic on the input 
hoard to open the termination and protecliiin relays and In 
alert tlie instiiiment ('PL' lhat the input is trijipetl. Tlie tri|j 
poijil is noniiiially ±7.2V, or about +27 dlini. constrained by 
the ilissipalion cajiabilily of Ihe terminations. 

It is important thai the detection diodes be reverse-biased 
for all noniial input signal levels. Tlic diodes only become 
fonvard-biiLsed al signal levels apjiroaching overload, pre- 
venlinj; nonlinear conduction cunenls from being (hawn 
from llie input and causing distortion. 

Calibration 

Till' cahbration signid is mtroduced info the input after the 
liroieclion relay. This is less lhan oi>liniuni in Ihe sense thai, 
ideally, the calibration sign;d would replace tlie inpiil signal 
for as much of the uipul circuitrj" as possible. With this 
approach, the input temiinatiorm are not included in the 
calibrated input circuitry. 

The principal adx atilage of this approach is that the input 
terminations maintain their suite relalive to liie input con- 
nector during {■alilitation. This minimizes disniiitions tn the 
system comiected to the insiruHLent itiput. Also, the protec- 
tion circuitry a-ssociateri willi ihe input terminations ren\ains 
in effecl during calibration. In the contrasting cfesign in 
which the input rerinuiations are inr hideii in the calibration, 
a ti'inporajy lerminai.ioii musi be switched across tlie input 
to niainlain the load on the system lieing measured. This is 
virtually impossible lo do without a transient disruption of 



the load, I o which some systems are seasitive. Also, protec- 
tion of the tcmpnraiy temiination is a problem that may 
require duplirating the trip circuitry. 

Since the input temiinalions are not calibrated, Ihe calibra- 
tion lenninalion must niimicihe iiipiii at-ctirately. Fortu- 
nately, this can he d<nie vety well ujj lo 10 MHz, 'ITie resisinr 
and return loss liming induclor simcliire of Ihe mput termina- 
tion is copied exact ly, wilh a small enipiricaily determined 
lumped caiiacitance added lo model Ihe capadl.mce of Ihe 
input lip to Ihe caliliration signal entry poirii. A transmission 
line deliming series resistor inunediately liillow iiig the cali- 
bration lemiinalion is also empirically delermined, and 
models the series resistor following tlie input termination, 
Tliese resistors aie tlie subject of the following section. 

Wide Bandwidth in a Higli-Impedance System 
One of the best wjQ^ to achieve good lineitrity and disltirlion 
pert'onnance over a given handwidlli is In design so lhal the 
bmidwidth of interest is a relatively small pari of ihe overall 
bandwidth of the system, nisloriiiiii olieii increases draniali- 
cally at frequencies near die bandwidth limit ofa ciri'Uii, 
TIius, a low-distorl ion ilesign may reasonahty employ circuits 
with ten limes the intendeil application bandwidth, 

Such is the case whh Ihe IIP 8911 OA inpul. Kesigned to be 
used to 10 Mllz, some of ils constituent blocks have band- 
widlhs on Ihe order of 100 MIIk or m<ire. However, wilh 
some signal runs on tile input board on the order of six 
inches in length. Inansrnissiun line efl'ects cannot be ignored. 

The noiTiial solution is to use a doubly tcnninaicd transmis- 
sion line design. Tliis design csills for carefully conirolled 
transmission hne impodanc(-s malclicd on both the source 
mid receiving ends. However, two factors ]jreckide the use 
of doubly Icrminaled transmissions lines in ihis appliiaiion. 
One is the li-dB signal loss al Ihe source-lo-iine divider, mid 
the other is Ihe iiiabiliiy lo .source the required current to 
drive inalcbed riD-oliin or even 100-ohm nmismission lines 
and maintain distortion performance. 

Instead, a hybrid slnicltire is used. Umg signal runs have a 
small series back-inalch resistor al the driving enil, Some 
rims also have capaeilively coupled temiuiaiion resistors al 
the receiving enil, These circuit elements contJTii the peak- 
ing in frequency response that woidd otherwise occur. b\it 
do not cause a iaige signal loss oi" diaw large ciuTcnts lliat 
would adversely affect dislortii.in perfomiance. 

Attenuators and Input PET 

The lirsl elemeiils in tlie ranging architecture of the input 
ari' a pair of^O-dB arti^iuators. These are designed for a 
1-megohnt system (prcsenling 1 megohm to the preceding 
circiiitty when loaded witli 1 megohm). 

The atteiiualors transition from resistive divider to capaii- 
live dividers in the lens-of-kiloIiei1z region, Eacii attenuator 
re«|uires a timable-capacilor flatitess atljustnient. The lirsi 
attenuator also has an input capaiilance adjusimcnl to 
balance ihe input capacitance for the attenuated and 
nonalieniiated settings. 

Ac coupling is jirovided by a 0.1-[iF capacitor, giving a low- 
freiiuency roll-off al noniiiially l.li Hz. The ac coupled and 
dc coupleil paths are carefully balanced for capacitance to 



34 Ilf I'pnitiiT ttH3 Hp»-leTl-l^ckiini .tiiiini,il 

©Copr. 1949-199B Hewlett-Packard Co. 



ground mthout this careful halaiire. a frequencj^ resjionse 
difference would esisi berween the two paths ai high 
frequencies, iinaccoimted for by ralibralioii. 

The input FET is the only discrete amplifier stage in the 
input. A source follower with cuiTent -source bias is con- 
structed from a matched pair of JFET^ in a single package, 
loss (drain current at zero gaie-io-sotu-ce voiiage) matching 
between the iwo deiicre to ±5% requires an a((justnient to 
tJie ciirrenl source to obtain OVdc aaoss tlie stage. Signal 
gain is nominally ahoiil -i dB for the stage. 

The FET device used, a special high-iraji.sconductance. 
high-l|jHS lypf"- 's capable of maintaining disiorlion perfor- 
manre well in exces.s of yo tlB at 10 MHz vvitli ihe-]2dBm 
signal levels found at this point in the ciiruit. 

The source follower configuration is susceptible to oscilla- 
tions if tlriven from a reactive source impedance. Gale- 
stopper resistors (small -value resistors placed riglit at the 
FKT gale) ensiu-e thai ihe stage is stable for any soiuce im- 
pedance presented at the input. Iflli olinis w;ls chosen lie- 
cause models showcti it absolutely guaianteeil stabilitj' 
while negligibly affecting the overall noise figure for Ihe 
input, which is dominated by the prpainr)lifier. 

Preamplifier 

Wiite-liaiiciwidlh current -fee clliack upei atioiial amphfiers are 
used for all of tlie remauiing signal ainplificaiion tasks in the 
in))ut circuit (see Fig. 3). These vety higli-s[>eed, high-fidehty 
devices allow ampiilication stages to be built with very wide 
bandwitlth, relatively inilopeiulent of the stage gain, because 
of their current -feed bacl( topology, 

This gain- bain] width independence is exploited in tbc vaii- 
able-gain preamp stage, which can be set to gains of 13.4 dB 
or 3.4 dB, Ihiisfjiving tlie finiclionalhy of a !l)-dB pad in the 
circuit for ranging. For Ibe small input ranges, it is important 
to achieve as inucb signiil gain in this stage as pii.ssilile to 
have llie besi noise performance. The 13.4-dH gain setting is 
used, providing about a 17-dB noise figure for llie stage. Tills 
dominates the sensitivity peiformance ofthe input. J^u'ge 
gain leads to a (|iiicl [ireanip stage becaii.se llie diiniinaiil 
noise iiLecli;itiisin for tlicse am|)lifiers is inverting injJUl c'ur- 
rcnt noLse, whicli gciierales an output noise for llie stage thai, 
is independenl of gain setluig. Referred back to the input, IhLs 



fixed output noise becomes a smaller etjuiv^ent input noise 
with larger gain. 

Signal Return Ground 

A special signal return ground Ls used for the uiput cable 
througii the preamp stages. This ground is connected back 
lo the main chassis ground at only one piiinr liie jioini where 
the input cable shield comes onix>ai'd. This is retjiiire*! for 
reasoiui similar to those promplmg the input connectors to 
be dc isolated at the front panel, thai is, currents generated 
by other mechaiiLsnis in the instrument flow across the 
board ground planes in this area, generalitig siuall but signif- 
icant spuiioiis voltage drops that cannot be allowed to add 
To Ihe input signal. Bringing all couneciions to tlie signal 
i-etum ground back to a smgte chassis ground connection 
point prevents other currents from flowing across the signal 
return ground, generating spurious voltages. 

.^er the preamp, the input signal is large enough not to he 
adversely effected by siiuiious ground cuiTents, and the 
boaix! ground planes are used for signsil return. The inehi- 
sion of this separate signal return ground results in apprnxi- 
malely a lO-dB reduction m the level of switching power 
supply related spurious signals, adding enough spec-ilication 
margin to guiuantcc producibility. 

DC Offset 

A 12-bit digital-to-analog converter sums dc into the dc off- 
set stage, which is a tiigh-speed ciirretil -feedback opera- 
tional amplifier Tliis is done under I'onirol of tbc instnunent 
CPL' atid aul.ozero software routines, and conipensati's for 
dc oflsei all the way to the analog-to-digiral converter. Care 
is taken not to introduce noise from the digital side and to 
maintain freiiiiency response flatness in the summing stage. 

Switchai)le Gain Amplifiers 

Tlie ivmainder of die i;uiging is accomplished in four ampli- 
llcalion stages fogetlier called the svviicliable gain aniplill- 
ere. Tliesc .stages have gains of S dii, li dB. 4 dli, and 2 dB, 
and can each be switched into the l ircnit or bypas.sed for a 
0-dB gain for tlie stage. Again, high-speed current-feedback 
operational ain|Dlifiers are employed. 

To niauitain ilistortion perfontiaticc, diese stages aie only 
used one at a time, Usuig one or bodi 20-dB attenuators, Uie 



Rller Drive Oulpul 




FiR. 3. K\\:i\ntf, Input fi:m\ niiii [iitl.piit stages, 

Dpcenihcr IHIW Hewli'll ["aflcnrit Joiuiiul 35 

© Copr. 1949-1998 Hewlett-Packard Co. 



UJ-dB \'ariaI)k'-Eain [jrcarnp, and imu nr unne of Ihe 8-(lB. 
lj-clI3. -t^lB, iuul 2^IB stages, landing in 2-dB slqis is 
iurhifvrd from -30 dBin to +28 dBiii. 

"IVigger Output 

Al Ihe output of Ihe switchable gain amiilifiers, ranging is 
fomplele. A! Ihis jioint, ait iiipiil signal al fiiU scale (in iUiy 
rankle is at a fixed size of about cLBm with as much of Ihe 
input signal's bandwidth preservc-d as possible. 

From tliis paint, liie inpul signal lakpH two paths. One path 
leads lo the trigger boaril and the other lo the ;inaJog-lo- 
digiljil con\eit.cr. The trigger tircnitrj' demands a rclali\ t'ly 
lar ge signal since higli-speeii compai"ali.irs willi Fixed lliresh- 
okls are used. Fortunately, signal fidclil.v demands are 
relaxed iiere. 

The trigger .signal first passi's Ihrough another variable-gain 
stage \piy siniilaj' to tlie pream|i. 'Die gain is set aecording 
to the dither mode used liy the analog-to-digisal eotnerter. 
For tile half-scale dilher mode iised by the staiiilalone HP 
8EI410A, the higher 12-dB gaiit setting of the trigger varialile- 
gain aniplilier stage is usetl, raising the trigger signal level l(j 
+7 dBm, For the i[iianer-.-si ide dither mode used by lite HP 
89440A RF vector signal analyzex the lower 6-<lB gain setting 
is used. 

The final trigger oiitpiii buffer .serves to lijiiher i.sokiie the 
inpiii circuitTy from the liar.sh environment ol' the trigger 
board and sums in the output of ;in S-bit digital-to-analog 
convcrier for dc offset adjn.srniein on the trigger sigjial. 
Depeiiiliiig on the range, on the order of dU MHz of ;j-dB 
bandwidtli is preserved from the uiput connector to the 
Irigger oiitpiil. 

The trigger bfiard provides ad just a! lie- level uiggeiing, o\pr- 
range detection, and half-i-ange detection. The lialf-range 
detection eircnitiy in inartit'iilar must be isolated fj oju the 
input circuits, since for signals above ap|)roximiitely (i dB 
below the I'jinge, the biilf-riinge comparators generate 
-stiiiare waves ai the input signal frequency, pmvidhig a 
.strong .source of oild harmonic energ,v. which coiild cause 
distortion il';illowe(l to couple hack imo the input. 

Filter Drivers and Anti-Alias Filter 

High-speed current-feedback operational ampllRers iti ati 
inveiting paiallel configuration drive the ;nni-aIi;Ls filter The 
paraUel ;unj)lifiers" outputs ;ue sujiuned into the filter iiipiit 
through ll)(l-ohm resistors, presenting a 50-olini source 
impedance to the tilter. 

The anti-alia-s filter is a nine-pole, eigln-zcio elliptical design. 
The filter comer is at approximately 1(1,2 MHz. ■niestr»p- 
band edge is at approximately 15.4 MHz. (Note that 15. fl MHk 
is the lowesi frequency that can alias l)ack into die 1(I-MH/ 
band given the 25.ii-MilK sample rate of Ihe analog-to-digiial 
conveision.) Tlie overall input Ireijuency respoase, includ- 
ing the effects of the filter passb;mri, Ls fiat witliin less tlian 1 
dB, which Ls easily corrected by calibration. Stnp-band at- 
tenuation is lypieally bottei' tlian !);) dB for all I'nijuencies 
beyond Uie slop-band edge. 

The filler Ls (iri\'en from ^0 ohnis and is tenninaled in 250 
ohms. To maintain signal Fidelity, special high-lmeajity 
inductors are employeiL 



A filler byi3!Lss path c;ui lie selected iF anti-alias filtering is nut 
desireil. Tills jiatb is switched al both enrls by high-isolalion 
relays to prevent signal leakage in thLs path from tleci easing 
tlie stop-hand attenuation of the filrereii path- 
Output Amplifier 

Like the filter driver, the output amplifier is a ]iarallel config- 
uration of a pair of high-speed cuiTei it -feedback operalioinil 
aniiilifieis. Tlie.se togcllicr provide a 50-ohm output imped- 
ance driving the 5U-ohm SMB cable path to the imalog-lo- 
digital convener This path is terminated in 5(H) ohms on the 
analog-to-digital converter end. The 5(l-ohni back-match on 
the ilrive end absorbs t he refieclion from the near opeii- 
cucuit termination and jirevents standing wave.s. Tliis ter- 
mination sy.stem provides minimum fl<- Iciading, improving 
lineaiity and distortion performance. 

Large-Scale-Dilhered ADC 

The AIX' is one of Ihe key enabling technologies for the 
development of a wide-infonnaliun-lrand width, wide-dynani- 
ic-raiige vector signal analyzer. As the critical block thai 
bridges the analog aiitl digital worlds, the AlK' domiiiale.s 
several impotlitnl iii.stninient s[>ccifi cat ions .such iis Ilie wid- 
est infonnation liandwidlh and the acliievable noise ;md 
rhstorliun d.viiamic range, Kecenl Irenils in eoinmereial 
AlX's niake it po.ssible lo use digital signal processing lech- 
[liqiies al RF informarion liaiidwidths. However, Ihe line;irity 
iimiiatious of conunercial converters make tliem tm.suifabie 
for the jirecision measiuements that mmiy traditional ana- 
lyzei' customers require, Tlierefore, the ADt's for analyzer 
applications have customarily been de\'eioped in-housc us- 
ing ixoprictaiy teclmiques sucii as dithering to achieve su- 
perior Imeaiity and signai-lo-noise ratio (SMi) al liiglier 
sample rales. 

To avoiti the long development times associated with the 
design of a fully custom AUt". the IIPSiJ^lUA converter ile- 
sign effort leveraged recent trends in commercially avadable 
liigh-speed ADCs through lui infonnal iiuraliel develojmienl 
elTort with ;ui extenuil MX' vendor This fast -track a|pproacb 
iLsed the highest -speei I commercially available 12-bil .ADC 
luider development as an embedded component in an IIP 
lai-ge-scale-dithered circuit design being developed in paral- 
lel. The resulting (■on\crl.er has unprecedented linearity per- 
formance for liigh-specd converters, pro\idcs sample rales 
to 2"i,() MSa/s, and achieves the necessary dynitniic range for 
widebiinil vector signal analyzer inst.tTinienlalii.in applicat ions, 

HP has considerable experience in the development of 
custom Alir arcliitectiu-es that use small-scale dithering. 
The benefits of small-scale dithering for achienng improved 
spurious perfoniiance arc well-known. Small-scale dithering 
provides a means for reducing high-order .spurious meclia- 
nisms. Llilhering in con.junciion with stibseqiienl digital 
filteiing makes it practical to extract signals far below the 
least -significant bit (.LSB) step size of the converter, 

Lai-ge-scale dithering not only reduces high-order distortion 
products but also seeks to achieve a significimt improvement 
m Ihe low-ortier distortion iierformance as well. This comes 
at the cost of some signal overliead associated with the level 
of dither sigiuil applied, Tlie HP S5i41(JA ALIV is Ihe first 
knovMi practical application by HP of large-scale dithciing tis 



36 llti't-mhiT lBi);f Hewlpn-I^tloni .li mrnal 

©Copr. 1949-1998 Hewlett-Packard Co. 



-70 



•a 

z 

s 

t -»+ 



-laD 



Undnhered Hard Rdoi 



Distortjon lm;iTovemen( 



Dllheied Hi^h-OnlEr 
DisinniDn ImpimtniEnl 




Second HamiDiiJc 
and IniemodulniDn 
DiltDilion SO dSc at 

Thjfd Ha manic 
and Inierniodulalion 
Disionion SSiBcaif, 



Ditfieied Hard 



Dislonian FIddi 

Nmb: DlslgrDon IdScI ^ OisiprTion IdBfsl - Fundanienol IdSbt 



-30 



■ — 1— 
-2D 



-10 -5 0 +5 -.flnc 

Nomiiral Clip Level 
full-Scale 



Funds mBDlai inpul Atnpliiuile IdBhl 

an external eniiancement lo an embedded ADf coniponenl. 
Tlie goal was superior low-order iinearil.y ai. high conversion 
rates. 

The fonvertpr lias two niodes of operalion: quarter-scale 
and half-scaie dither levels relative to tfie converter's rull- 
scale ievcl. The two modes iire optimised for the HP 8!I440A 
and HI'89-llOA, respee lively. Fig. 4 depii-ts ihc resulting 
low-order and high-unler disionion jierronnancf for the HP 
8!)4!0A ADC at half-seale dithered operaiioii. A! nominal ftill 
scale, the HP SiMlOA ADC [irovides ty|)ically SD-dBt secinid- 
order and S.5-dBc Ihird-order di.suirtion (^,^^la^ni(■ riuige witli 
5 dD ol' overhead. The higli-onler di.sinrlion a.s.sociaied willi 
ihe hard floiir, wliidi is fixed for mulilhorrd (■unvertei's, is 
signifirantly lowered, allowing '■soft disionion" hehavior at 
input signal levels nl her than nominal full scale (see ".ADC 
Bii.s, llistortion, and l )yn;iniic Range" on page ,iS for a dis- 
cussion of llie liard llooi and hard ;iiid sofi disluilirm). He- 
cause of IJie son dislortion charac I eristic of thediliiered 
converter, ihe performiuice me.-Lsured in dBc remains nemly 
constant over 10 dli uf input level ranfie. This llexlhilily in 
rhe tiominai operatinj; poitil allowed heller optimUalioii of 
Ilieim-rall in.stnunenl di.storlion perfonnaiice. The nnused 
overheitd in ihe HI' 8941()A is availahle to users for higher- 
SNR nieasurenH'Tils. 

In conti'ast to small-scale dillier, llie applicalion of Uir^P- 
scale dither ptises some signifieanl design challenges. The 
high-speed cireuilry assoi'laled mill dilheiing miisl exhiliit 
eslrenieiy fasl sellling limes. The large signal swings assoei- 
aled witli Ihe large-scale dilher levels nmsf lie sellled well 
within Ihe ;Sfl.tH)-ns conversion time lo avoid compromising 
fhe overall convener signaMo-noise ratio. To provide set- 
tling errors approximately 7()illli]own leijnires Ihe analog 
eircuilry to haveasetlhng lime con.slant of ft ns, Mrmwer, 
Ihe close proximlly of high-speeil switching comjiojient.s lo 
vacioiiH precision linear devices and wideh;ind anijiliriers 
necessitated a lareful KF layonl. Paniculm- anetiliun was 
paid 10 groimdlng and plai enu'ni lo minimi^ie high-speed 
relh'clions juid achieve tlie rtHjuired RF isolation. 

ADC Block Diagram 

Fig. sliows llie hlocli iliagrani of Ihe large-scale ililhered 
ADC. Thi' dilher signal eoasisis of random rioisi', which 
must be iLiitorrelaLed wilh the inpul signai. A random IJ-hit 



Fi«. 4. ,\D<^ disionion dyiiMnic 
ranjit' tiir halT-scaie diliientl 
operjilion. 



sequence Ls generated by a pseudorandom mitnbcr genera- 
tor with a [>eriod of 2*'-l samples. At a sample rale of 25,6 
MSa/.s this vields a period of about sis hours, ensuring that 
the periodicity of [he dither is well below any measurenieni 
freqnency of inierest, Acljacent dither aiinples iire designed 
lo be higlily uncorrelatcd to ensure thai liie power spectral 
density of the random noise is flat wilh freqnency. 

The sampled rantlom noLse sequence follows two signal patlis 
in the block diagram. On the analog input side of the biock 
diagram tlie setiiiencc is coiwened lo an ;malog raiidom noise 
represeniation l.iy Ihe high-speed dilher DAC, which ninsal 
Die smupic rate of Ihe convener. The analog l epresentation 
of the di tiler signal is then combined wilh Ihe analog inpui 
signal ai a wideband sumniingjunctinn im))ieniented with 
very wideband, low-distonion operational aniiiiifiers. The 
combined sigjial consisting of ihe inimi ,signal phis aiiiled 
random noi.se dither is applied (o Ihe track-atid-boid input iil 
the analog-lo-digital conversion block. 



Analog 
Input 



Summing 



► Junclion 



n-Bil 
Digital 
Sublraclcr 



Aralag 
DHher 
Path 



To Digrlal 







DigllBl 




Dilher 




Palh 






Fig. 5. Illiji'k (liiigram (if the large-scalL-iliihiTi'El anuiog-iu-digilal 
('(inverl er. 



©Copr. 1949-1998 H sw I att- Packard Co. 



Ilf'r'i'jlil"'! IN'rnii'uloll l';h.'k,-|trl.liiiirTi:il 37 



ADC Bits, Distortion, and Dynamic Range 



The nufflbet ot output tiUs ts ofien regarded as an ai:cijrate indicaiion of itiE 
rlvnarrat range ot an ana log- 10- dig II 3I convener In faci. Ihis maasurG tan be quiie 
mjsleadiog As ADCs Irerd loward more hils a\ higher speeds. sWliL- measures of 
converter nertormance sodi as mteyral and differgnlial nonlineaiEly and [he num- 
ber nl hitr^ atp giving way lo rivfiamic mEasures sui^h as sigiial-lo-nuise lalin and 
disinitntn dyramir: range Such dvnamii: measures oflen are far more useful in 
evaluating convener performance, paiiicularlv m aiiplicaiioiis where conveners 111 
con|unciion with digital signal processing are replacing traditionally analog imple 
meniatinns To help users better inlGrBret the dynamic perfnrmance requirements 
fnr iheir applications Itie specilicalinns of the HP BSfllDAand HP smOA ate 
written m terms nl dynamic measures such as signal-to-noise lalin and distortion 
dynamic range instead ul bits 

Often the fl-dB-per-bil rule is invoked in estimating the riynamii; range paienlial nt 
an ADC For many convener architectures ihis may be an overs 1 m pi liica nun An 
obvious example is a sigma-della cenvenEi, which may only use a single bit and 
by nversamplmg techniques achieve up lo IH effeclive bus ol signal-IQ-noise 
dynamiL lange Mnrenver, eonvEne'S with large numbers of bits at Lhe output may 
in fact suffer Irom inherent noise liinitalions thai litnil theit peiforinance Id fewer 
effective bits 

A related misconception Is to presume Ihst a convener's ability to extract signals 
islimitedby itsresoluiiunorlSB stBpsi?e For ADCs witfi d I diered architectures 
followed by digital lillennglha lesolulion is typically nol limited 10 the convene! 
step size The diinetirig, which randomizes the guantiiatiun otthe convenei. works 
in can|unction with llie inhereni time averaging of digital filters to allow the ex- 
Iraclion of signals fai below the step sue of the converter For smaller efttjctivE 
resnlulion lianriwidths of the digital litters, the resultant greater processing gains 
provide higher resolution limited only by the at:curacy of the digital filters. 

1 his concept is analogous to the reduction in noiss power that occurs for narrower 
resolution handwidlhs of analog IF fillers The digital fillers in the HP SgnOA and HP 
89M0A provide up to 73 hits o( reseiulinn corresponding 10 the narrowest resolution 
bandwidth of I mHi This means the ditfieied ADC and digital fillers can lesoive 
signal; to -HQ dBc (dB relative to the fundamental ampiiiudel. Dn the lowest 
input range setting of -30 dSm ttiis coi responds to -17l] dBm of sensitivity 

Similar arguments can be mads for evaluating the distortion dynamic range of a 
cunvarlei merely on tlie basis of bus at the convener output. Often tlie actual 
linearity may he far worse than the B-dB-per-bit rule would imply By contrast, lor 
ditftered architectures the lineanty typically far exceeds lhe lineanty suggested by 
the number of bilslsee "Wlial is Dilbenng?" on page W| 

Hard Distortion Floor 

finofher di.siinaion must be made between high-otder and low-order distortion 
mechamsms. whose characierijaiions can he a source ol confusion in dithered 
CDoveners Most unriilhered ADCs suffer from a hard floor limitation associated 
Willi lhe high-order disionion products generated by the staircase transfer func- 
tion This hard floor has the character 1 si ic ifiai as the tundamenial input signal 
amplitude is lowered, the amplitudes ot the disionmn products remain relatively 
fixed l^osi ADC distortion specifications ate writteo in teims of dB spurious 
because the distortion pertormance is dominated by higher-order spurious mecha- 
nisms These spuiioiis pioducts remain fixed in amplitude as the fundamental 
amplitude is lowered, thereby reducing lhe dynamic range in dBc 

This IS in contrast to analog impanenis and diliiered CDnvertert;. whose distortion 
IS usually dominaied by low-order mechanisms Dithered converters exhibit low- 
order or "sof! distortion'" behavior because the reduction of fiiglt-urder machamsms 
significantly lowers the hard floor, allowing The low-order distonion mecharasms 
to dominate As the signal amplitude is lowered, low-onJei distortion product 
amplitudes also decrease relative lo lull scale The amplitude at which ihs distor- 
tion products no longer cunlinue to drop and the dynamic raoga plateaus is called 
lhe hard distortion floor This floor is often measured m lamis of dBfs relative 10 the 
full-scale level of the convenei and may often he relerred lo as ihe linearity ol the 
converter This is depicted in Fig 1 Unditheted.theHPBgillOAhardfiDorwould 
be aroond -6B dBfs DiThering reduces liie actual hard floor in appruKimately 95 to 
100 dBfs 



low-urdei distonion pn]ducts are usually measured in dflc (dB relative to the 
fundamental signal amplitude) Because the disiurtion performance ol the HP 
BMIOA convener is dominated by loW'Ordei mechanisms the dis tuition is specified 
in dBt: at nominal lull scale 

Distonion order oiigmally derives from the oidef of ttie term in lhe polynomial 
expansion of the iransler (unction. II also dictates the amplitude and Freguency 
behavior ol a panicular distonion product A dBc specilication gives dislortion 
performance relative to the earner for a given amplitude Tiie traditional rule for 
deiBrmining lhe distonion ampliiude behavior tela live to the fundamental is 1 dB 
of second-order distonion amplitude reduction per dB ol lundamental amplitude 
reduction. 3 dB per dB tor third-ordei. and so 00 First-order distortion changes by 
1 dB per dB of reduction and therefora Ihe dBc specification remains constant 
With changing fundamental amplitude Because the dBc specification is riepen- 
dent un lhe absolute level of the fundamental, many amplifiers and mixers are 
specified in terms ol inteicepi point, which is lhe ilieoreiicai signal amplitude at 
which tfie tundamenial and Ibe panicular dtstonion term are equal 

The order ol a distortion product also dictates the Ireguency behavior of the distor- 
tion product in relation lo the fundamental. A 1 -Hz shift in fundamental frequency 
causES second-order distonion products In shift by 2 Hz, third-ordei by 3 Hz, and 
so on Examples of ihird-nrriet distonion include third -harmonic distortion or third- 
order inieimodulation 




I 

1 1- 1 1 1 1- 

FundamBntal Amjilitude 
I IdSml 
ADC Clip 

Level 

Nnminal Full 
Scale 

Fig. 1. Behsviur a! vanous aiders Qt low 'urdar disunion 



3S lici ,-iiit.ri I'irrtlli'wirii P.-L.-tarrl.lMiiriinl 

©Copr. 1949-199B Hewlett-Packard Co. 



Second -Hafmonic and Sec Dnd -Order 
InterniDditlaEicHL Distoition 



Thiid-Hsirminic and Third-Orilei 
InleimodulalionDislDrtiDn J 



-t- 



-5 a ts 

Nontinal Full 
Scale 

Fundamenlal Amplltuda IdBIs) 



frg. 2. SpKitied low ordet dintirEion ot the HP 8341DA ADC 



AHhDugh li is genwatly tnje ihai boifi the Frequency Bnt) itie aniDliiuae oresri af 
betiavioi are consisieni foi a given distorticra pioducl. itiis is id! atwa/s !Hp rase 
ivhen 3 diScanTinuiIv is invotvKl Jlii aianiple is cfissow disionion m a class B 
arrWlifiet oulpul siage, the second hatmomc may noi adiibil SBcond otde' ampliiiide 
tehavior Similar dfertsare seen m AOC5, which are fufwtenwmaHy high-onJer 
wrth niimeiQiis disraniimjities asortaiM wifli itie staircase MnslBf funciian 
Dilhenng lechniaues seek 10 refluce w eltminjie the eften ot diseonlirKiilies 

fig I illusneies iHe clilteieni^E heiween fien) and sofi Oisrariion in lerms of dBc. 
and sNjivs Ihe bedavior of several oraen (ri safi llow ontel disinrtiDn the scfi 
djsto'liwt ufiaracteriSlic foi llie HP B94IliA ADC Dtedotinanlly pjihitiils firsl -order 
amijlilude behavior anO 15 specifreO as shewn in Fig 2 for Soih sKniri-harnmnc 
and Ihinl-hannonic distorlran and secijndorder and tfiird-ordei imermodulaiBn. 

Manfred Barti 

Customer Support Engineering Mana^ 
La^e Stevens Instrumertt DMsmn 



The rlifjital jjatli of the pseudoranriora noise gcnenitcii- is 
roiii.ed to a hisli-specti i:3-l)it digital! siihiraclur Lmmpdiately 
following ihf aiiiiluR-io-digital ronverekui. Tin.' siibtractor 
consisw of 4-ljiI pipeUnud adcipis with Inokaltead cany. It 
suhtrarts coiTe-spoiiding values of dither on a jier-sani pit- 
basis from the fon^ened digital rppresfntalioii of the injiiil 
signal plus flitlipr The appropriate numijer of dplayn in the 
di(!ilal patii of the i,xin%'erit'r t'nsuro that corrpspoiidiiig 
values of dithpr ari- correctly subtracted, theiehy rrmo\ing 
the rlither and leaving only ihe digital representation of Ihe 
orif^iial input signal. 

In practice, it is a desij^i challenge to ensure lhai i.lie sub- 
traction step is perfrimied exactly so that no residual iirisiib- 
liaded pseudorrUidoni noi.'ie degrades Che overall Al)t' signal- 
Ki-noise dynamic rmige. Several mechanisms can coninbiile 
1.0 Ihe converter noisp, such iis inaccurate subtriiction of (he 
dilher, tJie seftling time of the laijidly rhmiging dither sigiuil 
in analog components, and fecdthrtmgh of the dilhei' signal. 
Ill addition, distortion errors in the rhibcr UAC manifest 
llu'mselves ;is dilher siihtracliiin errors which tletraci from 
the sifinal-lcmoise ratio. Seltliiig times associated with the 
analog (.■ompoiients .such iis the dither DAI', Ihe AfX' track- 
anil-liold r-iivuil. atiil Ihe wideband amphfiei-s nuisi be mini- 
mizeil 1.0 ensure that tJieir noise coiilribution.s are below the 
I'onverier's own noise. fJiiher feeiltlirougli is minimized by 
designing for high istilation. Fig. ti shows a Pareto chart, of 
Ihe noise floor componenis in the large-scale-dhhered ADC. 
The individual incise meclLanistns were reduced lo below tlie 
inliereiil noise of the vendoi-suijplieil MK.' component, rr'- 
.sitlluig in a sijsiial-tu-noise ratio of 127 d[5c/IIz. For half-scale 
rhthereil mode, the sigmil-to-noise ratio is 124 dBc/Hz, takijig 
the aildilional '■^ dli of dither overhead into ac'crnml. 

ADC Correlalor 

Small-scale ththered converters cmi often perform Ihe siib- 
trai'tion step with miiiinial noLse penally because Ihe dither 
signal levels are on the order of a few LSBs. Some small- 
scale dithered conveners ignore the suhtiaction step eiilireiy 
because any residual noise left by subtraction erriirs is negli- 
gible. For large-scale dilher, the precision of ihesiiblraditm 
sle|) becomes crilical, beraiise the hirgi- sign;il levels 
employed often exceed Ihe magnitude of the in|iiil signal. 



Accurate subtraction is ariiievpf! by using a tligita] correlat- 
or/accuniulalor a.spail of a Iow-rrr'i)tiency feedbark loop to 
generate a feedback error signal. The feedback enor signal 
is converted into iin iinalog enor signal by the automatic 
gain control (AGO) DAC The analog enor signal arlJu.sta the 
amphtude of ihe dilher signal al the output of the dilher 
DAC to ensure exact siiblractiim. 

The operation of the correlator can be described mathe- 
matically. Al Ihe output of the ADC. ihe digital representa- 
tion of the kill sample of the input signal phis dilher can he 
described as: 

Aouti( - Dh(I + e) 4- Si,, 

where D]( is the kth dilher value. S(( is the kt.h input signal, 
and e represents the dilher gain enor. Following Ihe digital 
subtraction of Ihe dilher. the digital output signal consists of 
the digitized version of the iii|)vil signal phis the residual 
dilher eiTors; 

iJoiiti, = I)|,e 4- S^. 



E 110- 



.e 120 



tffl-ScalB 

□ilhei 
Overliead trt-Seale 
1 Dilliar 
Overhead 



llllU 

Total SNA ADC tinear/ Dilher Dilher Dilhsr 
Componenl ThemtBl Shilling feed- OislDrllDn 
Ih rough 



Fig. 6. Fari'to chwrl. of iriiliviilkial riiUici' iiiiise rTiecJianisiiis. 



©Copr. 1949-1998 Hewlett- Packard Co. 



I'l'ivnilitT li)ti;l t!™li'ti I'liihiifil.liJiifiial 39 



D, from 
PseudorandBiii 
Numbfif GenaratQf 



Correlalor/ 
Digital Sublracloi 



AceiiiniilBtar 



AGCDAC 
ZeFQ-Dider HdIiI 



Filler 



OAC-ADC 
Gam 





1 









m 

The correlator multiplies each output saniplc by tlie cone- 
sponding dither sample, yielding: 

Cnrri, = D^e + DkSi,. 

Tlipsp values are fonsecutively added In the roni.ents <if the 
<ligilal ac euiiiiilator. yielding aii arcuiuiilatiiig average whose 
expt'C'leti value is given by; 

Ace = E[Dte + D^S;,] 

= EfDSe] + E[DkS,<] 

Beeause the dhher signal is uncorreialed wiTh the input 
signal, the expected value of their product is zero and the 
long-temi average value of the accumulator is: 

Ace = E[D^e] 

Thus tlie expected value of the curreliition simijjloR is yiropor- 
tioiial to tlie diliier gain error, yimimation of these siunplos 
in the accmnulator acts as a di^tfU integrator. The feedback 
dither error signal consiHls of the 8 niost-signirieant bits 
(MSB) in Ihe accuniulatur. It is lei! to an 8-tiil AGC DAC 
whose analog iiiodtilaies the aiiipliliide uf the dither 

signal a1 Ihe otiipui of the high-speed dither DAC to drive 
the dither subtraction errors to zero. 

Dither Gain Control Loop Analysis 

The dither gain eontrol loop is a mixed analog and digital 
control system. A simplified diagram is shown in Fig. Ta. 
Lumping the correlator and accumiJator gains into a single 
feedback gain |1 and ignoring Ihe s-domain effects of the 
zero-order hold iLs.sociaied wiili the AGC DAC and die loop 
shaping Tilter resiills in l.he simplified z-domain loop shown 
in Fig. Th, The transfer function of (he dhher error derived 
froin the simplified output error signal in the z domain is 
given by: 

E,Az) ^ PfAp - n 
z - (1 - A„ - 

For the correlalor loop the combined system g;iJii of the 
dither DAC and ADC, A,,, is f^proximately 1 and the feedback 



Fig. 7. (a) Sim[ilirifil diagram iif 
tiie niixed aiinliifi/diBital I'oiTelalor 
loop, [h) z-dniTiairi simiililical.i'ill. 

gain |i is d. For A;, = 1, the ditlier error goes to zero. For A^ 
approaching 1, the diUier errors are proportional \ii |i. 

The bit si/.e of the accumulator dominates the feedback gain 
eon.stant \\, which determines the lune constant of the loop. 
The elTect of the tiither gain loop on the input signal is to 
produce a small amniml of amplitude modulation. Tlie mag- 
nitude and fieqtiency ol the AM sideliands ari' made negligi- 
bly small by clioosing the ai'cunuilalor size to be sufficiently 
large CM bits). This corri'sponds to an appri.ixirnaie |1 Viiliie 
or2'^''"'^'fora r2-bit sample size, or modulation sidebands 
thai are 144 dB down. In arldilion, one ran estimate the loop 
liandwidlh from ihe hxip fi|iialinii: 

BW(;J-dB) = 

For |i = -144 dB = IQ-l-'l'^'" the loop bandwidth is U.G Hz 
yielding a time constmit of 2 seconds. These estimates are 
within an order of magnitude. 

The long time constafit associated with the dither en'or re- 
duction loop would manifest hseH'in the system at power-up 
as a noise floor in the freijueney domain lhai slowly lowei s 
ajid settles out at the system noise floi.ir as llie dither gain 
loop drives the dither subtraction errois to zero. Although 
the decay is exponential, on a log attiplitiule scale in the 
frequency domain Ihe sell ling time appeases as a lineiu- phe- 
nomenon in lime. To overcome the slow settling at jjower-np 
an arcunuilalor initialization step is implememed. I Miring 
periodic calibrations rhe lop 8 MSBs of the act unmlaior are 
stored in the iusimment nonvolatile RAM. ( in a .subsequent 
power-up, tJip stored vjiliie is preloaded mto the MSB por- 
tion of the accumulator, thereby providing inunediate ADC 
operation widi optimal sigiial-to-noise ratio. 

Hard Floor Mechanism 

Alliiough dithered converters substantially lower the hard 
floor liislortion plateau, they eventually encoimter a mecha- 
nism that limits the depth of the hai-d distortion floor. One 
surli mechanism is miivanted coupluig of the dithered .A.DC 
digital out]iul (o the analog input. This is explained hemisli- 
cally by considering the MSB or sign bit of the digital 'Uifput. 
This fligital signal b synchronoas with the Fundamental input 



40 Dpwmlwr 1903 Hpivlplt-Pflrkartl .lournal 

©Copr. 1949-1998 Hewlett-Packard Co. 



sign^ frecjupnci,' because it toggles in phase willi the sign of 
llie input si^al. The MSB can be considered tu lie licit in 
(tLsiLirtion haniionics related to tlic input fundatnenial, TTiese 
harmonics renuun l unslant relative to the input signaJ level, 
and ilej>eii(tiiig on tlie convener ouqjut-lo-iiipu! Isolation, can 
couple back into the input Thererore, the reduciion in dis- 
ttirtion seen in tliUiered ADC.s as the input signal is lowemi 
is limited by litis distortion feedback ineehanisn. 

Tills iiiechaiiisin was characleriied with the converter oper- 
ating iii the HP 89410A witli llie help of the autdcorrdalion 
measurement feature ofilie iiisirument. Because this mech- 
anism is correlated with die input but occurs at different 
time delays relative to the input, aiitocoiTelation pro\"ed 
helpful in tinco\ ering specific areas in the hardware w here 
the mechanism was dominant. Correlation peaks of the in- 
sirunient noise floor were obser\-ed at specific delay rimes 
corresponding to mulriples of the satiiple clock delay follow- 
ing the AlK' conversion. The most notable peaks were asso- 
dalt'd Willi delays corresponding to ihe output bns between 
tlie AIX" oiiipvil and the digital filters. To reduce the effect of 
this mechanism on llie distortion hard fioor in i he ADC ihe 
digital outputs were clianged from TTL to ECL, which has 
lower voltage tliresholds. 

Digital Implementation and Diagnostic Modes 

Tile iiiiyurily of the digital timclioiialily of the liuge-acale- 
dithered ADO is implemented in two electronically program- 
mable gate iUTiiyH. Tlie gate array algorithms can be dynami- 
cally reloaded ikiring instrument operation. This Facility is 
used to provide Ihe lialf-wale and quan.ei-scale dillier iikhIcs 
of operation for tlie Hi' a9411)A and IIP aa44(IA, respectively 
Flexibility in choo.slng tlifferent modes of ojierarion allowed 
Ihe tlilhered ADC U< l>e tailored to the individual overhead 
and dyniuiiic range reiiiiireiiienis of each insti iuneni. Thi.s 
capability is ;Jst) ii.sed lo provide factory service tet liiiiciiiiis 
a variety of ADC iiiode.s tor diagno.slic servicing of not only 
llie (Ulhered ADC but the iti.sirumeni signal processing liani- 
ware as well. Tliese ineliide viuious lest signal geiieratois as 
well as dilhered and iindilhere'l modes of operation. 

Digital LneaJ OKcillator and Decimation Filters 

Mift signals are sampled hy the aiialog-tiMligital converter 
section, ilie digital signals ;ue jiassed on lo the digital loc;i] 
oscillator and decimation filler (I.O/lli-') block. The eon- 
vetted data slream i.s first routed ihnmgh an inlennediale 
assembly between the two AUC channels and Ihe Lf>/DF 
block. Tills bulTer/swilch assemldy finjvides a data multi- 
plexer so that .sij;nai sources other than the analog front 
ends — digital iiiput.s, for example — i':m drive the LU/DF sec- 
tion. It also allows rinther digital signal coiuiitioning such as 
time gating to be performed on the data before il reaches 
the niters. For diagno.siic puiposes the digilaJ .source output 
can be routed through Ihe liuffer/s witch assembly to drive 
Ihe 1,IJ/DF section. This mode allows liolh Ihe digital source 
and the LO/DF assembly to be thoroughly exercis<-d during 
diagnostic tesls. 

The biifl'er/.switcb assenilily also performs ECl.-to-TTIj signal 
level traiLslalion on die iiieoniiiig .^DC data. One of Ihe de.sigii 
challenges encountered while Imildiiif! Ihe HPSiUKIA in- 
volved the backplane .signals uiiil the melliod hy which the 
AD(.' drives its dala tiirongh the backplane and onto llie digl- 
lal LO/[)F assembly h was found that Tl'L drivers on ihe 



AIX: assembly combined with the c^acitance of the back- 
plane made for a noLsj', high-eurreni switching combination 
thai fed tioise back itito the .\DC signal -conversion circuitry. 
To eliminaie this source of spurs, tlie ADC dri\-ers were 
changed lo quieter ECL buffeis- The smaller noise margins of 
ECLJc^c required carefiil shielding between the ADC data 
bus and the surrounding TTL signals of the rest of tjie back- 
plane to prevent crosstalk. Tiie ECL buffer oiilpnl and ECL 
receiver input temiinarions are matched to Ihe impedance- 
controlled signal lines in the backplane to ensure optimum 
transmission line chardcteristics. 

.\fter the buffer/switch assembly the converted ADC signals 
from both input chaimels get passed on to the LO/DF assem- 
bly This block provides two channels of frequency .selective 
liand analysis. Each cliaimel proiides one complex fre- 
quency sitifler and two sets, real and imaginary, of digital 
decimation filters. 

Tlie digital LO Ls a projirierary Hewlett-Packard high-speed 
IC that consists of a precision titiatlraiure local oscillator, 
two mixers, and two low-pass rdiers. It lakes in real input 
data and produces low-pass-filtered complex output data. 
This opei arion is known as zoom mode. The local oscillator 
consists of a precision 40-bil phase accumulator followed by 
a sine/cosine generation circuit, The phase accumulator is 
designed to give decmial frei^uencies for typical sample 
rales with a uiinimimi resolution of 25 \lH^. allhough Ihe IIP 
S9410A reijuires a settable LO center fre(]ueney resolution of 
only I niHz. The sine/cosine generation hardware stores the 
first octant of sine imd cosine in two 2-">£i-element R(;)Ms. 
Tlie ROM data is used with lineiir inleipolation to generate 
two suiusoids offset by M degrees with worst-case spurs of 
-110 dBc and typical .spurs of-l^S dBc. 

After mixing the local oscillator's complex sinusoid with the 
injiut dala, the output is y = xe"J'^', where x is the inpul daia 
and (II = 2;tri,( |, This complex mixing allows the real input 
dala to he fiequency translated aixiund dc for fuilher base- 
band filtering and decimation. The input sigi^ at frequency 
fi„ is U-aiislaied tu fo„i = fj„ - fuo- 

The first set of low-pass filters is impiemented in the digital 
LO, These fihei-s can be bypasseii for fiill-span (nojuleci- 
nialed) data or used when smaller spans are required. When 
Ihe fitters are ased, the dala is biiiullimiled lo y4 lo prevent 
aliasing and the output is oversampled by two. 

To pro\ide user-selectable bandwidths, (he down-ton vertett 
complex baseband signal producer! by Ihe digifai LO goes to 
Iwf) identiciil iiropnetaiy ilevvlett-l'ackaiil digiial decimation 
filter ICs, one for Ihe real data and one for the imaginaiy 
data The recursive decimation filters can be thought of as a 
cascaded chaui of alternating digital low-pass llllci's atid 
decimaie-by-lwo blocks (sec Fig. 8). With ;in ADC sjiinple 
rate of 25,15 MHz, the available decimated output dala rates 
are 25,(i MHz, i2.S Mllz, 6.4 MHz, 3.052 Hz, These sample 
rates I'orrespond to alias-free analysis spans of 1(1 MHz, 5 
MHz, 2,^ .MHz, ,. ., 1.1(32 Hz. By using a proprietary re.sampling 
algorillim implemented in the main DSP, liiis Imiited set of 
analysis spatis can be expanded to provide any user-selected 
arbitrary .spati between 10 MHz and LO Hz, 

The filter cliips supjjort. tlu^e different modes of operation: 
l.iyijass, decimate, and interpolate, Tlie digital source, de- 
scribed in a following section, uses the (lliei clii[:s in ihe 



© Copr. 1949-1998 Hewlett-Packard Co. 



DtKHHiber ltM.1 Hewlm-I'ai^limil Juuraal 4 1 



Input fcom 
ADC 



24 




IDW-PSEI 

Filloi t.rt 



Low-Pnsi 







OncinwlB 


FiiiGii.m 





Low-Paj* 


Fillei 


Declmnli 





Data Oulpul Sulec'ion and Mitlliplaiine 




2X 



Low-Pasi 
Ftllet 



3! 



31 



Real 



I magi nary 



Fig. 8. Zfjrirti .-liiii deoiiiitiLioii fllLeriiig. 



iiitf'riiolale muilf or "tiiooz" mode. The ciigitaJ LO/DF assem- 
bly iLss's tlif filler i hipji ill liolh Ilit' bypass aiid dcciniale 
iiukIi-s. In l>vpa.ss nuidf, Uis' data Ls aimply i^isswl lluougli 
tht' fhiji unrhaiiged. Tliis mode is used for full-span lO-MIIz 
nu^asiirpiiients and for ihe nt-xl .sindler liasir s()aii. ") MHz, 
heraiisc tlic Tir-sl nili'i' sin' is in Uw '"Wl' for Ilie 'vMHz 
span, ihi> oversaiiiplfd dala iiassing thiuusii llu' derinialion 
nilcr clui>s is df i.'irn;!led liy twu externally in luirdwajv for 
ihi' rcsiillaiil out] ml samjile rale of 12.8 MH/. Forsjiajis 
stnaller ilum 5 MH/., llie digiial lllier i hijis rue l onTigLLri'd for 
the deciniale iiuide. The dala is processeil liLiimgli 2.1 passes 
of dei'irn ate/filler sleps. each |iass [■onesponding lo une 
deciniale/filTer stage in Fig, H. Eai-h pass result is fiiit[)ul from 
the digital fillers, biLt only (he pa.ss ofinlerest is deposited 
iiito the sample RAM. If desired, all of llie basie spans of 
interest (excejjt full spaii] c;m be Hftbered by the instnimeni 
al once, lo be iLsed in algoritlims such as 1/N-oetave analysis. 
The sequence of pass outputs is: 

... 1 2 1 ;3 1 2 I 4 1 2 1 ;J 1 2 1 5 I 2 1 :5 1 2 1 4 
! 2 1 H 1 2 1 H 1 2 1 :( 1 2 1 4 1 2 1 :3 1 2 I 

where pass 1 rel'ei*s lo the outpul of Ihe first rascaded 
dee i mate/filter seetion and pass 2 refers to llie oiitpul of liie 
sei'ond cascaded de c in Lale/ filter section. The sei|uenee Ls 
structured so that for evei7 ]jass k. there are two input pomLs 
coming from d\e previoas pass ( k - ! 1. Fur example, belw^een 
each pair of pass 2 outputs Ihcre are two pass 1 oiitptjts, ajid 
Ix'l ween eaeii pair of pa.ss ^ oulpiit.s there aiv Iwo pass 2 oul- 
piits. Tliis .seijueiiee follows naturally fioiii Ibe arcbilecl lire of 
Ihe cascaded deciinale/niter seclions. A deciniiile-l)y-L] filler 
ivyiiireb IWO inpul poinis for eveiy oulpitt poinl. Therefore, 
two points from the (k - l)th pass must be oulinil Ijefore tlie 
klh pass i-an calculate one output poiiii. 

Tlic low-pass filter is a compromise between optimum filter 
shape and calculation siieed constraints. Althougli it has 
fairly gi.iod passband rippie chaiacierisi ii's. the filter's pass- 
band I'e.spon.se dro(j]is to nearly -I). .5 dB at the iijjjier end of 
the analysis span. Similarly, while die filler sloji biuul pro- 
\ides good rejection, the transition band of the filter, reflected 
aliout the Nyquist fietiuency, falls into the upper fretiueneies 
of the mialysis span, liinhing the worst-case alias protection 
to -8(i.7 dB. Tlic worst-cjise passband ripple is about 0.2 clB 
and the worst-case stop-band rejection is betler than 111 dB 
(all passes). C'orreclion routines in the m;iin DSP account 
for the passliajid roll-off anonialj' and ensure ven, fial pass- 
l.ianti I'haracteristii's foi' die entire span. The 111! filter iniple- 
mentalion enhances calciilarton pei1bnii;ince wliile sacrific- 
ing linie-doinaui perfonnanee as a result of the filter's sharp 



cutoff and nonlinear phase characteristifs. Algoritlims im- 
plemenled in llie main flSP pro\ide the necessary overshoot 
and phiise coiTcetions for time-tioniain measurements. 

The IF trigger eirc'uil on the digital LO/DF assembly uses ihe 
complex digital data coming out of the decimalion Tillers to 
allow freiiuency selective triggering. As opjiosed to Iradi- 
lional lir<iadband Iriggerinii iiiecbaiiisms like ihe ;uialog in- 
pul trigf^er, Ihe IF lrij;ger looks for energy preseni only in 
Ihe frefjuency band selected. The IF [rigger circuit looks al 
dala coming out of the real and imaginary digital filtei-s from 
eilher channel ) or channel 2. A RAM is used to pimnde a 
conijitex-niagnilude map that is compared to Ihe incoming 
conijiley, band-selecled data. The map c;m be \isualized as a 
graph whh the real part of the mpiu dala along Ihc x iixis 
and llie imagijiaiy pari along the y axis. Tlie magnitude of 
Ihe complex input data is the squiu c root of tlie sum of the 
si|iiaies of the real and imaginary paris, wliich describes a 
circle on Ihe gra|)h. For a gii en trigger value, llie con e- 
sjionding magnitude circle is calculated ;md placed in Ihe 
nia|i. The RAM is progranuned to l)s for Ihiise complex pairs 
fallijig inside Ihe trigger <'ircle and lo Is for those values 
falhng outside. When the incoming data's x-y location in iJie 
map moves froiii within the circle, where the RA.M reails 0, 
to without, where Ihe RAM reads 1. the magnitude of the 
data has crossed over the trigger point, ttlien Ihe IF trigger 
circuit detects the change of map values from 0 lo I , it gen- 
erates a trigger to the instrament. With two bits ill the map. 
two .se|iarate circles can be lieseribed, allowing addiiional 
trigger ftuiction;ility such as hysteresis. The hysteresis algo- 
rithm requues that the inpul signal first fall below the pro- 
gi-ammeii hysteresis level (smaller<'iiTie) before resetting 
the circuit to look for a new trigger Uarger circle). 

Sample iiAM 

The sample RAM assembly is an integral component of tlie 
measurement and Iriggerini; functions of the HP Sri41UA hi 
addirion lo its niiiin fiiiiciion of capturing one or iwo chan- 
nels of data from tlie digital LO/DF assembly, it provides the 
haidware necessary for prel.rigger and posl-lrigger delays 
and block size accounting. 

Two sample RAM options ai-e a\'ailable. The standard config- 
uration ])ro\ides storage for 32, 7(i8 lU-bit siiniples (:32-bh 
real. ;H2-bn imaginary) for each input chaimel in two-channel 
mode, and 65,-53ti 64-bit samjiles in single-c-hannel mode. 
The ojitional configuration i)ro\ides storage for 512K (i4-bit 
s:miples for each inpul chminel in !wo-chiUinel mode, and 
IM C4-bit samples in single-channel mode. In single-channel 



42 llei'enilHT 1110:! ttcwlen-Pai'liaiii .Iminial 

© Copr. 1949-199B Hewlett-Packard Co. 



mode. Uip larger configuration provities about 41 tns worth 
of data at ihp full 2-^.(5-MHz sample rale and abniil H'l hours 
worth of data at the lowe.sl allowable sample rale of :!,(^52 Hz. 

Tile main challenge in desigtiing the sample RAMs was 
deli\-ering high-s[>ee<l data capture ( > 4(«J Ml i,vies'sl at a 
reasonable com. The sniaiier sample RA.M lioard is designed 
with high-speed SRAMs. The larger sample RAM reijiiired a 
different approach IJecaiise of ihe size of the memory and the 
relaii^-ely high cosi nf high-speed staiic RAils. The solution 
to 111 is problem was found hy using jnlerlea\-e(i banks of 
video RAMs, Tlie two-port \ide»i RAM has a higli-^eeil seiial- 
acress regisiei' inierfaced with a slower random-access 
meinoi>'. The daia from the digital L(1/I)F assembly Ls loadetl 
serially into the sideo Ri\M serial register ( up lo 512 samples) 
anti is transferred with one operatinn into the rdndom-aecess 
memory. The samples ciui then be accessed \ia the slower 
^^em bus by the DSP or system mifroj>r(K.-essor. 

8K samples per channel is siifficieni data to support Ihe 
display, which shows a maximimi nf ^200 lines of frequency 
datEL The larger saniple RA-Ms are used foi- the time capture 
measurement mode, in which the insminient captures a 
farge amoiml of i-onlijiuoiis data for laier processing. 

Digital Signal Processor 

The DSP assembly is designed around the Motorola 
DSPi)6l)02 IKEE floating-point (ligil,al signal processiir. Ilie 
DSPi*6(H):^ provides two 32-hli iiiemorj- ports and peak jier- 
formmire of 4S MFLOPs. Tlie two memory ports are ideal 
for FFT caleulalions, Ihe main fimelion of the DSR The DSP 
can become the bus iniLsler for Ihe sysleni, allowing it (o 
access data from ilie sample RAM diivelly, perfonn Ihe nec- 
essary dala processin;; operations, and llieii place the re- 
siills directly inlo the display assembly, By removing the 
CPL' from the main data flow path, the insiniinent.'s ihrough- 
pui is signifii ainly tncreiised. (_ln a typical fast average 
measiiremeni, ihe DSP can transfer and |)roeess .'IDO 
5l2-poin( complex wpecli'a (including confciions, FFTs, 
averaging and display fonnatting) per second. 

Display 

The display ;Ls.seiiib!y is basetl oii the Texiis Instruments 
TMS:i40:?(l graphics system processor (GSP| and a IM-byte 
bank of video IIAMs, Tlie GSP is a 32-bil. high-speed geiieral- 
puqiose processor I hai is opltnii/.ed for graphic display sys- 
tems. The display system is designed to allow dala points lo 
be placed liiteclly inlo display metnoiy by Ihe DSP Tlie GSP's 
program iheii processes these data points iuid constmcts a 
trace and the siirroiinduig aimotjition on the display inde- 
pendently, without iiiit.side pioi'essor iiitet\ention. This ai'- 
rangenieiil frees the DSP and the host CPl' from the need to 
control the dLsplay operations directly. The lestilt is up to 61) 
trace updates per second. Watcrl'ail anil spectirogram tlis- 
plays are limited only by the inslrunieiit s processing speed. 

One of the challenges in designing the riisjilay assembly 
came from the juialog video circuitry. Tlie \ideo RAMDAC 
:uid \ideo hiilTers shme a [iritiled circuit board with the DSP 
and display blocks. The large data buses coming onto the 
i>n;ird anil the iissocialed circiiiliy caiLse a lot of ground and 
power noise, which lends to show up on the video output, mid 
I PT. To reduce the effects of the digital noise, the atiah>g 



video subsection is placed on a ^lil ground and power plane 
"island" and ihe video signals are rouied lo the baclqilane 
between saridw iched ground planes lo provide shielding and 
a controlled 75-ohni irace impedanc-e. 

CPU 

The HP 894 IDA eetural processing imil assetnbly is t)asert on 
the Motorola Mf'58K( ;J2-bii microprocessoi lutd Ilie 
MriiSSSil floaiing-poinl maUi coprocessor Tlie f PI' handles 
many of ihe user interaction fiinclioas and the liigli-ievel 
control of the measurement system, TTie TPl " assembly also 
provides sysiem bus arbitralion anil IM bvles of maui ineiil- 
orv; and controls the fle.vible disk, Uie HP-IB tIEEE 4.SS. EC 
G25). the serial pori. and the keyboard. The HP 89410As 
system code is contained in 3M byies of in-circuit program- 
mable fltish EEPR(.)M to allow easy finiiware updates via 
the flexible disk tiiive. 

Backplane 

Anotlier challenging asp ec't of the HPSfl41l).\ design w;is 
dealing whh the large number of signals to be roiik'd from 
assembly to assembly wiihin (he iligital card nest, The card 
nest backplane niolberboard comtects eight digital logic 
assemblies and connects ihe display assembly to the inter- 
nal CRT die front ends fo the LO/DF assembly, and tlie 
source tf> the digital source assembly. Tlie backfjimie routes 
approximately 11)0 signals ihrmigli five l-'il>pin connectors, 
three 2t.)0-pm coimectora. and two ^JU(l-pin connector.s. The 
backplane had to be carefiilly designed to carry all of these 
signals with minimal crosstalk and ground bounce imd im- 
pedance<on trolled ECL video, and clock lines. The back- 
plane Ls constructed as a ten-layer board. The lop layer is a 
groimd plane. The next layer provides die Kt.'L, video, and 
.system clock Izaces and is sandwicJied between Ihe top 
layer and another ground plane. The two groiitid pimies, 
along wltii Ihe (l.(ll),^i-in<'h-wide trtices, produce a r)ll-ohni 
characteristic imjjedance transmission hue for the, signals 
on this layer The;ui;ilog video signals and two ADC ECL 
data buses are fiiither shieliled from t he TTL-level .system 
clock lines that reside on this layer The foiirih la.yer jiro- 
vides Ihe power plane while I hi' Inst six layers ;u'e u.sed 
for the rest of the digital signals on the backiihuie. These 
layers are pnrjiosely .separated from the groiuiil and power 
l>lanes to lower capaci till ice ami thus increase the iharacter- 
istii- impedmice of the signal lines. The higher impedance 
prevents ground bounce on the thgitiil assemblies when a 
large number of outputs on the bai. k-plane ai-e aciivated al 
the same time. The large buses such as the sysi.ent address 
ami data bus are placed on Ihe coiuponeni side of Ihe baek- 
phme (bott.oni ) to keep them as fiir away as possible from 
the ground planes, hi addition to the extrialeil froni-eiid 
juialog cai-il nest shielding, unol her .shield is placed between 
the front end and Ihe electrii'ally noisy hiackphine. 

Digital and Analog Source 

The HI' 8!t !lI)A .source is a DAC-driven, SD-ohm output 
impedance analog source willi capabilities similar to many 
standalone .scmrces. II |iroviiles a vjuiely of initpiU types 
inchiding sjngie-fre(|ueiicy ^inc. (iaussi;in distiibuteil ran- 
(Uim noise, periodic chii-)j, and user-defincrl arbilrmy wave- 
foruLs, The source is used in calibrating tlie IIP S!)4lt)Aund 
in diagnosing the rest of the instniment. 



©Copr, 1949-1998 H sw I att- Packard Co. 



Pei™herlim.Ttli'nW(-tVl(iinl.li)imwl 43 



What Is Dithering? 



Dilhe'itig is a meiiiod ioi rariduini2;riy llie quantiialinn smis of an ADC hy adding 
slimulus ihat is imcorrelaied win tlie desired siynal ai the analog iiipul of ihe 
cnrverier Fig 1 depicts a basic block diagram nf an exlBmally dutiared convener 
in wtiicli tlie dirher signal is sjbtracled fallowing Ihe ADC conversion There are 
i/arious types of ditfiering, which can be rliffereotiated in various ways, one hemg 
by the characteristics of the dulter signal Dither signals can be chatacterijed on 
the basis of signal lype (sucli as nnisel, amplitude {small-scale or large-scalel, or 
frBQuency [rtarrowband or broadband], as shown in Fig 2 l^larmwband dither 
signals putsideihe infuimaiiori bandwidth can be lEmoved following the conver- 
smn by digital filtering Diihei signals are also atien characlerijed on the basis of 
the probability disiribuimn of the dither ampliiuds (such as uniform or Gaussian 
for random noise) 

Dithering a converter can pravide significatit improvGmenis in the ability to extract 
signals heluw the resolution of the converter and in linBatiitng s converter's par- 
formanoe. The improvements vary with the degree and type of diihcr CnmiiinBd 
with the time-averaging properly of subsequent digital tiltetmg, diiheriiig eHet- 
lively removes or smDolhs the guanlizaliim noise inhBreni in the staircase transfer 
function of an ADC. 

This can be illustrated as follows Consider an Ideal ADC with a staircase Transfer 
hjnclion whose quantized ideal error transfer function is shown in Fig 3a The 
expectetl error transfer function resulting from dilhering with a tandom noiss 
Signal can be camputed by evaluating the enpHCted iiansfei function with the 
weighted probability distribution of the diiliei Fig 3b depirits a uniformly distrib- 
uted probability density function of random dithei with exactly I LSB of dither 
amplitude 

Tfie Qenetal eguaiiun lor computing the Hipecied value G for a transfer function 
with one random variable is givan by 



G = 



where el^l represents the transfer function and pl;| represenls Itje probability 
density function of the random variable z The expected value computes the aver- 
age of the transfer function weighted by the relative likelihood of z. For the case of 
dithering, in which Ihe random variable z is added lo the input signal at amplitude 
X, the resulting expeclerl value of the transfer function appears as follows; 



p(?)H()( + (|dt 



The funclion e(x-tz) corresponds to the transfer function in the variable f added lo 
the random variable ; representing Uie dither 

This resamblss a convolutDn. and hence we can illustrate the priKEss graphically 
as the iniEgratiDn of Ihe dither probability density function as it is moved past the 
ADC enor transfer function as sliuwn in Fig 3c For a dither amplitude of 1 LSB, 
the ihiegiaiion result is lato, yielding an ADC transfer function that is completely 



Analog 
Input 




Diltief Generator 





Pseudo- 












Generalof 



Digital 
Oulpul 




1 LSB 



lirlDrmaliun Bandwidth 

Frequency 

Fig. Z Tvp^ af ditfw in Uia frequency domaiit 




P tab ability Density 
Funclign 









H 


1— 


■ 1 — - 



Aa!2 Amplilude 



lb) 




Rg, 1. BIdcV diagram at anexiemallvdithaieaanalDg-io-digilfll crnivenai 



Rg.a.lalErtorttansfBrlunciiimotanlilealADt; ibtPrabaOility density funclion of unifcrrmly 
distributed randDindiiher with 1-iSB amplitude [cIConvolutionDf IslwiHilb) yialdsieic 



44 



Iii'irndiiT l!Hi;l H™li-ll-l'Hc li;irit.liiiii-iiJl 



) Copr. 1949-199B Hewlett-Packard Co. 



s + 



Undnhsrcd 




0 FfBqHHney (Hll lO' 

Rf.<, TlmB-narniDnicdislDniQimflhe same ADC wi;h large- stal e ditner anfl nodllher 

linear The hrgh-otde^ disiDrlwi lerms associaied wilh the pDlynomial Bipansmn 
of ihe quantized irarsfer function are effeciiuely renmiied by Uie dilherirg Ttiis 
resull IS true for dilfier amplitintes thai are imegral multiplBs of ilie ADC LSB. 

Low-ordet disiartron perf(3rTnance impravemenis can also be abiamed by ttre 
aoplication of large-scale dither In practice Ifie large-scale dittiei effectively 



smrotfis the inflections caused tiy Itie miegral nonltneantv of 1tie ADC ttansfei 
funcLon AlitiDugh diifiering a transfer chaiacienstic with a smgle secona-orfler 
or Dlird-order dislfllion lerm does not improve Ifiat term, it does improve the 
ImvcdBr distortion of transiet funoiuns whose polynomia' expansions also 
coniam hi^er-orfler terms, wtitch is the case ioi real ADC Iransfet funclions The 
coiwlulicffi geneiate liwer-ontei lems Utun t*» higner-ofBer terms, and Ifiese 
(wliinE wTifi the ongioal low-order lefflis or the iraisfei ignctiDn to make itwn 
smalle* This results in an overall imorovamerTt m lovK-oitlei rtisioriion riie degree 
of improvement is directly related to the rnagniiude of ifie dither signal employed 
Larger-scale dittie/ yields greater reduciion of low-nnler distortion Idi a given 
sigiial amplitude 

4 demonsirgiES this pfieromenon with a measurement iliHt comparK tfie third- 
(larmonc: distortion perfonnance of the HP B3450A mriverter for half-scale ditfier 
add unditherefl modes of operation Ttie third-tiaimonic disimtion rs significantly 
mproved by the laigB-scalEditheting. 

Manfred Sam 

Customer Support En g me arm g Manager 
Lake Stevens Instrument Division 



The source is composed of two assemblies: a digital source 
assembly and aji analuS source assembly. The (iigita! somre 
asseinbly contains a ■^2K-silmpk' complex ( rt- aJ atid imagi- 
nary] waveform memory buffer, a pseud oraiidoin noise gen- 
erator, and the same digital LO/DF cliipsel that is used in the 
froiii-eiid receiver seclioii. The cligitiil source assembly 
c reates a digital signal for the analof^ source assemlily. The 
analog assembly contains a wavefonii DAC, a reconslnic- 
tion filter, dc offset circuitry, an outptil amplifier. 10-dB step 
attenuators, and ft'ont-end calibrator circuitiy. 

All source output signals are created digitally on l.lie digital 
.doUn:e assetnbly. Tliey are geiieraied via tlie .source satitple 
RAM. the pwndorandom noise generator, and the digital 
local iiscillalor IC. as shown in the block diagram. Fig, 1. 
The source signal How is e.ssenlially ihe reverse of I he front- 
end receivt-r section. The signal palh stiirts with the source 
wavefonn memory, where the complex digitid signals are 
stored to be fed into the real and imaginary digital inlei-jinia- 
tion fliten*. The address generator lo the waveform tlMl is 
either a linear cimnter or a pseiHlorandom noLse source. The 
address comiter is useil wilh repetitive or single-block wave- 
forms such as periodic chirii and user-ileriiied source t,y])es. 
The pseudorandom ttoise addressing mechanism is used lo 
generate noi.se oiitpuls. The distribution of the random noise 
is tleterniltied by the waveform loaded into Ihe source RAM. 
For example, if a Gaussian c'urve is loaded inlf.i the wave- 
fonn memory, the random sampling of Ihe curve by the 
pM'uikirandom noi.se address generator causes a Gaus.sian- 
distribuled noise signai lo be fed into the iligital interpola- 
tion filters. The pseuilordiidom noise address generator is a 
mfudmal-ienglh sequence lhal repeats itself apjiroximately 
every G hours at Ihe hill output sample rale ofari.ll Mllz. 

The digital inteiiiolation fillers are used lo increase the input 
(wavefonn) sample rate by a factor of a*^, where N is pro- 
gr;miMtable from 0 lo 23. Sitice the output DAC siimple rate is 
always i!5.(i MHz, the digitized wavefonn store in the sotirce 
RAM caji have an effective simiple rale as low as .'1,05 Hx. 
Tliisalkiws the Hi'8iMll)A to have exiremely low -frequency 
waveforms and veiy largo effective blix'k sizes even Ihotigh 



the waveform RAM is only 33K samples long. Also, since the 
DAC runs at one sample rate, ottty one reconstruction filter 
is neetled. 

The complex interpolated signals are routed into a digiial UJ 
chip where they are mixed from dc to any center fretiuenty 
up to 10 MIIz, By viiiiie of this complex mixing ojjpratitm. 
tlie resultant output signal lias a real two-sided spectnmi 
ar<nmd the positive and negative bO frequencies. In this op- 
eration, or "mooz" mode, the output is y = lie|xcJ"''). where y 
is the input, Re represents the real pail of Ihe number in the 
brax-es, and id = 2jifu). This frequency inmslation causes the 
source's complex wavefonn signal at frequency f;,, to appear 
at the outi)ul. frequency f„„( = f,,, + fLo, 

Tlie real tligital soiiri-e signal y is ihen converted into an 
analog waveform by a 12-bil DAC nmning at 2'i.G MHz. The 
reconslntcticm filter follows the nulput of the DAC, limiting 
the bandwidth of the sigiuil to 10 MHz w hile correcting for 
tlie sin((jjt/2)/(itjt/2) rolloff result iiig from Ihe zero-order hok I 
effer-i of the DAC. Tiie recimst mctcd signal is then summit 
with the outpiu of a dc offset IIAC iuid die result is buffered 
with a 211-dB-gam output amplifier. Tlie amplified .signal goi^s 
through the attenuator section to provide coarse control of 
the source level before being oiiiiiut on the IVonl-panel 
source BNC. 

TVigger 

The HP 8!I4H1A prondes very flexible triggering modes to 
supiJoil complex measurement.s. The IIF' HtMlOA can trigger 
froni four siniri es: the external friiril-piuiel trigger input, 
either input chiuinel, or die internal soiiri'e blor k start signal. 
The trigger level and slope are user-de finable. The trigger 
can Ije controlled by an exteniiil arm signal on the rem- 
panel ami B.NC connector. I'relrigger and post-trigger delay 
and arm delay are available. 

Tlie HI' S!l41t)A h;LS a sample rale or25,f. MHz. With a 
10-Mllz input signal, up to 140.0 degrees of signal can be 
missed while waitmg for the next sample lime after a trigger 
condhioii is met. Tiiis aninnnt rjf imcerlaiiity wrjuld negate 



©Copr. 1949-1998 H ew I att- Packard Co. 



tJiTcnituT tUll.i Hrwk'll-eackard.lijLjmal 45 



I 111' iihase infonnat.ion Ilmi is soiiseriil Tor many nieasurp- 
[imits. The soliiliim is lo measure tlie parluil saiiiplc lime. 
The time belwcen Ihe owum-nce of I lie Irigscr (.■undltiiins 
ami the next s;unple |joinl is im'iLsiii L'd ami used ti.> c nrTeri 
liie sampled data. A pulse slajls when the trigger eondilii 'lis 
ai c fuifilled and ends when two sample points have been 
taken. The pulse c-liargcs a capacitor at a fitsl rale. When Ihe 
pulse ends, Ihe capacitor is discharged al a slower lale. The 
fast chai-ge raic is alioul 7^31 I lines the slow charge rale. This 
effectively stretches the pulse by a fac tor of 7^1. 

Tiiis strelrhed jmlse gales a coimler running ai G,4 MHz. 
When the stretched pulse ends, 10 bits cjf pulse length infor- 
mation is available for the software (o use for corrections. 
The conilguiat.ioii gives a resohiiiim of 211 picoseconds, or 
lJ,77de^ee of a lO-MIIz input signal. 

During a ealihralioii lyele, the partial sample generalorLs 
calihraled ti.v geiienitiiig sine waves of known jihiise iHa- 
liimship to die sample dock and measuring the resuliing 
partial s;uiiple values. This has shown the circuitry to have 
good lineaiily luiii repeatahilily. 

Frequency Reference 

The sijecifi cations of the HP 89410A require a clean, low- 
itoisc refei ente oscillator capable of lockuig !o a ciislomer- 
supplied signal or nveii oscillator Previous FFf analyzers 
dou'i share this re(|\ilreiiienl because tlieir sample rates jire 
much lower than llieir crystal oscillator clock rreqiieiieies. 
For exatnple, tlie HP 35fi70A (lynainic signal analyzer derives 
its2(i2-kHx Siimple rale from a40-MH/ crystal oscillalor Tliis 
150:1 ratio results in a 4.'i-(ili noise reduction. This appmai li 
has llie effect of limying niediocre reference perfomiance 
under the uipiit noise floor. 

Since the HI' Hf»41IIA samples al a itmch higher rale than 
previous-generation analyzers, a differeni appn)ai'h had to 
he taken. The design is modeled after Ihe SO-MH/ refeivnce 
develo|ied for the HP 35RaA, a hybrid FFT-mialog .swept 
speetiiim analyzer,- The key components of this design are a 
clean sampling phase detector and oscillator.'^ Tlie IIP 
89411IA reference is a crystal-contr olled VCO niniiinf! at 
MI iz tuned by a phase detector samphng al 4(K) kVl/.. The 
external lock sigiKil is divided down by so dial a lU-MH/ 
lock signal becomes 401) kHz, Tire sampling phase ctei.ectnr- 



RllA'r is lunert lo allow locking down loa 1-MIIz lock input, 
The iil.2-MHj; signal is divided by 2 ;md routed lo atlboards 
in the inslrtiment in quadrature. The quadrature signal is 
developeil by inv erting and delaymg by IIJ nano.secomb. The 
iO-ns delay allows the data Imes lo settle l.iefore latching. 

Another board lot:ks a l(>MHz cryslal o.stillator to Die refer- 
ence and prochicvji 48. 64, and SO MHz. These signals are sent 
to other boards and used for riiT. IJSP. ( IT, and system bus 
clocks. Ilavijig ;\li i kx ks in the inslninieni jihase locked al- 
lowed Ihe design team to loi ale ajid address fixed crosslalk- 
indnced spurious signals railier than spurious iiipula that 
drift and hide, 

Ac kn o w ! edgm ents 

M;uiy people i iintribiiteil lo the success i)f the HI' SfMlOA 
hardware devi>loj)meni. Ki^v members of the dc'sign teiun 
included project nuuiager Larry Whatley. Glen !\ircly, ilim 
Canlhoni, Moot.s ii, Daji Foilune, Inna L;mi, Ch^u'lie Panek, 
;md Rusly Ames, niechanic:il designer, Don Hiller is recog- 
nized for bis hel[i with die initial ADt ' investigation. Early 
proditciion engineering (est support from Jernnie AtkiiLson 
aJid Roy Hart well helped sniooth the introduction of the 
prodiici. Many honis of overtime were contriliuleil by proj- 
ecl |i;u-ts admhii.siraior I-ienee Slociimb and by David ,k'ghmi 
tor vendor relations ajid buyer suppoil. Throughout the pro- 
loiype develo|iment the design team rehed heavily on tech- 
nician support from Bruce Beyer. C olin Eiickson. anil liuss 
Milc-hell, Printed i. ircuil layout support was pros ided by Jeff 
;\nyim, Liivonne Fogel, .MIyson Kiley, Natalie Kcliiiiiiard, 
and Steve Wtile, Wilhout Ihe contribtil ions and efforts of 
these people the project would not have acliieveil as great a 
success. 

References 

1. .I.S. Rjjsli'in. !■( al. "ilardware Design for a I'yiiaiiiic Signal Aiia- 
iyzer," Ih-irlrll-Fiick<iril Jiniriiiil. Vol. ;55, nci. 12. Decemijer 1984, 
pp. 12-17. 

2. K.C. Carlson, el al, "A lO-Hn-Io-l^fl-MH/, Spectrum .^lalyzerwitli a 
nigilal IFSpc'tiiiii," Ht ii'U-ll-Pt\cl,niil ■huinioi. VoL 42, aa. li.-lune 
1!!M1. pp. l l-li(l. 

:3. '1'. ilillstroni, "Uesifin Method ileitis Lijw-Noise, Wnlc^RanRc Crystal 
(")scillariirs,"mv. March 17, in.SR, 

4, M. Barlz, "liyge-Scalc nilhering KiUiances ADC Uynaniii' Range.'" 
Mia-mraops &RF,Uay 



4(! ni'l-eillhiT VM] Ilpwl"tl-Parkard .toiiniEll 

©Copr. 1949-199B Hewlett-Packard Co. 



RF Vector Signal Analyzer Hardware 
Design 



Based on the HP 89410A baseband vector signal analyzer, the HP 89440A 
RF vector signal analyzer extends the frequency range of both receiver and 
source to 1 ,8 GHz with a 7-MHz information bandwidth. Alt of The vector 
capabilities of the 10-MHz baseband instrument (up to a 7-MHz information 
bandwidth) can be translated to any frequency from 0 to 1-8 GHz. 

by Robert T. Culler, William J. Ginder. Timothy L. HiUstrom, Ke\in L. Johnson. Roy L. Mason, 
and James Pietsch 



Tlip HP Sf'440A radio froqiiencv (liF) vector signal analyzer 
consists of i wo componems. The first is die iii- 
temiefliafe freijiiency (IF) sectiDii. which is identica] to the 
HP S!:j410A HI-MHz hasehand verlor signal aaialy/er ck'sciihed 
in die article on [jiific :J1. Tlie second component is die IIP 
8!)M0A RF spc-lioiL which extends I he vector signal analysis 
capabilities of tlie Ijasebaiid analyner to RF freqiiendps. This 
article concentrates on die RF section. 

The 1.8-GHz RF section contains a iriple-convereion hetero- 
dyned receiver, a sduree dial mirrors the receiver, a ioc'al 
oscillator, frequency references, and iledicated processor 
control. The hlock iliagrain is shown hi Kig, I. Seveial jispecls 
of (he design (iifferentiate il from Iradiiional RF analyzers: 
The RF st^ctinn maintains an 8-MHz IF Ijandwicllh designeci 
for fjooci flatness. However, ui a vec tor wijinal analy/er i)olh 
aiiiplihidc and phase accuracy are veiy iniporlani,, so sopliis- 
licated vector IF c-ali lira lions were developed, 
'niie-rms power' deieciion and excellcnl ainplitiidc accncacy 
combine for a powerful nicasiirenirm capabilily 
A local oscillator (LO) fceddirongh cancellaiion circnil 
improves Ihe LO feedthroiigh siiij.slanlially, [ireserving 
dyiianik' range al low inpm frequencies. 
The RF local oscillator is nol coiistraini-d to provide fine 
frequency resolution, [Tlie biLsel)and section j provides that 
with ilB (hgital 1/ is). This degree of freed. ini had signifii'ani 
unpad on design efficiency iuid developmi^al liaie. 
The RF source has all Ihe capabihties of the ha-seband 
source, incliichng sinusoid, pseudorandom, cliir[>. and arbi- 
trary waveform source tyijes. bul translated li.i R|-' frequen- 
cies as high us 1,S f;Hz, This provides an excellenl viilue to 
users who need a flexilde source al RF ft equcwies. 
DeveiopmeiK lime was a lop priority. New melliods were 
developed to Jiiiiinlain ilesijjn nexibility ;nid rechice risk. For 
example, receiver, source, and LO "jjlales" featuring RF 
coaxial vias were developed — a low-cost sohiiioii dial Jil.so 
reduced technical risk by distributing ;md isolating critical 
blocks. Extensive RF and microwave simulations using the 
IIP Microwave Design System (MDSl greaily improved 
l.iu-n-on rales and elinunaied a protolyjie cycles 

Receiver 

The HP 8!J440A RF section receiver is a triple-conversion 
down-converter wit h a filkjlini input impedance, h translale.s 



signals with a maximum 8-MlIz bandwidih between 2 MHz 
ajid 1.8 GHz to a final IF centered at 6 MHz with a baiid- 
wirilli of 8 MIlz, The Ij-MHk IF output is comiecied directly 
to Ihe input of the HP 8!)44»A IF section. The IF section sup- 
plies a signal to cahbrale the IF tillers and amplifiers of Ihe 
RF section receiver. The freciiiency response of the 2-MHz- 
to-l.S-GHi! input is calibrated al the factory and stored in 
nonvolatile meiuory. The receiver is a couventional up/dnwn 
convcjTer with Ihe first fF centered at 2,44lj GHz followed 
by a down-conversion lo an EF centered al 4li MHz. Tlie final 
IF is centered at B MHz and provides gain iuid a biiflered out- 
put lo the IF secfion. 

Con\'entinnal swepl spectiiim analyzers use a log detector, 
which produces a sijjnal jiro])oiiiona! to ihe kigarillim of l.he 
power at liie deleclor input. The input jiowcr is a function of 
the resolution bandwidih when noise is the (loniinanl input, 
and is a function of the level of the input sigufd otherwise. In 
either case, log detectors have only 80 to 90 dB of nsable 
dynamic range. ThersToic, a \ariab!e-g;Liti arnplifiei' may 
pnx'ede the log rlelcctor to incre:ise or rleciease Ihe in])iit 
level so thai il falls within the d^mamic range of the detecior. 
By coiitrasi, there is no variable IF gain ui (lie HP S9440A RF 
section receiver with (he exception of a gain actiuslmenl and 
a 1-dB gain stej. in the filial tl-MHz IF Tins l-dB g;uii step is 
used with the 2-dB iupiil range steps in the IF section re- 
ceiver to conipensale for temperature dependent gain varia- 
tions in Ehe three IFs of the !iF section. Tlie detector in the 
IF sei-tion is an analog-lo-digi(al convener (ADC) with a 
dynamic nmge of llifi dB ( the ratio of the fiill-scale input 
power lo tlie noise power in a l-Hz bandwidih). The signul- 
to-noise ratio (HNR) of Ihe KF section is appnwinifitely V20 
dB (the ratio of the maxiniiim input for a 70-dB disloiliim- 
free dynamic range to the noise [lower in a 1-Hz bandwidth). 
Tlie receiver's dist oil ion-free dynamic r;mge and noise fig- 
ure ;ire dominated by the perfoniiajice of the first converter 
and first IK Adding variable gain beyond these stages 
elianges only the signal level bul not ihe HNR. As a result, 
vaiialile IF gain offers no iiii proven lent in dynamic range. 
The user only needs to iMuirol the in|nLt level to Ihe fust 
ini.Ker, which is accomplished with an input step attenuator 
prec:eding the firsi mixer. 



© Copr. 1949-1998 Hewlett-Packard Co. 



IJtl-i-iiilJiT tMW;)il.-",icll-pHrtiai,l,li.iirtljy 47 



DC 10 ID MHz 



DC-lo-IBOO-MH; 



Inpul -CT T3-||-<V 1 



0 iB 
Slcf) Alirnu- 

Sleiiil 



itmo-uHf 

low-Pass 
filter 



Diisctional 



Coupler 



Firal 

Conversion 



FIrsi IF 
Fillei 2U2 
10 24BO 
MHi 



SoconJIF 
Filii!r42 
(ij SO MH/ 



low -Pan 
Fillor 13 



FtcdtliroiiBli 
Canoellalion 



Second 
Conversion'^ 



Third 
Conversion' 



► 



S9iWA 
IFSccllon 
Input 



Benrijiass 




Local 
Otiillaiar 
7m 10 4242 

MHi 
11'MHi Sleotl 



ID Isalalion 
Amp I ill ei 



Final Convorsioii 
Modul 





•4- 


bOO-MHf 

am 

OiorllMor 




m 




IS 









Second LO 

Amplitier 600 MHi 



WMHi 



3D-<IB 
Slsgi Alton u- 
alor[IO de 

Slap 1 1 



iDW'Pait 
Filler 



DC-io-1800-MHj 
Source Oulpul 



OHIO 



W ZCoitvaision 



Fiis(IF 
Filler 42 
to SO MKl 



AM 
MddirlBlDr 




DC 10 ID MHz 



Q Froni 
HP S9440A 
IFSectian 
Oulpul 



Pig. 1. Cieneral block liiagr.-iiii iifthp HP S!)44()A RF serlion. 



The HP 89440A has one acidilioiial feature not found in 
tradilionai swept anal.viiers. A feed-forwai^ri Lfl feertt.hrougli 
nulling rircuil has been added to rediice the level iif 1,0 
feedtJiiougli in Ihe lu'sl IF. Beyond the second ronvericr, the 
second-IF filters remove the feedthroiigh term. Wiihotit this 
LO feedtJiiYiugli nulling, LO feedtlirongh referrefl lo the input 
could be 20 dB higher than a full-scale input signal. This 



could result in residual responses and increased second- 
harmonic distortion at input frequencies below 15 MHz. 

Cyanate Ester Printed Circuit Boards 

The inpui altenuatni- and several niher RF i-ecei\'er. I.O, and 
RF source boaids ihat operate at frequencies beyond I GHz 
use cyanate ester printed circuit board material. Cyanate 



48 [>pfpmher UW;! newlf tt-Piirkari! .Iniinia] 

©Copr. 1949-1998 Hewlett-Packard Co. 



ester was chosen in place of the standard glass epoxy 
printed circuil board (HP FR4 ) ijet-ause of its lower loss 
tangent, whit-h results in lower losses in the board. The di- 
electric constant of cyanate reter is 4.0 at 2 GHz while glass 

is typically specified at ■tri. All cyanate ester boards 
in the RF section are O.oao inch thick rather than the stan- 
dard 0.060 inch. The thinner printed circuit board niaieriai 
has two advantages. .Many rccei\"er. LO. and source lnjards 
in the RF section use surface mount parls nith niicrosirip 
ciJiistniction, Wth tlie Ihinner printed cireiiii board material, 
ground vias are shortened by 0.030 inch, reducing parasitic 
inductance in surface mount components needing a return 
to ground. In addition, microsirip transmission lines arv 
narrower, A 5(t-olim microstrip irai emission Ime is nominally 
0.060 inch wide on the fl.0;30-tnch printed circuit l>oard mate- 
rial while the same line on O.OfiO-inch printed i-ircuit board Ls 
0.1 ID inch ^vide. Tile ctisadvantage of O.U30-inc)i jirinted cir- 
cuit board is less rigidity. However, all of the cyanaie esier 
buai ds are momiied on "plates" and are well supported (see 
"Microwave Plate .Assembly" on page 50). 

Attenuator 

The main signal pulh of the HP 89440A RF section starts 
with a step artetmator assembly lhal provides 0 lo 55 dB of 
attenuation in -^-dB stops and is followed by the llrst con- 
verter. The step allenuator has an input for a cahbralion 
signal from the HI'8y4I0A, a mode lo temiinale the user 
inpul ciuring calibration, and a bypass mode to bypass tiie 
RF section. The bypass path connects the RF section re- 
ceiver input connector directly to fhe IP section receiver 
input for freijiiencies below 2 MHz. 

First Conversion 

Following the input all.enuatoi- ls the firsl. converter. Input 
signals are converted I o an IF centered ai 2,440 (iHz. The 
fu-st mixer is a vai-iaiil of the single- balanced design used in 
the HP 85000 spei-|run\ ajnilyzer II is preceded by a 15- 
seclion low-pass filter with a cutoff Ireijiiency of 1.8 GHz. 
Tlie input low-pass filler eliminates input image freqiieiides 
(4.S9 GHz lo (),(i9 GIlz] as well as spurious components 
fspurs) resulling from uul-uf4iantl inpul.s. The First-convener 
LO supplies a 20-dBni sigriiil between 2.4.52 GHz iu\d 4.24^! 
GHz, which is attenuated Uy-i dB ai the I.Oport of Ihe mixer 
to improve match. Following the mixer is a microstrip dircc- 
tiorul coupler, where Ihe LO feedlhrough caticellation signal 
is introduced. This is followerl by a diplexer ami a l.G-GHz 
low-puss filter I not shown in Fig. 1) lo ehminale mixer prod- 
uct.s and LO harmonics which can produce residual re- 
sponses when mixed with the LO of the seconil I'onverter. 
The entire mixer is buih un Duroid board (Rogers Corpora- 
tion) which lias a rlielectric constanl of 2.;W ±0.05 and a loss 
tangent of 0.001 at 1 GHz, The Ijoard thickness and dielec- 
tric constant are tightly specified so that primed circuit 
board microwave filters, couplers, and Iransmissiori lines 
with repealahle performance can be produced, Karly in Ihe 
design it was recognized lhai skin effect losses in the inpul 
attenuator and the input low-pass filter preceding the fu^ 
mixer would result in fret|uency dependent loss thai is about 
4 dl) al l,K (illz. This unflauiess can be caliljrated ami re- 
moved, bul it results iu a displayed noise fioorihai is unflai, 
and it I'educes Ihe effective dynamic range of the Al)(.: by 1 
dB. An ampiilude e(|ualizcr (not shown in Fig. I ) was added 



between the attenuator and (he inpul low-pass filter to elimi- 
nate tins effect- This has the addetl benefit of reducing the 
le\"el of muiiiple tones in the first IF at low input frequen- 
cies. Multiple tones are present because Ihe sum and differ- 
ence products and ihe LO feedlhrough are only sejiaraied by 
the input frequejtcy and are not eliminated by the firsl IF 
filter if the input frequency is low. 

The equalizer, fhe input low-pass filter, the 4 5 GHz low-pass 
filter, and the diplexer were designed using MDS. 'Hie mixer 
con\-ersion loss was also sunulaled with MDS and the mea- 
sured results were in excellent agreement wllli the simulated 
results. MDS eliminated at leajit one design turn of ihe 
printed circuit boards. 

The diplexer (a traveling- wave directional filler) following 
the first mixer is a stripline design that terminates die first 
mixer IF outjiut in .50 oluns at all frequencies. It has a hand- 
pass frequency response lhai reduces the mixer sum proiluct 
ai the first IF amplifier- The diplexer is implemented with 
two 8.5-dB stripUne coiijders and two quarter- wavelength 
coupling anns. The insertion loss at 2.446 GHz is less than 
1.5 dB and ihe:i-dB bandwidili is 150 MHz. The conversion 
loss fi-om tiie input attenuator to the i.iid-GBx IF output of 
the first mixer board is typically 14 dB. 

LO Feedthr<»ugh Cancellation 

Wilhout cjutccllation, LO feedlhrough referred lo the input is 
typically -10 dBm while the ma-xiimim mpuf (0 dB input 
attenuator) is -:J0 dBm for 70 dH of distoition-free dynamic 
range, LO feerithrough cancellation reduces the LO I'eed- 
through at the Ih-sl IF amplifier by inon? than 20 dB. A sam- 
pled LO .signal al I he input of Ihe LO |.ior( of the first mixer is 
amphfied and split with a quarh-ature hybrid. At the output 
of (he quadrature hybrid are two equ;il-iuagnitude .signals 
lhal have a pha.se difference of 00 degrees. Bach of these 
oiitpuls drives llie UJ port of a double-balanced mixer con- 
figured as a current-conl rolled attenuator. The mixer IF 
ports are drii en by separate current sources capable of 
sourcing or sinking up to (i mA of current with l-^A resolu- 
tion. Tiu- .signals froin ihe mixer RF jxuls are summed by a 
Wilkin.son combiner whose output, is coupleil inUi fhe IF 
port of (he first mixer with a 14-dB microstrip coupler. Dur- 
ing calibration of Ihe RF section, the IFseclion acljusts each 
current source hi jiroduce a signal equal In magnitude and 
opposite ui .sign lo Iliai of the LO feedlhrough a( Ihe IF port. 
The use of quadrat lire signals in t3ie LO feecilhrough can- 
cellation circuitry reduces the interaction between the two 
current source controls so lhal ideally they are uidependent 
if the iwo signals are in perfect quailrature. 

First IF 

Following ilie firsl converter assembly are two cascaded 
low-noise GaAs MMIC {microwa\*e monolithic integrated 
circuit) amplifiers which have a combined noise figure of 
5 dB, a gain of 14 dB and an ouipiil 1 bird-order intercept* 
greater Ihati 25 dBm. These iunplifiers must be able to han- 
dle all mixer products leaving the first conversion a.s.scmiily 
including Lf) feedlhrough ;uid both ihe sum ;md difference 
firsl-mixer producLs, 1 ligher-order mixer products as well as 

liiird-ordBr miereepi is Ilie ttieuradcal sistial IovbI ai vvhith the tundamctnisl and iha ilmd-ut- 
am distotiad are aqua! 



© Copr. 1949-1998 Hewlett-Packard Co. 



Dw'inljtr ItlSlKIIciviell-pBclmril.IniinMl 49 



Microwave Plate Assembly 



AI the mceplirai of llie HP89440A RF veciar signal aiialyzEr pnjieci. il was known 
thai tlie packaging technology used tor the high-trequencv portions o( the analyzer 
circuiTry wnulii play an important role m the ni/erall riertormance ol Ihe instritrrBnl 
A nufflher ol packaging techniques were considereil, including Hie conventmnal 
appmach ol independently packaged ciicuiis connected with semirigid cable 
assemblies Given ttie oi/erall goals of shielding effectiveness, ease cif assemhiy, 
and relaliuely taw cost, it was decided to try SDHielhing ditterBnl Ifie tiigh'treguEncy 
microwave plate assembly nackagmg sctieme that evalued for the HP B94flQA 
satisfies these goals, provides good flEiibiliiyfar inlEgiaiing oihei luncimnality. 
and provides a physical support structure lor the individual modules Tliere are 
three such microwave plate assemblies in ihe HP BS440A. the HF source, the RF 
receiver, and the local oscillator. 

The physical implementation of the packaging scheme is rather simple (see Fig ^\ 
Printed circuit assemblies ate mounted with screws to the lacea ol a D 375-inch- 
thick aluminum plate Assemblies that require RF electrical iniercorneciion on a 
given microwave plate assemhiy are mounted on opposite aides ol the plaie. The 
RF connection is supplied through a hoie in the plate iniu which a conductor pin 
and an msulator ere inserted A 50-ohm charactejistic impedance is maintained 
through the hole in the plate by sizing the diameters of ihe hole and the pin ac- 
cording Id Ihe dielectric constant of the insulator IPIf E|. A miOTSinp-to-cnaxiai 
transition is fonned as Ihe conductor pin is soldered into a hole at the end of a 
microstnp on the pnnted circuit board Good local ground contact at the transition 
is maintained by a mullltingered ground ring that is seated in a shallow counter- 
hore m Ihe plate and is in contact with the pnnted cirrruii board ground plana at 
the perimeter ul the hole. Two screws located near the hole secure the printed 




Shield Can 



Rfl. 1. Microwave plate assemBly packaging st'tiBme 



LO iiarmnnics are remrjveti by the 4.5-GHz low-pass Hller on 
tiie first rotiversioii assoiiihly. The siiiu ami (lifrerpiire jiri)tl- 
itcts are amplified hy this hoaril, aiid the thirri -order Iwii- 
tonc dislorlion products resulting ftoni tliese tones appear 
as Lhi rd- harm oil ic disloriioii lo Llic user. At input fi-equeneies 
above 60 MHz the diplexer reduces the sum rone by G <iB at 
the input to the ntst IF amplifier, so Ihe liurd-ordtTtiisrortian 
proiiueed in tlte first 1 F is a eoiK eni only at lo« input iVe- 
(liieneies. Following the first-IF amplifiers is a four-section 
coaxial filler with a \7-UH/. liaiidwidth and tut insertion loss 
less liiaii 3 dB. This fitter has a eeiiter frequency of 2.-14B GHz 
and greater tiian SOdB of fejeclion ai the Intage frequency 
of 2.358 CHz. 



circuit board fu ilie plate and guarantee that the fingers of the ground ring are 
compressed Good raiurn loss is achieved witli this interconnect solution for fre 
guencies up to and beyond 6 GHi. A low-pass hlter version of the conductor pin 
was developed thai is physically interchangeable with the standard pm to sup- 
press transmission of higher-frequency mierleting signals (greater ihan 5 GH?) if 
teguiretl Compared to the conventional approach, ibis method ot RF irteiconnec- 
tron raflucBs lliE tiumber nf eipensive RF connectors and cable assemblies rs- 
quired Whan shielding ul the individual printed circuil assemblies is requited, a 
shield can is screwed down nvBr tha assembly The aluminum plate acts as the 
sisth wall of the shield can A good RF seal is provided by i:unductivE elasiDmei 
gaskei matenal m the hp ol the shield can For some assemblies, sliield-can reso- 
nances present a problem Poiyiton is liied to the lop ol Ihe shield can to suppiass 
the resonances m tliose cases Dc power is supplied to the shielded assemblies 
with standard screw-in teedthrough capacitors The feedthroughs do an excellent 
|ob of prevenling HT leakage into nr out of the shielded assemblies 

Flexibility IS provided by the packagini) scheme lo allow inouniing of different 
types dI devices onto the plates For insiance, several coupled coaxial resonator 
filters are used in the HP 83440A These filters incorporate the plate mio their 
design The tiller hnusing is screwed to the plate, which functions as the cover lot 
Ihe filler. Input and output coupling rods used in the filters are integiaieii with 
prinled circuit board launches as described above to provide a CDnneciorless 
interlace between the filter and a printed circuit bnard A signilicani cost saving 
was realized on these liltets as a tasuli of this design 

Additional lunclionality ts mcluded in Ihe plate design in the lutm of leatutss 
milled into the plate An example is shielding compartmeiiis tn isoiaie the individ- 
ual sections of the step attenuators in the RF receiver and the BF source, TTie step 
aitenuaiur printed circuit assemblies are screwed down to the shielding structures 
milled into the plate. 

Certain aspects of ttte printed circuil hoard designs are optimized for use with the 
micrnwave plate packaging scheme. Coaxial-to-microsuip launch geometries are 
optiniiied for best reiuin loss The side of the punted circuit board toward Ihe 
plale IS mostly ground plane Tin plating is used instead of SMOBC-HAl (snider 
mask on bare copper — hot eir leveled) In ensure that the boaiiJ his flat onio Ihe 
plate for connecliDn to other modules, pnnied circuit boards have bulkbead- 
mDunt SMA connactors soldered to the ground plane side Each connector is 
pushed ihiDuah a counlerbored hole in ibe plate and lis mounting nui is installed 
Conductor pin geometry and screw hole locations were chosen so that a standard 
flange-mnuni SMA connector can be used to perform measurements on the 
individual printed ciicuit assBmhlies, 

This high-frequency packaging approach is attractive for low-volume Instruments 
Unit cost IS relatively low and minimal laniing charges are incurred 

Roy L Mason 

Deveiopmeni Engmeet 

Lake Stevens instrument Division 



Second Cniiver»«ion and IF 

Following Ihe first IF filler is Ihe sceond converter assembly 
which down-eonvetis the lj.^4G-tiHz IK to (lie second IF cen- 
tered at -Ifi MHz. Tiie secr>ii<] 1,0 is a l>.4-i;Hy. signal suppiieti 
by the reference. It is amplified lo 10 dBm, filtered by a two- 
section eonibliiie liller to eliminate an>' sidebands al 50 MHz, 
and tlien amiilified lo 13 dBm before application lo tlie sec- 
ond mixer LO pon Ihrough a '■i-iiB attenuator. The 4(5-MHz IF 
output of Ihe seconil mbier is amplified bj' a lo^v -noise am- 
plifier, resulting in a signal level equal to Ihal of the input 
signal (-130 dBm full-scale], The noise figure al this point is 
nominally 2'S dB. Following I lie second convener arc three 
ident ical cascaded second-IF filter assemblies iniplemenled 



.50 Iti-. i'ititirr IllSi llr»li-ll Piii'k^ml .l.iiiiiid 

© Copr. 1949-199B Hewlett-Packard Co. 



with capacitively cfmpled resonators. The filter design is 
based on a Tdiebycliev filter n ith 0.1 (IB of riijpie. The filter 
is proflistorted aiid iia.s an in-wrlion loss (ir4 <iB. The filler 
lias a minimuni rejercion or 27 (IB at 38 MHz (edge oflhe 
111 ird-con version image band), sfj llie three cascaded st^es 
have a minimum of 80 dB of reje*-(ion. So tliat eai li filler 
board will have a nominal gain of {t itB a low-noise amplifier 
[wecedes each filter. Three identical Tillers were chosen over 
a deslfgi in which the entire filter re.sided on a single board. 
A singie-lioard design would reyiiire shielding between sec- 
lions of (he fiiter and aligiunent of the filter would reniiiire a 
complex adjustment procedure. Using smaller identical 
boanis, iKc alignment procedure is identical for each board 
and simplified by the faei ihat there are ojily four sections lo 
ai^iLst. Shielding is provided by the walls of (he card nest in 
wliirh the hoards reside. The three cascaded filters have a 
MB bandwiilih of fi,.5 MHz and a combined peak-tu-peak 
ripple less than \ 2 dB. 

Third Conversion 

The final stage of the RF section receiver is the third- 
convener assembly which translates the IG-Mliz second IP 
to the final IF centered at ti MHz. A lO-MHz LU is provided 
by the refereticc at 3 dBni. Following the conversion are iwri 
wideband ojierational amplifiers, which proWde the final 
gain and buffering before the sigmd is sent lo (lie IF section. 
There is a 1-dB giiin slep rhal can be switched hy the IF sec- 
tion and a manual gain atliuBtnienI lo compensate fornomial 
gain variance in the mamifactiiring process. 

Local Oscillator 

.Since the IF section has the ability to tmie with millihortz 
resolution to any fTe()i:ency within its dc-to-If)-Mflz input 
bandwidth, the LO for die RF serlion does not have to n?pli- 
catc this function. The RF seclinii {lown-con veils the 
!8U(I-Mllz iii|iiii spiui In within the fi'iijuency r;mge of the IP' 
section and the digital LO nines U> the desired center frc- 
i]uency in Iradiliunal spectrum analyzers, muhiloop LOs are 
designed lo realize niillihertz re.solul ion. Multiloop designs 
mean at lea.sl lliiee phase-locked loops in the main 1.0. The 
step loop provides coai-se fre<iiiency resolution over a wide 
range of frequencies, The inleniolalion loop tunes across a 
narrow span but with liigii freriueiicy resolution. The .sum 
lonjj combines the iiiitpiits of the Iwo loops, hi contra-sl. the 
HP 8SI-MIJA RF section has a single-loop ],(> that limes in 
1-MHz steps. Tile eliiiiinatian of the sum and iiitciT.icilalion 
loops inejms signirieanl savings in eonipiexily ;uid reduced 
developmem risk within the U) section oflhe IIP SIJ440A. 
This triidc'-off is not without conseiiiience because a coarse 
Ui resohition reduces (he analysis hainlwiilih of the uisU vi- 
menl. The IF Ijandwidih of the receiver chain sujiports an 
S-.MHk iinaly.siR Iwmdwiihh but ati arbilriiry center fieqiieiiey 
at the input ctm only be placed within +0-5 MHz of (he IF 
cenler frequency because of the slep size of I lie LO. Hence, 
for an Jirbllcary center frequency liie S-Mllz IF bandwidth is 
reduced to a 7-MlIz m;L\imuiu aiiiilysis bandwidth. 

The frequency range ofUie IXt in the RF section basically 
sljuis at the fin^l IF frequency and times lo a frequency 1.8 
GHz ai)ove that. A i-oiniuercial VIG (yttrium iron ganicl.) 
oscillator was selected to cover 2.4 to 4,3 GHz. The niilput 
of [|ie VIG oscillator goes to (he LO di.slribulioii amplifier 



and is also fed back to the synthesizer phase-locked loop 
(see Fig, 2). 

Since the ontpiil frei{uencies in\'<>lved are beyond the reach 
of programmable <-ouiilers. the feedback path includes a 
down-conversion slage. The LU frequency' range of 2.452 to 
4.242 GHz is down-conv eried with one of three oflsets (2.4. 
•i.O, or 3.C GHz). The offeei frequency is chosen lo produce a 
down-eoin ened signal between 42 and 642 MHz. which is 
vriihiii the raiigeof iheRFseciion programmable coiuiier. 

The value of the programmable counter lT<i> is chosen to 
ihvide the counter input freiiucncy dorni lo 1 MHz i S = 42 to 
i>42t. The oulpul of the diviiler is phase-detected against a 
t-MHz signal derived from the 40-MHz reference. This estab- 
lishes llie 1-Mllz step size for the LO. Tliesign oflhe phase- 
locked loop must be switcbable bec-ause the YIG oscillator 
tunes above ami below the offset frequencies. The sign is 
switched hy swapping tlie reference and feedback signals at 
(he phase delecior. 

To compensaie for the wide range of the loop gain (because 
N ranges from 42 to 6421. a programmable gain block with 
2o-dD gain varialion is added to the loop. Finally, a UAC- 
ilriven coai?ie I iming signal is used (o steer tlie YIG oscillator 
into die lock range. 

The LO is distributed across foi ir printed circuit, boanis and 
one microwave plate assembly. Tlie circuit boards contain 
the fiOO-MHz reference, the 40-MHz reference, t)ie frequency 
coimler, the pliase detector, and Ihe VICi driver. The circiiils 
on the microwave plate aie fabricalerl wilh cyiinate ester 
printed circuit lioaiils and include the I'requeiicy multipfiers 
and Uie YIG down-<?onversion. 

LO Distribution 

The LU dislribulion amphfier is hiiili aiouiid a packaged 
GaAs MMIC amplilier designed by Hi"s Microwave Technol- 
ogy Divi.sion. This amplifier has dual outputs which are used 
lo suj)|ily the LO signal to both the RF receiver and (he RF 
source sections. 

LO Offsets 

The three offset frequetwies (2.4, 3.0, and 3,6 GHx) are gen- 
erated by mull iplying 600 MHz by integer values. The 600 
MHz comes diKu Ihe LO microwave plate a-ssemlily anil is 
split I o provide signaLs for both the offset multipliers and I he 
second-LO miiltipiiers. P-i-n diode switches select Ihe paili 
to the aclivaled offset miilliplier and each palh iiiis ils own 
final stage of amiilificaiioti lief'ore the miihi(ilier. Hchotlk>' 
diodes aie li.sed ;i.s the harmonic generating devices in each 
of the multipliers. At die oulpul of each multiplier is a two- 
section coaxial filter lo suppress the acljaceni (iDO-MHz har- 
monic. The second LU (2,4 GHzj is generated in a .similar 
manner. 

YIG Down-Conversion 

1 lown-ci inversion is implemented with a 7-dBm microwave 
nii.xer. Becausi' of die low-level signals involved and the 
wide livriueucy ,s|ian of Die output IF 1 1(1 MHz lo 700 .MHz), 
the iioi.se ligiue of die postconver.sion amplifier is impoitiuit. 
The broadband noise wilhin the 700-MIIz span is sampled 



© Copr. 1949-1998 Hewlett-Packard Co, 



Dfivintif r lltn;) Ili'Wli-l.t-I'ai kiirii ,l(]iinial .'i 1 



To Receivei 

Flist 
CDnvBrsian 
4 



lO-MH; 
HBtBfBncs Out 




I *<^^-» 



NuMi^Jor Gam 
CEim;iei]5AliDn 




riGOscillatoi 



LOOIslilliutiDn 
AmpliTier 





Ja SoutCfl 

Final 
Conversion 



1 1 _ ♦ _ W-MHi 




V-/ BefBiBnee 



Fig. 2. Ill' 8iW40A RF sect.iori local OKfiUalor. 

down by the cUfiitaJ d:\i(icrs ( 101og[70(l MIiz/1 MIz| = 28.5 dB 
noise gain) and ciiri coiilribiile to the phase noise pedestaL 

10-Bit IIHF Counter 

A l()-hii roimter is retiuired to accommodate all integer divide 
numbers from 42 lo 642. Since higli-si>eed conimerciaJ roiinl- 
ers are liniiled to S liits, a prn|irietary coiinler circuit wa-s 
developed using ECL integrated circuits. This impleineiila- 
l.ion allows ihc use of inexpensive comniprcia! I'omponents 
lo achieve niiixinnim count frequencies of over TOO MHz, 
whereas standard de.'iign imiJlementations only permit a 
500-MHz maximum input frenuency. 'I'hiy iinprovejncnt was 
critical in realizing the design efficiency of the LO block 
diagram. 

YTG Drivers and Timing 

The yiG oscillai<»- has a main coil lor coarse tuning and ;ui 
FM coil for locking the phase4ocke(l loop. Bolh coils are 
driven by voltage-tocurreni converters and high-current 
drivers. 



Third LQ 
4aMHi 



The main coil is controlled by a 12-bit DAC, |Dro\iding better 
than 1-MHz fri'iiiiency resolution. Because of the exlremely 
high gain of the main coil (20 ( illz/A), uoise fillers arc re- 
quired. Ordniaiily this would siSnificiUilly affect LO switch- 
ing time, so a speed-up c'irciiil was designed lo precharge 
the filler elements to Iheir final values, greatly improving 
settling lime. 

The main coil tolerance is far too large for the a\"ailab!e FM 
coil tuning range, so an automatic YIO tuning calibralion is 
peilomicci pcriodit'ally, initialed by lite instrument calibra- 
tion timer This improves Uie absolute accuracy of Ihe main 
coil from ±400 MH/ lo ±2 MHz- 

Frequency References 

The (iUD-MHz reference is the soiu'ce for ilic offset frequen- 
cies, the second LO. and the third LO (40 MHz). It is based 
on a phase-locked GOO-Mllz SAW (surface acoustic wave) 
oscillator. It is designed so ihai il does not contribute to Ihe 



52 Dccpluher ir>rn TlewSplt-Pai'kard .Imimjil 

©Copr. 1949-1998 Hewlett-Packard Co. 



LO syslem phase noise. The loop is locked to ihe 40-MHz 
reference. 

The 40-MHz reference prriviiies subharnionic ( l-MIIz. 2-yiHz. 
5-MHz, 10-M}!« I reference locking for the in.smmient and 
acts as a cleanup loop for the user's external reference, if 
present. Il contains a 4(l-MHz vollage-coniixilteti crj'stal os- 
cillator and a phase-locked loup with a^mplinspliase de- 
lector, hi Ihe absence of a iiser-pr<»ided external refereni'e, 
the reference locks to the iiiteniaJ high-stabililj' ovenized 
l()-MIIz reference. 

Tlie lO-MHz oveni/.ed reference is the widely used HP 
10811, wliich has ultralow phase noise and extremely high 
temperature stability. 

RF Source 

The HP 8!J44UA h;is an ojjtional RF source to provide stimu- 
lus signals for a \'ariet.v of test jiiiiposes. Output signals pro- 
duced by the soiu'ceare iti the 2-MHk-Io-1.8-(.HIz frequency 
range when the insmmient is in rhe RF \ eclor or demodula- 
tion modes. The HP 8914t)A is not a tradil ional swept ana- 
lyzer, so the source provides se\ eral signal types in addition 
to tlie standard sine output to satislj' varioii.s jneasiu cmcnt 
needs. Available HP Ky440A source output signal types are 
sine, chiip, pseudorandom noise, ;tnd arbitrjuy (see "A Versa- 
tile Tiacking and Arhin-ciry Source" on page ">!). l>ne or more 
of these somce rv]ies are available at baseband in most 
modes using the RF source byjiass iiath. t'hiip or sine .signal 
levels avjulable from the RF Sf juree are +13 to -27 dBm. 

As shown in Fig. 1, the source circuitry of the HP 89440A RF 
section is basically a frequency converter for the source 
oulput signal from the HP 8S)440A [F section. The circuitry 
llial generates Ihe IF wecliori source oiitpnl .signal is de- 
scribed in the aiticle on jjage ;J1. The soiu'ce signal originat- 
ing in the IF section is a chiip or noise signal centered al fi 
MUz. a sine wave belweeji 2.5 and 9.") MHz, or an arliilrary 
signal. 1ji Ihc RF section, this suurcc signal is fed lt> tlie llrsi 
conversion assembly, wdicii [iiovide.s pjirt of the .signal 
Bwilching funclionahly to bypass ihe RF section source or 
to route a ciilibration .signal lo the receiver. Amphtude mod- 
ulalioji circuitry allows Ihe signal lo he modulateil al a maxi- 
mum h equency of about I MHk. Thi.s can lie useful for 
impre.sslng certain types orsyrichroiiixaliou signals on the 
RF source output signal. The signal i.s l.lien mixed with a 
40-MHz fixed signal from Ihe RF section reference to upcon- 
vert il lo a l(j-MIlz IF center Treqiieiicy. Following Ihe first 
conversion assembly are I wo ca.sca<led 4(>-MHz IF fiiler as- 
semblies identical lo tho.se used in the RF receiver. To ac- 
comniodale tlie wideband noise and cliirji signals fnjm the 
IF section source, the IF bandwidth of 1 lie entire RF source 
is approximately 8 MHz. This bandwidth also facilitates off- 
setting die frequency of sine waves up lo^i.i) MHz from the 
tuned center frequency. 

Tiie 4r)-.MHi; IF .signal is pjis.sed lo the second conversion 
assembly where it is again upc<jn verted, this time lo a 
2.44(i-GHz IF center frequency, by a mixer whose LO port, is 
driven with 2.4 GHz. The fixed 2.1-(iliz signal lo drive the 
second mixer originates in Ihe RF seciion reference and is 
aiii|)lified by the spcond-U) amjilillcr a.ssenibiy local I'd on 
the KF source assembly. Besides amiilillcaliou of Ihe 
2, 4-GHz signal, the second-I.O amplifier itssembly provides 



reverse isolation to prevent Uie source IF signals (particularly 
the 2.44f>GHz IF) from leaking back ihrough the 2.4-(.;Hz W 
distjibuiion circuiir>' and into the receiv er fF. If this were to 
hajipen. receiver sensiliviij- would be compromised when 
the source is funcTioning. 

Following the second mixer are two IF amplifiers. The oul- 
put of tlie last IF amplifier leaves the assembly and goes to 
tJie 2.44(5-GHz second-IF filter. This ftiur-section coupled 
coaxial resonator bandpass filter has a 17-MHz bandwidlh 
and is identical to Ihe receiver IF filter. Tlie filter is reijuired 
to provide adequate rejedion for the 2.4-GHz LO second- 
mixer feedthrough and the mixer lower sideband product 
centered at 2.'S'A GHz while maintaining reasonable inser- 
tion loss. Physically, ihe filler is optimized to take advantage 
of ihe tiiicrowave plate packaging si' heme which also helped 
minimize ils I'ost (see "Microwave Plale Assembly" on imge 
6<l). The filter is tuned by ai^jusling four self-lockuig luning 
elements using a simple, iio?iiteralive tuning procedure. A 
timing port on tlie liiter housing aids in the procedure. 

.After the 2.44(K;Hz IF filter, additional IF amplification is 
proviiied by two mnplifier stages on the IF gain itssenibiy 
iiefore Hie signal is applied lo the final conversion module. 
An existing HP Microwa\'e Teclmology Division design, the 
final conversion module was leveraged because its funclion- 
alily is a good 111 for the RF section sotu'ce. Tlie final conver- 
sion module uses a GaAs MMIC to mix the 2.44t>GHz IF 
signal with a variable LO signal of 2.4-52 lo 4.242 GHz to pro- 
duce the ba.sel)aiid output .signal, which is then apphed to a 
tliin-filni low-jiass tiller. As in Ihe caseof tlie 2,l-r,IIz LO 
signal, sufficient isolation is needed in Ihe variuble-LO dis- 
Irilnition path to Ihe RF source lo ensure that the 2.4-4lj-GHz 
Si.uuce IF signal does not leak inlo llie receiver IF. Some 
reverse isoladou is afforded by a GaAs MM!C LO driver 
wilbin die tinal conversion module, bin ibis is insidficient by 
itself Ad(iitional isolation is supplied iiy ihe isolation aniph- 
fter stage located beiween the final I'onversion module LO 
input and Ihe main LO distribution ampliller out])Ut. A GaAs 
MMIC output amplifier brings Ihe sigri;il lo Ihe projier ouiiml 
level. Anoiher thin-film low-pass tiller follows Ihe oui|)ui 
amplifier, Tlie (iaAs devices inside liie final convension mod- 
ule retjiiire their dc supplies to power up and power down in 
a prescfibeti sequence. Since the RF seciion main power 
supply does mil provide Uie proper sequencing, a local 
power supply iLssembty on llie RF source accomplishes this 
along with voltage rpgiilation and current limiling for all of 
ihe final conversion assembly dc supplies. 

Aslep alteuualor assembly is located al the output of the 
final conversion assembly. The steji allcnuaior assembly 
applies 0, 10, 20, or -il) dB of attemiadon to the oulput signal 
fur mnplitude corilrol. The IF section provides fine ani|)Ii- 
lude conlrol. An amplitude equalizer on Ihe step allemialor 
assembly helps ('orrect for roll-off in die final conversion 
assembly and Ihe sti'p atlenualor assembly. A signal gener- 
aleil in ihp IFseclion can be routed ilirough Ihe RF source 
swilching circuitjy of ihe fii-sl conver'sion a,ssemlily iind the 
step attenuator as.senibly to Ihe RF receiver iupiil fur re- 
ceiver IF calibralion. The output of the Resource can also 
lie routed lo Ihe receiver input for caliliration and leveling of 
tlie RF source oulput. Tlie liF. source ( ircuilj-y c;m be liy- 
[la.s.sed so that the IFseclion source output is available at 
the RF source output comiector, 



©Copr. 1949-1998 Hsw I alt- Packard Co. 



l)i-i'i-iiibi>r imi Ili^wlPlt-rui.'kard.Iiiiini.'il 53 



A Versatile Tracking and Arbitrary Source 



Ttie measuremenl pnwer ol the HP BSIIOA and BWQA veciw siijnal aiialy/ers is 
greaily enhanced by llie inclusion of a versalile signal snurce Ai/ailable uulpuls 
are iwo tracking signals— psB lid utandam noise anil pHrindic chirp— and Iwd 
indapendenl source lypes — sine and arbiliary The ciicLiils ihai generate these 
signals aie described in llie accnnipanvint] article and in tlie orliclB on page 31 

lha [isBiidoranrtDm naise is generated wilh a sequence lariglti a! 2^^ - 1 samples, 
wbicb IS about six hours long in ihe widest span anrl praportionaiely lunger as rhe 
span IS reduced Its probability density funciiDn apfir us i mates a Gaussian piobaUility 
ilensity lunclion In a i^ddth nf at least i3 standard deviatiuns Tnese pruperties rnake 
It iunclien lite a trOE Gaussian random noise source lor uitmally all applications 

The periodic chirp is a swepi sms wave that covers the span being measured by 
the receiver Because it is generated as a swept sine, il has relatively constant 
amplitude when uhseruad in Ibe time dornam, with the ejrceplion ut bandliiniting 
tiliBi tinging As a result ol being 'Tlaf in the lime domain, ii is not absolutely flat 
when observed in the lrequeni;y domain 

The arbittary source uses timE-domain data from a usar-seleirted stored data 
register and reconstructs the waveform with lha original span and center Ire- 
gueni;y The stored data can consist ot measured waveforms, data generated on a 
computer and downloaded to the instrument, rjr ttie result o( user malh on euher 
or both The frequency of downloaded data is set by the headers that are loaded 
with It This capability makes it simple tonieale, for enample, an independent [not 
ttat:kingj chirp source To record the chirp, the receiver is connected to the tracking 
chirp source and the fi<ed span and center frequency are chosen The time data is 
stored and istben available as an arbitrary source, at the chosen span, center 
frequency, and number of lime points 11 is wnrth noting that the arbitrary source 
data can he complex, that is, il can represent an l-Q (in-phase and quadrature] 
signal with the center frequency and span specified m the data register header 
intormatiun Thus ii is easy lu obtain data cummunicatioris waveforms and other 
hard-io-prnduce stimulus waveforms as long as the time-length constrain]; are 
acceptable A number of these wavelorms are included on a disk supplied with 
the instnimenis and can replace a number ol ejiemal waveform generamrs 

Both the arbitrary source and Ihe sine source can be placed anywhere within ifie 
D-tD-10-MHz HP Bg41QA Itequency range and anywhere within 3 M\ii 13.5 MH; 



for sinsi of the HP a944[)A cemer frequency in vector and demodulation measura- 
menl modes The cbirp and random sources are designed to cover the frequency 
range being measured when the instrument is in Ihe vector and demodulation 
modes All four of these source types are generated in hardv^are — and software 
m the case of cliirp and arbitrary — that to a great eilent mirrors the signal pro- 
cessing in the receiver Fig. 1 on page 4B illustrates this symmetry very clearly, The 
local oscillator ILO) frequencies and IF filter trequencies are all Ihe same— the 
signal flow is simply in the opposite direction This is also seen in rhe IF section 
(see Fig ] on page 32] Ihe digilal-to-analng convertet (DAC| in the source per- 
forms the opposite function nt the analog-tO' digital converter |ADCI in the le- 
ceiver The 10-MHz reconstruction filter after the DAC suppresses alias compo- 
nanis in a manner very much like ihe anii-alias Idter preceding the ADC One 
shght exception is that the source reconstruction filter contains additional peaking 
in the frequency domain to match the implied siniiiil/2|/mt/2 attenuation of The 
DAC This difference anses because the apenure ol the ADC is rnuch narrower 
than the full sample width apeiTuie of the DAC 

Before the DAC, the sQun;e is also a mirror Image of the receiver. The decimating 
digilal filter and LO ICs are designed to work in reverse, forming a complei inter- 
polation hitei. For the mdepentlent source types, the Oandwidth and LO frequency 
of the digital source hardware are set independently from those of the receiver 

The source BAf^fl contains the waveform to be output, but at baseband, before all 
the frequency translations of the u pen over si on cham In the case uf the sine 
source, this is simply a devalue — the upconversion shifts this D-H? signal to the 
frequency desired 

The chirp and arbitrary wavefoons lot the source RAf^ are cemputed In software. 
To iniplemerit arbitrary source spans given the fixed hardware sample rates, re- 
samplmg must Be performed, just as in the case of the receivei The source resam- 
pling filter uses the same filter coefficienis as the resampling hiter in the receiver. 

For computational simplicity, this filtor has an alias-protected bandwidth of only 
one founh of its incoming sample rate It is riesigned to operate with mcnming 
samples that are two nmes ouersampled in the lime domain, that is, with an extra 
sample puml inteipolated between each pair of stored data register samples. 



Tlip first cniiversion assembly and tht' two llj-iMIIz IF filter 
assemblioH rcsiilc in the cani nest slmt liire within the RF 
section. Tlie rest ol' (he HF source {.-ircuitry resides or the 
stmree microwave jjlate rissembly, which is a siip])oil, 
shifliiing atiil interconneclioii stmcliire (.see "Microwave 
Ptalt' Assent III .v" mi p:t^f ^lO). Twn-.sided, r).f)f)(l-jnch-llik-k 
cyatiale ester prinled l irciiit homil wa.s iiseil in iJie RF 
source because of its RF performance. 

Controller 

The i'oiilrol of (he RF section is vested in a resident 
Motorola MCliSHCli microcontroller, wltich controls the 
iiaitiware seLiip, comnittnicates with the IF section via 
RS-2;12, stores HP SM-1411A caliliration ilata iti Dti-sli memorj', 
imd prosrammat k-Lill.v |ierriiitns llie VUi liming calil nation. 

Calibration Contributions 

The HP 89440A Ls one of the most accurate RF analyzers 
ever produced by Hewleti-Packard. Al room temperature, at 
any frequency within the 2-to-IISOII-Mllz mea.siirement band, 
and at any level from 711 dB below full scale lo fidl scale, HP 
8U-t40A level nieasiirements are typically accurate witliin 
±0.5 dB. Level accuracy is inipoitant because a vital applica- 
tion for the IIP 8!)44flA is true-mis jjower iiiea.'juiements on 
complex signals. Since lite HP 8!I441)A is a veclur signal ana- 
lyzer, the relative phase accuuacy (deviation from linear 



phase) is ;Usn hnpiatanl. To make accurate vector nieasui i-^ 
ments ;md perfonn accurate demodulalion of complex sig- 
nals, the relative phiise over narrow IVernieticy spans iniLst be 
accurate within a few tenths of a degree. TliLs amplitude and 
phase accuracy is acltieveil tlniJiigli extensive self-cuiil.i rati rat 
coujiled wilh an extensive factory characterization. 

Tlie HP 8S1110A self-calihralion routine calibrates both the 
IF section ;md the RF section. Since Ihe ciilibration of the IF 
section is identical lo llie c;dibralion iil'lhe HP8!I41I1A as a 
sejiarale instmmenl, only the HP S9440A RF .section .self- 
caiibralioii is dismissed here. Self-calibration can be set up 
lo occur automat ically at predetermined intervals to com- 
pensate lor temperature diifl. In addilion lo the calibration 
reiiuired for amplitude and phiise acciirticy, the HP StMlOA 
self-calibration performs many other ruiictions including 
source acetiracy calibration, I'roitl-eiid dc offset compensa- 
tion, trigger calibration, and first-LO feedihrougb nulling. 

Caiibrator Hardware 

yome jiails of liie HP S5I440A self-calibration require a 
precise calibration signal. An internal calibratiim sigruil is 
generated l^y taking a single hit frrmi i!ie source RAM, re- 
clocking Ihe muisilions. and clipping it to a precisely con- 
trolled amplitude. During the self-calibration the calihi-ator 



-'>4 Iii'ivnihtT I1i!'-I llrnlrtl-rm-knril.Iiiiiniiil 

©Copr. 1949-199B Hewlett-Packard Co. 



(n the receiver case, Bie oirefSKupled daa is genetgied by Simp(» nm performing 
ihe final decimanwi aoeradon ai The ouipui of the digital filters In itie source case 
this two limes OTEijamplea data is generated in software Dy inserting 3 sample 
pQpnt nf value lem tjeiiiveen each pare ot siDrefl data pomis Iliis simplistic inier- 
palaimn leaves a huge alias cnmponeni centered ai the original sample lete. now 
half the samnlB laie This is the siarOara inierootatiw imljigm as ssn m 
IftB trequBlcv domajn arid is solvMt try additional iillefirig live frgqusncy response 
tiKHen fat ttus filiei is a raised ct^ine in the Irequency dotnain This intetpolaiion 
tiller IS senaratE ftom the tesamplirg filter, which follows it. 

The source software also corrects the chirp and artmrery A-avefoms for the fre- 
queticv response errois Itoh amplitude anri phasel of The digital and anslng 
tecoJisif union filters The analog co'ieciiDn 15 for the nominal frequency response 
Inol indiyidually measured] ot the reEonstttiEiian fitter m IheHP89410A, thelilters 
in the HPa9440Aftl^ section arenoi cofrecied tor The lesampling filter also has 
amplitude errors which are cotrecied for 

The correction and oversampling are both perlonned in ihe frequency domain an 
overlapping blocks of data. This allows the use nt frequency -domain correciion 
data and makes the raised cosme Dversamplmg filler easy to implement The 
ouetiapping block approach conserves memory and removes si?e limitations from 
this portion of the signal processing Ihe source length limits are determined by 
die maximum siored data register length and Ihe size of the source RAIul. 

The source RAM also places anoihet interesting limit on the periodic chirp and 
arbitrary source wauaforms In spile of the arbitrary span capability of the ream- 
plrng process, the length of one period of The source, in sample points, 15 limited 
to an inieger number ot source RAM samples. This is because the source RAM 
follows the resampling rather than precedes it To assist the user in chirp measiite- 
menis, the instrument defaults lo a "chiip periodic" resolution ot span choices when 
the chirp source is on (this is easily defeated if necessaiyi This gives receiuei lime 
retards that are exactly the length of the source penod for true penodic chirp 
measurenients 

The pseudorandom soutce data is generaied Entirely In hardware using the source 
RAM as a Gaussian probability density lunciion lookup table, rather than as a 
source of actual sample points Because of this, it is nui corrected for frequency 
response errors other than the sin(ijit/Z|/ui[/Z cortection included in the analog 
reconstrjctipn filter Resampling is not available, so the spans are limited to the 



fador-Df-two choices implBnientea cy digital rKansiruciion filsis Thisisnol 
a majDr problem — the source span 15 chosen 10 include the receiver span 

The chirp end arbilrary source types have a single-shot capability in addiuon 10 
the normal repeating, periixfic output mode When this mo(fe is Chosen, ifie 
source IS toggeiefi by ihe fneasuremeni trigger This gives wily one smnce Durst 
for each rnssuiemsil, which lan be positiortefl relelrve to ihe measurement by 
adiuSting the trigger delay This is funcunnai in all bijl the source trigger made Ithe 
measurement is triggered by the source in this moOel so the analyzer can be se! 
up ID rrtalte a single-burst measurement condrtioned by an enteinal or iniBtnal 
irigger The sourw can be summed into an emsiing agnal. such as a TV wavetotm. 
aflewtrig a waveform to be inserted onto a particular TV line if Ihe appropriate TV 
line Ingger is input to the analyzer's aitemal trigger port One rninor artifact is that 
there is 3 variable laterrcy Iftpm z&ro to one source FIAW sample) between the 
tiigger signal and Ihestatt of reading out the contents of the soutcellAM 

Another ualuable. application-driven teaiute is ihei when the source is forced id 
the 0-to-!D-llflH; mode while the HP B9440A 2-io-iam-MHz receiver mode is 
engaged, the source outputs its waveform across half the vector span, which is 
equal to the demodulation span With the second input channel option, this allows 
direct Irequency response measurements of modulation systems, including phase- 
locked loops. In this latter application, the source can be used m inject an error 
wavelonn into a portion of the phase processing circuitry. A reference response 
can be measured at another place in the phase circuitry using Ihe second input 
channEl, which also operates at half span Hie actual phase can ihen be measuied 
with the RF receiver in phase demodulation mode with the instmmenl center 
Irequency placed directly at the RF frequency being analyzed. This can also he 
applied tp other modulation systems 

Acknowledgments 

Ihe source capability adds cortsiderahly to the measurement applications ot Ihe 
HP BSfllOA and HP B9440A A number uf people contributed a great deal 10 this 
Bucaptional tunclionality, notably Jerry Weibel, David Kelley, Charlie Panek. and 
Hoy Mason. 

DonHiller 
Design engineer 

lake Stevens Instrument Division 



IH iiili'niiilly rnnncTt.eri tn thp input chiuuicl ;iiifl the input 
iviiiains It'niiiiiatfil — no iiuiiiijiil cuiuiL't.'liorLS yif it'iitiirt'd. 

The caiibraloi' oiilijut level is calibrated at tite factory or 
fielii senir e center by ctjmpafins a known signal at llie in- 
put (siip|)li«i liy the facl.Diy or sM-vice t.fKt. station) with the 
calibrator signaJ. The difference is stored in noiivolaliie RAM. 

IF Section Vector Calibration 

Tlie HP WI4 !()A st'll-raliliratioii rimtine generates eompiex- 
valued coi recfion data for the IF section itipiil chatuiels over 
a dc-to-lO-MHz frequency band. These corrections are valid 
for mpastirenients made al (he IF section's input connectors 
when ih(> an;ilyzec is in Ihe dr--io-ll.l-MHz baselituitl receiver 
nioiie. Tlif correction data is comltined with ttic kinumi re- 
sponse of the digital filters to pnjvide complete calibration 
of both aniplitiido and piiase. 

The correction data is comptiled fioiii calibraliun data nb- 
tained by passing a calibration signal through the IF. The 
Ciilibralion sifjiiiil is H^'neraled rroiii a 25(>-bit liinaty Heqiience 
clocked at 25,(1 MHk. Tliis prodin ey a comb s])('clnini with 
spectral lines sijacctl e\i'r>' 11)0 kH/., each with knowTi en- 
ergy level and phase relative to Ihe source IriRger. Only 24 of 
the spectral lines are actnally ij.sed to niodcl llie riequency 
response of the IF. Uy hmidng ihe jiiiinbi'r of frequencies 



tnea.stired, the amount of memuiy rei|nired iii .siore ihe cali- 
bralion data is reduced. Al.so. the binmy sequeni e tlial gen- 
erates the comb spectjiun can be opliniined lo nia.'i inline rhi' 
etiergy level al the cabbration freiinencies, Ibereby improv- 
ing the SNR of the calibration measiirenienl , Tin- fi wiueji- 
cies measured are not e\'enly spaced, but were clioseti based 
on the chtiract eristics tjf ilie tillers to lie characterized. To 
compute coixeclion data, a .spline routine is used lo inleipo- 
lale between the calibration points. The calibration data is 
complex, so the real data and the imaginary data are inler- 
jmlaled sepiuately. Befoiv inleiiinliHion. excess jjhase caused 
by delay is removed from the ilala. Tliis improves Ihe accu- 
racy of the inl erjiolation by reducing Uie order of the data. 
In other words, the spline roulines are inleipolaling ti lower- 
cH'der curve. After interpolation the delay is reiiiiruduced 
into die correction dal.a. 

Wlien meastihng the calibradon signtd, sotuce triggcriiiii 
and time averaging are iisetl It.i reduce lite noise of tlie inea- 
surement. Alter Ihe fust me;Lsurenieiit of tfie calibration 
signal is made, tlie bit sequence is inverted (<jnes liecome 
zeros and vice versa), and another little-averaged measure- 
ment is made. These results are combined ajid i oiii])ai'ed tn 
Ihe known speetiiun of die calibration signal to jirodiicf Uie 



© Copr. 1949-1998 Hewlett-Packard Co. 



Detuniber lisa HtwIeti-Pat-kuril JuumuJ 55 



vector corrections for Hit' input cliannel al the specific 
rrcquencies genei'ated by the calibration signal. 

Tilt" vector corrrclions arc measured for each inpiil clianne) 
configuration thai is likely I o produce a difierenl frequency 
response. Six primary c aliljrations are perfonned. Four sec- 
ondary calibrations arc |)erfomicd on the LF section's 20-i1B 
attenuator and Kl-or-;3-dB amplifier The secondaiy calibra- 
tions are similar lo the primaiy calibrations except that tlie 
nieasinenienis are only made al one fretiiiency. The resnlla dI' 
the six [>riniaiy caMbrations anti the foui' secondarj' calibr a- 
tions are sloreci ui nonvolatile RAM and r;onibined in a vaiiety 
nf ways to compute a correction vector for each input range. 

)F Section DC Offset Calibration 

Each IF section input channel hits a dc offset DAC i hat is 
used to compensate for residual dc- offsets in the analog input 
circuit and in the ADC. The autozero calibration measures 
tlie residual dc and adjusts the dc offset DAC to nunimize 
the amount of dc nft'set. This calibration must he ])erfn!Tned 
for each different ennfiguratinn of the inpul ehatmel's active 
elements. Twenty dc oftsei calilxations are performed for 
each channel. 

IF Section Source Calibration 

The IF section's dc-to-lO-MIlz soiu-ce is calibruLcd by inier- 
iially conneciiii); the source to tile previously calibrated iii[)iit 
cliajiiiel. The uncorrected source is prugramined for a sine 
wave al 1.5 MH/ and the level <if ihe signal is measured. The 
resulting amplitutit- correction fa<ti.)r Is apjilied to .source 
levels entered by the operator. 

Tile source luis a dc offsel DAC lo provide uscr-selecled dc 
offsets. The gain ofihe lic offset DAC is measured and this 
correclion is applied lo ihe source dc offset value entereii by 
llie operator. The source dc offset DAC is alsfi usetl lo com- 
pensate for any dc offsets in tJie somre DAC ai^d associated 
analog c u cuits. The calibration routine finrls tlie setting of 
the source DAC that will produce zero volts dc at the output. 

Trigger Calibration 

Three se|iarate calibrations are required for the IF section's 
trigger cu'cuits. First, tlie trigger dc offset DAC is used to 
compensate for iuiy residual dc offsets in the trigger circuits. 
The callliration routine miLsi determine Ihe correct .settings 
for [his DAC. Since the tic offset of the trigger caimol be 
measured directly, il must be inferred from another meiisure- 
nienl. To determine the dc offset, the somce is programmed 
for a !-MHz sine wave and internally connected to the inpul 
<-haJiiiel. Input triggering is used lo meiisure the amplilude 
of the sine wave al Ihe I rigger point. Two measuretnents are 
made, one using a positive trigger slope anil one a negative 
trigger slope. If tliere is no de ofTset in the trigger then the 
two measurements will have equal amplitudes but opposite 
polarilies. 

Each of the trigger types (external, channel, and source) has 
a different amomil of delay relative to the input signal. In 
addition, there is a different delay for eac h receiv er mode. 
The Iiigger calibration routine must measm-e these delays so 
the correct trigger point can be detemiined. To measure 
delay, the cahbralor is progranuned for an SOO-kliz squaie 
wave and is internally coimected lo the input chatutol, Tlie 
pliase of tlie calibration signal is measm ed using each trigger 



type and tlie corresponding delays are stored in nonvolatile 

RAM. 

The partial -I rigger delay coimli'r allows the aiiiilyzer to (ie- 
lemiine Ihe c-on'ect trigger point oven if the trigger puiiil 
occurs between .^DC samples. The pajtial-trigger tlelay 
counter is based on an analog pulse stietchcr that must be 
characterized before stable triggering can be acbieveci. The 
two characteristics lhat must be determined are the mini- 
niitiii number Ihe counter vrill return frjr zero trigger delay 
relative to llie .sample c lock, and the maximum number re- 
tunic^d for a trigger with sbghtly less than one sample clock 
of delay. To ascertain these values, the internal source is 
programmed lo generate a sine wave and this signal is inter- 
nally ccmneeted to the inpul channel. The maximum ajid 
minimnm rmnibers are read from the [iartial-trigger lielay 
counler as the trigger point is moved relative to the sample 
clock. The I rigger point is a<l|uste(i by changing Ihe phase of 
the sine wave. The maximum and miniiiuini uunihers are 
used lo find tlie coefficients ofa firsl-oiiler equation and this 
equation is used to compute Ihe delay for any other counter 
value l eiunied. 

RF Section Vector Calibration 

The RF section calibration provides amplitude and phase 
corrections lhat aic \ alid for measiuemenls made al (he in- 
put coimector of the RF section when the IIP 8y440A is in the 
2-to-iaoO-MHz receiver mode. The HP B9440A self-calibration 
routine generates vector correction data for the analyzer's 
7-Mllz IK The IF section's input ch;iimei is consiilered part 
of I he IV for this calibraiion — the correction vectors pre- 
viously calculated for the IF section's input channel aie not 
used. 

The RF section vector calibration is alniosl identical to the 
IF section vector calibration. The calibration signal is inter- 
nally connected to Ihe RF injuit with the RF section timed to 
6 \lHx. The RF section W) adds an arbitraiy phase rotation to 
the measured data and the self-calibration routhie must de- 
tennine the amomit of phase rotation for each measurement 
Euid correct the data tiefore the results are averaged. Since 
changes in the RF allcnualor do noi aO'ecl llie frequency 
response of the IF, Ihe vector caliiiration is only performed 
at the Ul-dti attenuator setting (-20-dBni range). 

The RF section attenuators are calibrated by using the inter- 
nal calibrator mid the IF seel inn input channel to measure 
attenuation relative to the 10-dB at teniiaior setting. Only five 
attenuator combinations are measured and these measure- 
ments are combineil in a variety of ways to produce gain 
coiTeclions at all 12 attenuator settings, 

LO Feed through Nulling 

Two D.-^Cs in the RF section must be a^usted to minimize 
die amoiml of receiver first-LO feedtlirough. Minimizing the 
LO feedthrough l ediices low-frequency residuals and re- 
duces cUslortion problems caused by having multiple tones 
m the IFal low frequencies. The LC feedlhrough is mini- 
mized by a circuit that samples the LO and adds a small 
amount of Ihe LO to the signal in Ihe first IF Both in-phase 
and iiuaciraiure components of the LO are added. The D.\Cs 
are used to adjust the in-phase and quadratiii*e components 
to null the LO I'eedthrough. 



56 npi'crnber UlfW liewlpll-P^'kr-ird .kmmnl 

©Copr. 1949-1998 Hewlett-Packard Co. 



The calibration software aftfiists the DACs to prodiice a 
mininial amouiiT of LO fceiiihri.)iigh «ilh the RFseciion 
tuned lo 6 MIIz. A! this frpqufiii.-.v ihe LO fccdthrougli term 
shows up ai 12 MHz. requiring the aini-aJias Giter in ihe IF 
section to be disabled for the ralibratioti measuremeiil. 
Since the feedlhrongh term is the only signal present, alias- 
ing is ivit a conreni. To find the optimal DAt" settings. The 
calibration routine uses what is commonly referred to as a 
golden section search.^ The search is carried out first on 
one DAC and then on llie other. However, since (he in-phase 
and quadrature-phase signals used lo cancel ihe feedthrough 
term arc not in perfect quad rah ire, there is a certain amount 
of interaction between the two DAC's. Several iterations are 
required to find the optimal DAC settings. To minintize the 
search lime, the DAC settings obtained tii the previous 
caliti ration are used as a starting point in the searcli- 

EF Source Calibration 

The RF source is calibrated by internally conneciing the 
source to the RF injiii: aiid nieiisuring the uncorrected 
source level over the ISOfl-MHa frequency range. The result- 
ing correction factors are stored in non\-olafile RAM and are 
applied to the user-selecfefl source level. 

RF Section Factory Calibration 

To achiei e a +l).5-dH typical accuracy, llie flatness of the RF 
receiver must he extensively characterized at Ihe factory 
and at field service centers. This i.s aceomplislied by using 
the lest setups shown in Fig. 'S. In the first steji ( Fig. 'ia), the 
tracking and flatness of the power spUtter and ealjles are 
chaiacierized by conneciing it second power meter channel 
to the end of the Test cable. The signal generator is pro- 
grammed for each calibration fretiiiency and tbe difference 
between t he reathngs of ihe twi: power mcler cliannels is 
stored. This step transfers the flaltie.ss of the secmid power 
meter channel to die test setup with very little degradation. 

In Ihe second setup (Fig. 3b), die test cable is connected to 
Ihe KF input ajid the gain of the RF section i.s measured at 
each calibration freiniency. The level of Ihe input signal is 
measured by reading the power meter aiifl correcting llic 
reading using the stored results from the iirsi step. The in- 
put to the IF section (which is ;dways at tlie same frtHjuency 
and at nearly the same level) is measured with the HP 
WI440A in dc-to-l()-MHK receiver mode. The ratio of the in- 
put signal level and the IF signal level is Ihe gain of the RF 
section. Tiie absolute accuracy of tlie g.iiii nieasiu-emen! is 
not important since only the flatness of Ihe RF secfion is of 
interest. The actual gain of the RF section is calibrated by 
the self-calibration de.sciibed previously. 

The RF flatness is measured for all attenuator settings of the 
RF section atid the results are stored in nonvolatile RAM 
within the RF unit. In all, over 1801) calibration points ai'e 
stored in the RF section. At [lower-up, Ihe IF section reads 
tbe RF section calibration data mid uses this data lo correct 
level measurements. Storing the RF calibration data ui the 
RF section allows any IF section to operate with any RF 
.section. An HP In.stninient BASIC program, nmning in the 
HP «iM.!UA, allows service centers or customers to pertbrm 
this calibration easily. 



HPBSMOAVeclor B UG Cable 

Signal AuItmi 




Exumal 



Signal Geneiatni 



SsnnrC Ninwl^llDAdapter Type-N i:abJc 

I 1MBAtlenuatoi-'-ijn|i-J5E!n Type- N Cahla 
— PawerSplitter 
Power Outpui 
, Sensor #1 



PawrerMsiei 



HPBMMAVeclor B UG Cable 
Signal Anefyier | 



|u — H w 




1 


0 I 




* 



Exlarnal 
Reference 



,F Signal Generalnr 





_, -,J -J 

. JJ -r 

' jj _i 

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J-'-— ^.-^ 

1 £> 

JJ JJJJ— 'T 



Type-N Gable - 

Output 

ID-dBAttenualDr'' Input Type-N C abia 

- — Power Sphttei 
Power n . . 

Sensor 



Power Meter 



Outpui 



Fig, 3. ScliipK fur charHct.eri/J(ig the receiver tlalnsss or I In- HP 
Si)44l"IA RF section, (a) Snlup for L'hHractoriziiig the trai 'Icing and 
DHtiii'ss rifltie puwer splitter unci cahlre, (10 Setup fur tneasuiiiig 
Ihe aiiiu (if tile RF .wtitUin us a fimiitinn uf fiequwicy. 



Performance Verification 

Tlie factory calibration, coupled with the aiit.onialic self- 
(.■alibration, produces an instmment with jierfomiajice that 
can be very difficuU lo verify, Tlic aniplitiitle accuracy and 
IFflamess of ihe HP 8!I44(IA are verifieil using a met bod 
identical lo Ibe factory RF cahbralion shnwn in Fig. :Jb. 
Usuig this rnelliod, the HP 8944(IA's measured level accuracy 
at rooni lemi.ierattire conditions is typically better than the 
measurenieni unceilauily. 

To veriij- the vector perform an ce <jf the IIP 8fJ44tlA, Ibe devi- 
ation from hnear phase (relative phase error) within one IF 
bandwidth must be measiu-cd. Ideally, we coultl ciilculate 
the worst -case relative phase error from tbe worst-case IF 
flatness because the IF con'eci ion data consists of complex 
vectors. The amplitude and phase corrections are tiol inde- 
pendent and any phase en'ors would have corresponding 
amplitude emirs, hi practice, however, Ihe scalar- RP llal- 
ness calibration fiata is also used Lo apjjly second-order cor- 
rections to the IF levels. This means that, it is possible for 



© Copr. 1949-1998 Hewlett-Packard Co. 



rifr-iniilH'r imHi'wtrll-l'ae'kanl.lniirrial 37 



Vector Measurements beyond 1.8 GHz 



Ihe HP B941DA and HP a9440A veclnr signal analyzers provide urpreceilentBil 
analysis tapabililies for lookma at cnmplei signals Driven by a need fo'tnore and 
larger spectiitm iequiremenls, many communJcaTion schemes are moving tn liighar 
frequencies The HP 8341 1 A provides a way 1o apply Ihe powerful analysis capahili- 
lies DfitiB HP Ba410A to signals thai lieabnw 1 BGH/ II doesnaiha^e tlie level 
gf inlegialion or the complete set of leaturBS providEri by the HP BMOA, but it 
does allnw a user lo view ani) analyse signals above 1.B GHz as ihey have prob- 
ably navEi done before The HPB9411A piuvides Uiis capability by translating Itie 
atixihary IFIiniermerliaie frequency} Dutpu I from oneof a number ol RF and micFfr 
wave specitum analyiers to a center frequency within the analysis range of the 
HPaS'llOA It will directly translate the IF outputs of analyzers such as the HP 
710DD Series and HPaSBSA/B. CiisiDmerswho already awn rjne nl these analyzers 
can now ertend then instrument's capabilities by combining them with the mea- 
surement capabilities of the HP In addition, an integrated solution can be 
consirucled using features of tlieHP89410A such as HP Instrument BASIC, HP-IE 
(IEEE 488, lEC 525) comraiiBr capability, andbuilt-m firmware features describing 
fidlemal down -converters such as the HP 8941 1 A. 

The HP B941 1 A IS fixed-frequency down-convenai that translates a band of signals 
up to 7 MHz wide from a center frequency ot 21 4 MHz to a hand centered at 5.B 
MH; As the block diagram, Fig 1 , shows, it does this in two frequency conversion 
steps and provides conversion gam and image filtering for the signal band of 
interest Its internal oscillators are phase-locked io an external ID-MHz reference 
Irequsnty !□ allow high -quality magnitude and phase measurements on a variety 
of signals The HPa94nApackagehas the same footprint as the HP 89410A soli 
can be stacked directly below it. 

The HP 89411 As pBrformance goals were aimed at making it appear largely invisible 
to the user Its broadband nuise, distortion, spurinus, and phase nuise perfor- 
mance are similar to ihe HP a94inA's. In a typical system the overall performance 
will be determined by the RF ot microwave spectrum analyzer 

To support the HP 89411 A as well as other down -converters, several firmware 
features are incorporated into the HP B941 DA A menu under the instrument mode 
key allows the user to dehne some of the attributes of an external receiver such 
as the HP 89411 A. These attributes include the tuning range, IF bandwidth, display 
"mirroring", and enabling of HP-IB control When theffiiternal receiver mode is 
selected the instrument's x axis and markers are labeled with the actual mput 
frequency Selecting the mirroring function instructs the HP 8341DAid mirror or 
flip the specitum display Tins capability is provided to undu the mirroring that can 
occur as a result of the mixing scheme used in some RF and microwave specttum 
analyzers The HP-IB control capability is diractiy compatible with all HP 71 DOG 
Series spectrum analyzers and with the HP 8566A/B When changing frequencies 
the user simply enters the new center frequency and span on the HP E941 DA The 
HP B94inAthen checlts to see that the parameters are rtot out of range for liie 
defined external receiver setup, and then can optionally issue HP-IS commands to 
Tune the RF or microwave analyzer to the desired frequency The user can account 
for conversion gam differences in various setups by applying trace math to the 



I1PS9410A Vecloi Signal Analyser 



Ext 

SouicB Trig Ch I Cti 2 HP-IB 



Ref Out 



IF Signal 



S£MHz 



HP89411A 1.B'GH: Down-ConVBrtBi 

Dulput 


t 


V 

Ret In 



HP 71OO0 SpBcmim 

a-^ ic HP7090CIB 

°tS ^Si LnGal 

5:1^ tu. OscillaWr 



2 u 

^ E O 



0- ov? O- ^ 



Jt^. — Signal ln)iul 



Fig. 2. Cannedf an diagram for fWodijlar Measuramant Systairi hookup v<ilh BnHP7tOQD 
Series spactninr ana Iper, 

measured results If more complicated control functions are needed the HP 
B94iOAcan be configured as an HP-IB bus controller, and with an HP Instrument 
BASIC piogtain, an integrated down-conversion system can be constnjrted. The 
instrument connechons for such s system are shown in Fig 2, 

Acknowledgments 

The author wnuirt like lo thank those people who coriributad io the deveiopmeni 
of the HP 89411 A Thatch Hatvey did all of the mechanical design and kept track 
of many other details dunng the course of the project Charlie Potter managed the 
project and provided guidance and direction Ihe HP B9411A had a short develop- 
ment schedule and because ol this, much of the design was leveraged from uUiei 
products, in particular the HP B944DA RF section I woulrl Ilka lo thank Jim Pielsch, 
Tim Hillstrom, Bill Binder, Roy [^ason, and Julie Wernet lor theit assistance 

Joe Tarantino 
Design Engineer 

Lake Sffivens Instrument Division 



21,qMHz 
-20 dBin >\ 
I Rear Panal) 



low-Pasi 
FN ref 



OlQlBdB, * 
5-dB Steps 



■ 


Bandpass 




filloi Center 




FiBquBiicy ^ 


■I 


«5EMH; 


1 






Heference 
X Unlocked 
— ' Indicator 



10-MKi 
Low-Pes 
Fillet 



► 



S,E MH; 
-lEdBm 
V Nominal 
(From 
Panell 



10 MHz 



Circuils 



40MHz 



ia-MH; Reference 
In IRear Panel] 



10-MHi Heference 
□ utlRearPanBll 



Fig. 1. Blank ffiagrsm of the HP B9411 A 

down-cDnvenei 



58 Ufi-pniljer l£ttl3 HewltTtt-Parkaril Julinial 

©Copr. 1949-1998 Hewlett-Packard Co. 



the analyzers [F flatness to be significanily better thaii the 
measured phase acciuacj- would iniply. 

To measure tieviation from linear phase, a lest signal is 
needed lliai has three or more sjiei'iral components « ilh 
kiiomi phase relationships lielween Ihem. Since HP fifl4-4(lA 
phase measure me! lis lia\e arliitrar>- delay and ofeet lemis, 
ihe phase of a single (mne and the phase relaiionship be- 
tween two (ones are arbitrary'. Howe\er. [lie phase differenre 
between two tones relative to the phase dirferencc between 
one of these tones and a third tone is not arbitrary. For ex- 
ample, suppose ibe IIP 89440A is used to mea.siire aii amph- 
tude mofhilaied carrier. The tiifTcrence between the r.arrier 
phaw ajid ilie pb^we t)f llif upper AM sideband should have 
the SiUiie magnitude as the difrerenee between tiie cairier 
phase and the phase of the lower AM sidebani. but Ihe op- 
jjosiiesign. If the source has no incidental PM, then the siim 



of Ihe upper and lower sideband phase differences relative 
10 (he carrier phiise is a measure ottJie analraer's deviation 
from linear phase. A nielhoii similar to tliis is use»l at the 
factor>' to measure the deviation from linear phase of tlie 
HP S9+40A's 7-MHz IF baiidw idlh- 

Arkn ti w ledgments 

Other memben- of ilie design team were Gene Obie, who 
designed ihe 10-MHz and iHHi-MHii references, and Dave 
Rasmussen, who designeii ihe [jower supply and processor 
board and decelojiei! ihe itp section rirmware. Kric Wicklund 
was the project manager, aiifl Julie Weniet was our tireless 
project coordinator. 

Reference 

I. W, Press, ei at, .\'iimeiirul Recipes in C. Cambridge UniVBrsily 
r'ress.1988. 



© Copr. 1949-1998 Hewlett-Packard Co. 



Optical Spectrum Analyzers with High 
Dynamic Range and Excellent Input 
Sensitivity 

The diffraction-grating-based HP 71450A and 71451A optical spectrum 
analyzers provide the basic spectral measurement of optical power versus 
wavelength and advanced functions for measuring and characterizing 
LEDs. DFB Lasers, and Fabrv-Perot lasers. 

by Da\id A. Bailey and James R, Stimple 



I'lie telecommunications industry is one of the most lively 
and liilereBtinf! areas of Ihf c!et' ironies indiislry loday. Tlie 
development of higli-pcrfoi'inanee filit'i-oplic sysleins re- 
quires tiie ultimate perfonniuiee of eoniponenis sucti as laser 
sources, llliers, ii|)tical anipliilers. and receivers. Aecuralely 
ineasunng Uie [lerrornianre or lliese roiiiponenis and eoii- 
iirming Iheir opera! ioii in llie syslem is esseniia! lo prove 
tiic deMign. The optical spet'lnnn analyzer is one ut the most 
valuable tools for maJdng these nieasiu-ements. Tlie HP 
TMfiUA and 71 15IA optical spectrmn anal>'zers are designed 
to make spectnii measurements in rhi' laboratory and in a 
producl ion en^ii-onment. The HP 71451A optical sijectnmi 
analyzer is sliown in Fig. 1. 

Both analyzers can make spectral measurements between 
6(11) nm and 1700 nm on LEDs. Fabry-Perot lasers, distrib- 
uted feedback lasers, and erhiimi-doperl Qbor amplifiei-s. 
Those basic measurement capabilities ai e described later in 
tills article. A new liouble-pass nionorluomalor enables the 
analyzers lo provide the iiigli d.vuamic ranjie of dotible- 
monochnmialor instnirneiit.s (5ri dH at am Irom the peak) 
ajid the sensitivity of single-miinoehroiuaior iii.stniments 



fbetter than -90 dBm). Pig. 2 shows block iliagrams uf tlie 
IIP 71450A and 7145 lA optica! speclnim analyzers. 

The HP 714.')1A optical sjieelrum analyzer offers measure- 
ment capabilities thai go beyond basic optical spectral niea- 
sm einents by providing four measurement ports: mono- 
clnomator input, photodeicetor input, monochroniiitor 
output, and transimpedance amplifier input (see Pig. 2b). 
These ports allow five diflereni modes of opcraiion: 
■ OpticLiI sped rum analyKer mode. This mode provides basic 
optical speclmm analysis with precise amplitude accuracy 
and less than 0,5 ilii iiolarinatioii seiisil i\ity, 

• Preselector mode. This mode allows front-panel output of 

that passes Ihroiigh the monorhromator. Wavelengdi 
di\"ision muliiple.ved channels, inrii\idiial modes of Fabry- 
Perot lasers, and selected width.s of I.P'l )s or white light 
sources can be outpnt on f)2-um fiber Ibr further use or 
analysis. 

• Sljnmli&response mode, VVlien broad spontaneous emission 
liglit is applied to the moi;ochromaior input, (he mouocliro- 
mator output becoiiu's a varial>le-wave length source. The 
user can |iass the lii!bi through a device or filler. ;uid then 




fid I ii'i i'iiiljcr l!iy:t lltwlult-f'aclrartl.tLMininl 

© Copr. 1949-199B Hewlett-Packard Co. 



LUNI 


DouUv-Pau 




rnpul 







TnnsimiiEdBnce 



DptiBnBl 
CuRem SouicB 




i 



Is) 



Light . 
Input 



Ddu hie- Pass 



Optional 
Currant SoursB 



TtantlHi Switch 

0 0 ^ 



MonochroniBlat 
□utpui 



I Photg detector 
^Inpiit 



TiansimpedancE 
Amplilier 



Hi 


H 








Conliol 



Trims imped a nee 
Input 



Fig. 2. fa) Block diagram of Uie 
HI' 7I450A uplical spei:lrujii ana- 
lyzET, (li) Bti"'k dUignirii (irilie 
HP71-151A. wtiiPli fxt.'nils ihe 
capabililieE of the HP 714FillA. 



reimetl. it into the plujlodetei lof iripul: for analysis. Filters, 
fibers, amplifiers, isolators, swilehes, and other components 
can Ijp fliaracterizpd tisiiig this iimdi'. 

• Power meter nioiie. This mode oFiers direct access to the 
photndeterUir. In this innde, a tjiice of average [jowcr versus 
time is ilispiayed, allowing the user lo record any amplitude 
change over time or monitor amplitude while ai^iustmetits 
are made. LoJig-lerm diil'l can also Ix- monilored. 

■ Pho(oiietetl(ir mode. This mode is similar to (he stiimiliis- 
respon.se mode excejil lhai the tli'Vice under le.sl (1,'llT) is 
an optieal-lo-eleclrieal coniponejit. By comimriii); llie cali- 
brated response of the inienial pholodiode with (he mea- 
sured response of tlie Dl'T (via the iraiLsimpeifiiiice input), 
respotisivity versus wavelength can be calculated and 
displayed. 

The rest of this article dcficiibes the user inferlace and tJie 
advanced tncasiiremeiu pt ogt ajLis providefl with tiie IIP 
71450A and 714I51A <iptjca] specfnun analj'zers, Otiier articles 
in this issue descriiie the design and inipiementalion of the 
coniponejils in tiiese analyzers. 

User Interface 

TIte iLser hiterface of the IIP 7H50A £aid 71451 A optical 
specinini atial.yzers is designed to have the same look ajid 
feel ;t,s HP's KF M.rid jiiicrowave speetnim analyzers, Tiie 
histninieiit fuiu-iions are selected ftotii the Inait |iaiiel via M 
Boflkcys and 15 haid keys. The only obvious differeiiee lic- 
l.ween optical ajid KF and microwave instrtmient.s is lhal the 
signal information is displayed in wavelength I nanometers) 



for optical inslTuments ajid I'requeni y (Hi!) for RF or micro- 
wave insti-uments. Displaying sigttal infonnation as a function 
of wavelength has always been an optical tradition. 

Menu Keys 

liver 251) instniment funciions are availabh' from ihc 14 
keys located on Ihc sides of the insininieni display (Fig. '•i'). 
These runctii.ins arc grouped into seven measmemcnt cate- 
gtjries called llrinkeys. which are always displayed on the 
left side of the display. Pressing a fimikey accesses a group 



FitmliBys 



; ^^:3l-S3 Jtw n i593 



niplri 
larkf r 

TricBC 
ilite 



<sch; 


-5E 
d8/ 














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1 



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fPBK 



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ficnsunt 



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Fig. 3, All e\ani]iki uf Ihf anftta'.vs anil Umikpys llial ap|i|iiu' 1 1 it' 
I'riiiii jjiiiii'l (il"tlM> II!' Tl-l'iiiA iiiirl TH^ilA ^jiieiinitn aimlyKKrs. 



© Copr. 1949-1998 Hewlett-Packard Co. 



Defemljet ^^M^ lti-ivlL>l.l-l'Qi'kHrilJ(jLimal 61 



Optical Spectrum Analysis 



Dpiical sfiecirjm analysis (s tlie measuremBnt ot tiplicsl power as a turciion at 
wauelarglh Applicaiians include testing lasEi and LED lighl sources foi snettral 
puniy and puwer disliibuliDn, and leMing the transmission characterislits dI 
optical devices 

The spectral Width ol a light source is an important parameter in fiber-optic com- 
municaiion syslertis bacause rtt chramaiic dispersion, iMiicii cKUrs in the fiber and 
limits the modjiaiion barrfwidih cf the SYStem. The effer:! nf clirDmaiic dispaision 
can be seer in tlie time domain as puise broadening of llie digilai mtormetion 
wai/etotm Since cliiDinaiic dispersion is a tunction of the sqeciral width of the 
iight source, narrow specitai widlbs are des i rati le for high-speed commonication 
systems 

Fig 1 shows the spectrum of a Fabty-Perot laser The laser is roi purely mono- 
chromatic. Its spe::trum consists ot a series of eveiiiy spaced cohateni spectral lines 
with an ampliiude profile determined by the characteristics cf the gam medium. 

Dptiiai spectrum analyzars can be divided into three categories: rtiffraction-gratmg- 
based and the Fahry-Perot and Michelson mterteromeler- based optical spectrum 
analysers Di ft raction-gra ting-based optical speclnim analysers are capable of 
measuring the spectra of lasers and LEDs The lesolutmn ot these instruments is 
vaiiable. typically ranamg Irum 0 1 tim id 5 oi ID rim. Fabry- Perot- rule ([eiomaier- 
Based optical spectium analyjers have a fixed, narrpw resolution, typically specified 
in frequency, between IDO MHz and ID GHz. Ttiis narrow resolution allows them 



cue 
Sn Of* 

On air 

ii;s:nis 
On nil 



h-L- 

Ur mi 



111 J 



• 9in .ivpU ^ W%! Jl nt rUHrl - 1,61; n« 

piih Hiviiii ■ Wi{ n lit i>ttb sM ' -a.iiB aui 
vDda cpiclpg-^ J. I? nil liiVfl po-ii' ■ ^Sh 



SiBtLi 





(-oil. 


■HJ 












■3 f* 


J 






























if 


















if 




It 












vmm 






_ 






































































































3 rii 






■S 





ELIEEP 

COHl 
SUEE" 



iimisH 



Wavelength 

Rg. 1. Optical spaclruin ot a Fahry-Peiot lasei 



Tunable 
Bandpass 
Filler 



Input 



PholD- 
OeteiMr 



TranslmjieilBncB 
^^Amplifier 



la-Oisi<Bl 
Caiivafin 



Vertical 
Position 



Wa¥e!eii[th 
Tuning 





Horiionlal Position 



Fig.Z,Simplifiad blDch diagram ot an npllcal speclrum enalvTer 

to be osed far measuring laser ctiitp, hul can limit their measurement spans much 
more than the diffractiun-graling-basad optical spectrum analyzers. Micheison- 
mtetferomeier-based oiitrcal spectrum analyzers, wfitch are used lor direct 
coherence-lengih measuiemenls. display Ihe spectrum by calculating the Fourier 
transform of measured mierfetence patterns 

TliH HP 7145[1A and 714S1A optical spectrum analyzers are diffraction -gratmg- 
based 

Basic System 

A simplified block diagram of a gtaiing-based optical spectrum analyzer is shown 
in Fig 7 The incoming light passes through a tunable- wavelength optical filter 
Imonochrnmaloif which resolves the individual spectral components. The photo- 
detectpr then conyerts the optical signal to an electrical current prppnninnal to 
the incident optical power 

The current Irom the photodetBctor is r.onverted to 3 voltage by the trsnsimped- 
encB amplifier and then digitized Any remaimng signal processing, such as apply- 
ing correciion factors, is performed digitally The signal is then applied to the 
display as the vertical, or ampliiuda, data A motor roiaies the ditfraciion grating , 
tuning the wavelength of the optical filter The angular position of the diffracnon 
grating determines the Itoozontal location of tile trace as It sweeps from left tn 
tight. AiiacB of optical power ye rsus wavelength results The displayed width of 
each mode of the laset is a function ol Ihe spectral resolution of the tunable- 
wavB length optical filter 



of fimctiotis rallr?d softkeys, which are riisplaypd nil IIk^ 
right side cif the ciisplay. Pressing Ihe MORE siiftkey ilis|>lays 
ail aciditionaJ set of softkeys. Tliese aciditioiial keys tend to 
be Llio keys used less often. Thi.s meniL tiee stnicttire ailow'H 
easy access to all of the instmnient functions. 

Hard Keys 

To allow easy access lo fmictions liiat are iiseii most often, 
tlicrt' are 15 hard keys on the from pajiel [see Fig. 4). Tliese 
ftinciions control swr'cp vv'iivck'iigths. resolntion, iprerciice 
ii'\'ei. ;ind marker piisiliujis. Tlirei' limctions liave been aiiderl 
that aie not found on a niicrowavc s^])ectrmn analyzer. These 
riinclionsare aulomeastire lAUTDHflEAS key), sensitivity 
I SENS key), and aiitoalifin I AUTO ALIGN key). 

Automeasure. Wlien I hi' AUTO MEAS key isjin'ssed, the iijial^'zer 
searches llie frill wa\elcngth spim and locates the jjirgcsi 
detected signal. If a sigiuil caiuiot be foimd, the sensitiv ity is 
increased and the search continues. Unce a signal is found, 



it is jxjsilioned rm the screen by adjusting the center wave- 
length, sensitivity, and reference level. Tlie signal widlii is 
also mea-stired and the span is reduced so thai iuosl (or ail) 
of Ihe signal jwwer Ls displayed, Tlie ajn|)li1urle scale is set 
to 10 dB/ilivisLiin. 

The user can modify this operation by selecting a wave- 
lenglli span ;ind a final aniplilnde scale to he used upon 
compieiiiin ol'ilie automatic measurement routine. If multi- 
ple signals are present and (he signal ofinlei est is a lower- 
level signal. Ihe user can position the marker on tliat signal 
and the automatic measurement routine wil! aciiuire tlic 
peak closest to the marker. 

Sensitivitv, The SENS key is used to adjust the sensitivity of 
the insti iiitienl. Normally the opiical s|ip('iniin ;uialyzer autrj- 
maticiilly selec'ts the greatest sensiti\it.\' that does not affect, 
tile sweep speerl. The aensiti\ity function allows the user To 
select the sniallest signal amplitude to he ciis"piayed across 



B2 Ui-tfinlicr ItiaafloHli-lt-PackiinJ.tounird 

©Copr. 1949-1998 Hewlett-Packard Co. 



OPTICAL SPECTRUM AtUALrZER 





SPA! 




r NORMAL^ 

1 on/offJ 










r START H 


STOP 


LiU 


1 fEAK 1 

[search J 






GED 




' «m> 1 


flUTO 
ALIGN 


nn 

IcenterJ 









^^^5^^ Coniliiction Band 



i1g.4.ArepreseiLMtlun oTUieliai'd key-son Uie frtail panel of the 
HP 7I450A and TI451A speL-lruiri aiuiiyzers. 

the current wavelength range, hicreasuig the sensitivity 
caiLSPS low-level signals to rhaaige the amplifier gain. These 
gaiji changes requii'e pauses, which decrease I tie HWfep 
speed. An increase iji sens!ti\ity may also rerjuire a nar- 
rower \ideo bandwidth filter, which will also .slow the 
sweep speed. Adjusting the reference level to the highest 
signal level to be measured and the sensitivity to tlie lowest 
signal level to be measured will optimize the sweep speed. 

Autoslign. .-yigimienl rit [he optical spectnim anE^yzer is easy 
to perform, ttlien the AUTO AUGN key is pressed, I he oplical 
spectrum analyzer auromatically ad,jusis l.lie niec'haiii<'al posi- 
tion of the optical output fiber to en-sure amplitude accuracy. 
No manual ad,iustments are necessary, and the optical spec- 
liiini analyzer cmi use an input signal of any wavelength for 
alignment 

Advanced Measurement Programs 

The HP 714.')0.\ and 714.'ilA riplical spectnini analysers 
provide the capability to (io\vTiloa<i iuui execute custom pro- 
j;rmn.s, which are called advaJii'ed measiiiement jirograias. 
'Hiese prognmis provide one-bullon niea.tviremenl sniullons 
withoul nsing an exiemal cnmpuler. The programs fan lie 
downloaded from a disk or memory card anrf storiod in Ihe 
analyzer's nonvolatile RAM. They can be rim by pressing Ihe 
USER finnkey and Ibeii .selecting ihe displayed soflke.y. They 
can also be accessed remotely via the HI'-IB ijiterface. Three 
advanced measiiremeni programs are supplied with the opti- 
cal spectnrm analysers. These prDgr.ims automatically mea- 
,sure Ihe following light sources: 
Lighl-erniltlng diodes (LEl).'4j 
Fabry -Perot lasers 
Distributed feedback lasers (OFBs). 

Light- Emitting Diodes 

Lifibi eiiiilting diodes produce lighr with a wide spectral 
widlli. When u.sed in liljcr-oplic communication systems, 
they can be modulated at frequencies up to about 200 MIIz. 
LEDs have t!ie advanlages of low tem|ierature sensitivity 
and no .sensilivily iii liack reOeclions, Additionally, [he inco- 
herent emil ted light is not sensitive to optical interference 
from reflections. 

A light-emilling diode generates liglit by .sjjontajieous emis- 
sion. This occurs when an electron in a high-energy coririuc- 
lion biuid changes to a low-energy valence b;md ( Fig. ii). 



RBCombination 



T 



Photon 

Direciion Bandgap 

i 

Fig. 5. Siiiiiiliiiieous i-missitni. Musr i.'I<.'Ctfijtis iii-ivi-' from die 
I 'JiKiiicl 11.111 hand to tiic valciii-c iiami dtiriiig ri-™inbiiiiUion. 

The energy lost by the electron is released as a phoion. For 
a gi^ en material, discrete energy Ic els represent tlie tlifler- 
ent orbital states of the electron, Tile energy of the released 
photon is eijual to the energ.v lost by tlie electron, and the 
wavelength of the emitted photon is a limction of its ener^iy. 
As a result, the wavelength of the phoion is detennined by 
tiie maleriai used to make the LED, 

The spontaneous emission is caused by the recombination 
of electrons ft-om the conduction band to the valence band. 
The difference in energy between the conduction band and 
the \ alence band is called the baiidga]i energy (E^) and is 
expressed in imits of either joules or electron volts feV), The 
wavelength of the einilled pholun is determined by Ihe 
bandgap energy. The wavelenglii is expressed as: 

>. = he/Eg = 1.24 tun/Eg, 

where h (Planck's constant) is equal to li.&i x 1()-''^ Ws^, c 
(speed of light) is 2.998 x 10" m/s, and Ey (bandgap energy 
of the niaierialj is expressed in units of joules. 

Tile conduction-tiand electrons are generated by a forwan.l 
bias placed on the p-n jimction of the diode (Fig. 0}, The 
material on Ihc n-layer side of the junction has immobile 
positive ch;irges evenly distribuled throughout the layer, with 
mol.iile ncfjative charges, or electrons, lesponsilale for elec- 
trical current, flow. <.'onversti,y, Ihe material on Ihe |j-layer 
side of tlie jimction has immobile negative charges evenly 
distribuled throughout the layer wilh mobile positively 
charged holes, actuidly locations of nii.ssitig elecli'ons, 
responsible fur eieclrical eurreni flow. 

At the junction, the mobile electrons from the n layer and 
Ihe mobile holes fi'om the p layer recomhine and produce 



o*o'o 



OOO 

-0-0 



PhalDns 



\ 



ImniDliile Negative Chaiges 
Immobile Positive Charges 



Fig. It. Diagram ufn rorwvird biased p-n,|iuirtii;ii shoning Him lin';irioii 
111' nuiiKibitr uharRi'a ami muljile dirreiii charges. 



© Copr. 1949-1998 Hewlett-Packard Co. 



Dpt'cmliiT ll)!l:! HciA'li'll-rai'k.'-ii-Ll ,loiini;il ()3 



photons. While LEDs in use today consist of imilliple layi-rs 
or scmiconiluctor material, tlie light -geniTat ion process is 
the same. 

Tiie spectrum of a light-eniilting diode results in a broad 
dislriliulion of wavelengtlis t*eiit.eri^d about Ihi- wiivi'li'nath 
calculated by [he above ef|ualion. The .spectral width is 
often specified at tlie half-power points of the ."ipectrum. oi- 
FWHM ( lull witllh ai half miL\imiini I |i(jinl.s. Topical values 
for lull will til al half nia.\imunt range from 2(1 lun to Bfl luu 
for LEDs. 

LED Advanced Measurement Program 

The LED advajired lueasuremeiil program automatically 
measures many LED paiameler.s. Some paramctf rs such as 
mean wa\elength and spectral wifith aie measured by two 
metho(is. I ine method taltes into accoimt the entire sjtei-- 
Irum. while llie otiier takes into accoimt only a few points of 
the spectrum. .All the mcasureme iil.s are made al the end <if 
each sweep. Fig. 7 shows the nieasurenieiil display provided 
by the IIP 71450A and HP 71451A LED advanced nieiisiire- 
ment program, The following is an cKpliinaliou of each uf 
the values computed by [lie LED advanced uieasiirciiicnl 
progiani. 

■ Ti ilid i.iowcr. The total power parameter is the summation of 
the power al each trace point between two nscr-selcclcd 
poiju.s, iiiirnuilized liy the ratio of the trace point spacing 
and lesohilioii hiuidwidlli. This nonuidiKutioii is rwiiiircd 
because the specti^um of the LED is continuous rather than 
containing discrete spectral components as a laser does. 







itunni •- 
tmi . 


lC5 Its. 
B31.?5 n. 3 dB 


-IIIIPI 


n. 


as 11 


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31.3? n. 


ph d^nn \ Int )- 


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-H? dBi 


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■IK 



SltllLt 



COKI 

CEKICfl 

SPAN 



I mm 



I n I ■9'- 1 



Mean tWHM 



MBBn|3dB 




Peak Wavelength 



—FWHM— J \ 



ToibI Power 




Fig. 7. la) The apeclruin of a ligtu-eniitlitig diode. CbjTlie parameters 
proviilfd by the LED autumaliu measureiiienl pragtain. 



The total power of an LED being tested is deleJiiiined by the 
equation; 

N 

Total Power = P„ = P, fTS/RBW). 

1= I 

where RUW is Ihe resolution banilwidth and TS is the trace 
point spacing. 

Mean FWHM. Tlie mean wavelength of full width at half 
iiiaxirniirn jioints represents the center of mass of the trace 
[luiiiis. The power ami wavelengdi of each trace point are 
used to calculate tlie mean FWTIM wavelength: 

Mean FWTIM = X = Pj (TS/RBW) 

1 = I 

Sigma. This [neasiirempnt is an nns calculation of Ihe 
s]jpctral width of an LED b;iscii oil a (.iaussimi di.stnl)ution. 
The power and wavelength of each trace point are used to 
calculate sigma. 



Sigma = o = y S ITS/HBW) - kf/P„. 

Sigma is also used lo calculate the distribution trace 
(described below). 

' FWHM (full width al half maximum). This measurement 
describes tiie spectral width of die hali-power points of tlie 
LED, nssmiiing a continuous Gaussian ]jower distribution. 
The half-ptjwei' points tuc those j.ioints where the power 
spectral density is one-half that of Ihe peak amplitude. 

FWHM = 2.355 X Sigma. 

3-dB width. This value is used to describe the spectral width 
of an LEU based on the scpiiralion of Ihe iwo wavelengths 
thai each have a power spectral density equal to one-half Ihe 
jieak power spectral density, 'flie .'J-dB width is determined 
by finding Ihe peak of the LED spectrum and droppmg 
down ;J dB on each side. 

' Mean (3 dB). This is the wavelength tiial is the average of the 
two wavelengl.iis detemiined in iJie 3-dB width nieasuremenl . 

Peak wa\elength. This is the wa\'elenglh at which the peak 
of the LEIl's spcctnun occui's. 

Peak density ( 1 nm). This is the power spectral density 
(normalized to a 1-nm bandwidth) of the LED at the peak 
wavelength. 

Distribution trace. Tltis is a trace that is based on the total 
power, the jmwer liislrihuHon, iincl the meiui wavelength of 
an LED. This trace has a Gaussian spectral distribution and 
represents a Gaussian approximation of the measured 
spectrum. 

Fabry-Perot Lasers 

l^isers;ire capable of producing high output power and direc- 
tional light beams. When used in fiber-opric commimication 
sy^lems, semiconductor lasere can be modulated at rales iiji 
lo aiiout id <;H/. Howe\ er. lasers are sensitiv e to Teni|ierature 
and back reflections. Ailditionally, Ihe coherent emitted lij^it 
is sensitive to optical interference from l efiections. 



64 IJ.rpmtipr HiHvIrii-[';ii-!i;i|-r( .linini:il 

© Copr. 1949-1998 Hewlett-Packard Co. 



^^^^(^ CDnduclion Sanil 
^ 1 



RecombinBtron 



Cavity 
Length lU 



Pdotoii Bati^p 
^ Drreciion ^ 



Fig. 8. Slimulaliii L-itussioii is Ilie reltas*' of a photon because of an 
electron liole reconibl nation irlggprert by aiiollicr vtiuliin. 

The design of Ihe Fabry-Perot laser is simpler thaii the dis- 
tribufed feedback laser (described laler). Howei^, it is more 
susceptible Id chromatic tlispersion when used in fiber-optic 
systems becatisp it has a wider speciral bandwitlth. A Fahry- 
Perot laser differw IrtiTii a lii>ht-eiiiji;iiig tliode iu that it gener- 
ates light uiaiiily h,v sUinulaTed emission. Some of the photons 
are generated by spoiUaneous emission, as described for the 
LED, but the jii^ority of the photons are generated by stim- 
ulated emission, where pliolons itigger aildilioiial eleclrun- 
hoie rec<mihinations, resulting in adililiojial photons !is 
shown it! Fig. 8. A stimulated photon ira\'e]s iji ihe same 
direction and has the same wavelength and phase as the 
photon that triggered its generation. 

Stimulated emission caii be Ihoughl of as tlie amplification 
of light (laser is ati acnmyui for liglii aniplificaiion by stimu- 
lated emission of radl.ation). As one pholuii piisses through 
Ihe region of holes and conduction b;md eleclrons, addi- 
tional photons aie generated. If Ihe tnalerial were long 
enough, enough photons mighl be generated to pfofliice a 
significant amoimt {}f power al a smgle wavelenglJi. 

An easier way lo liuild uji power is m place a refleclive 
mirror al each end of the region ,)usi described stj that tlie 
photons I favel back aiul forih belween (he miri'ors. hnlliling 
up the ntimlierof phi>tons with each I t ip. These mirrors 
form a resonator, wliich is a re<iuirernent for laser operation. 

Laser operation has two additional requirements. One re- 
quirement is that for stinnilated emission lo occur, a greater 
imniber of condiiclion-band elci'irons Ihim valence-band 
electrons miLsl he piesenl. This is calleil a populaliim Inver- 
sion, It is achieved by forcing a high ciirreni densiiy In the 
adive layer of tlie diode structure. The secoiui i*e(iiiiretneiit 
is that the gain exceed the losses Smm H!)sorplioii and raili- 
ation. Pari of the radiation loss is the amount of light re- 
leased al the liLser output. As Ihe current increases, the gain 
increases. Tlie ciirretil for which stimulated emissions occur 
is tlie threshold ciirreni of the laser. 

The resonator is often just highly reflective, cleaved smfaces 
on the edges of the diode (Fig. 9). As the light reflects be- 
tween the mirrors, the photons of a given wavelength nnisl 
be in |ih;i.se lo add coiislnictively. The resonator acts as a 
Kabry-Perol interferometer berausr only lighl frjr wliich ihe 
resfinator spacing is an integral nnniber of half wavelengtlis 
will add constructively. As a n'sult, the .spectrum of a Fabry- 
I'erot laser contains multiple discrete- wavelength compo- 
nents. 

The possible wavelengtlis produced by the resonator are 
given by; 

r„.s = mc/21n, 



AsliM 





O' -o 


1 p 


n'rin 

* 




boo 


B 


o o 






\ / 

Rdlective 
Surfaces 



Lnei 
Output 



Fig. 9. Tlie reDeuuve surfaces al tlie e( 
a FHlir5'-P(;rol type reaotialor 



ges of the laser diode act as 



where m = mteger, c = speed of lighl. I = iengtli of cavity, 
and n = refractive index of cavity. 

The actual output power at each of these wavelengths is 
detennined by the laser gain and mirror refleclivity al that 
wavelength. As with the LED. the center wavelengtli can be 
determined from the bandgap energy. The separation 
belw^een the different wavelengtlis (mode spacing) can be 
determined from the separation of tl:e mirrors as follows; 

Mode Spacing = c/31n (Hz) or X-/21n fnm). 

Fabry-Perot Laser Advanced Measurement Program 
Tlie Fabry-t'eriH litser ;idvanceil me;Lsiirenienl jiiogiani auto- 
matically me;Lsures the paranieters oftlie Faiiiy-Perol laser 
al the end of eai'li sweep. All of \hc measurements ;u-e based 
upon the detecled trace peaks of the laser fsee Fig. 10). 
What deftnes a peak is controlled by Ihe peak excursion 
function. The i)eak exctu'sion value (in dB) can be sel by the 
user iuid is used to delennine which trace jjeaks are accepted 
as iliserete spectral responses. 

A peaks function is supplied in Ihe measurement program to 
verily Lliat a proper peak excursion value is being used. Ttie 
peaks function, when eiiabletl, displays a veHical line from 
the luillom of the grid lo each coiuiled spei;iral component 
nflhesigiiai (see Fig. 10a). 

A distrihurion trace fund ion is also supplied with the pro- 
gram. This runcliiHi ilis[)lays a trace lhal is based on Ihe Irjlal 
power, individiiai wavelengths, mciui wavelenglh, and mode 
spacing of the laser. This ir;tre can he a Gaussian, Lorentzian. 
or envelope spectral disliihution that represent continuous 
approximations to the real discrete spectrum. 

The oilier parameters coniput.eil by Die Fabry-Perol 
measurement program include: 
■Total power. This is the smnmallon of the power in each of 
the displayed spectra! componeitts. or modes, that satisfy 
l.ne peak excursion criteria. 

Total Power = P„ = Y Pj. 

1 = 1 



© Copr. 1949-1998 Hewlett-Packard Co. 



DirciiilJiT l!BJiJ ilrwiL'tr-t'iu.-kiiril .Jduniid liS 



m 







1 abr^'l'vrat 


lu*r Itll 












1M 




■a 1 n - 


























1.5B 



^"'^ RL -fl 33 «B. 



On iff 



Or. Qff 







ht — 


















f 

-J 



SMiEf 



CENIEfl 



lltLP 

On en 



fill 
la) 




Mean Peak 
Wave length Wavelength 

Peak Amplilude 




Full Widlh al Hall MaKimum 



Dislribution Trace 



Ibl 



Fig. 10. (a) TliP Kj>ertmni Dfa Fubry-Perol laser, fti) The iiar.miPlers 
priividtii by llie Fjibry-Perul Iflser iiipasureiiieiit |jri)gi-am 

' Mean wavelength. Tliis parameter represents the center of 
mass of the .spectrjU componenls onsereen. Tlic power ajid 
wavelenglli of each .ypeciral ei.tiiiponeiit iire used to calculate 
the meaii wiivp length. 

Mean Wavelength = /, = V P|X,/Pn. 

1 = 1 

■ Sigma. This is an mis fak'iilallon ol" (lie .speclral widtli of 
llif Fahij'-Peroi la.'ser hased on a (iaiissian (liHlribulion. 



Sigma = /'^P\{h - >.)7p<.- 



I t'WHM (lull width at lialf maximimi). This parameter (ie- 
scrihes Ihe spectral width of the half-)itiwej' pitiiits of the 
Fal)r>-l'eroi kuser. itssiiniing a continiiou.s, Ciaussian power 
UistriliiiiiiHi. The half-power points are tiiose where the 
power spectral density is one-hjilf Ihat of die peak ampUtiide. 

FWIIM = 2.355 X Sigma. 

I Mode spacing. This is the average wavelength spacing 
between the Iniiividual spectral compi inenis of the Fabiy- 
Perot laser. 

' Peak amplitude. The power level of the pea!( spectral 

component of the Fabry-Perut la.ser. 
' Peak wavt'lenglh. Tliis is the wavelength ai vvliii li the peak 

spectral component of the Fabiy-Perot laser occurs. 



O'f 

SI? m 

Bn OK 



BPtifii; 



HELP 



F«ir 



Uft Itnr Ifil 

■ode sltsei= t.31 n* pvA* avfi 
nop tiinif - ? S9 n« hiiitf.iditi 
cnl.' aH^el = 





i-sMHi — 












4 
















































































































































i 

















CON' 
SuEtP 



-•cENIEn 



SPflti 



Di, Bit 



PES* 
E»CU«6N 



Fig. 11. The iipL'[:lruriL uf a distributed feedback laser. 
Distributed Feedback Lasers 

Distributed feedback (DP'R) lasers aie simi!;ir tci Fabry- 
Perot lasei's, except that all but one of their spectral com|ju- 
nents ;ue significantly l ediiced (see Fig. 1 1 1. BecaiLse its 
sjiec'tnun hu.s only one line, the sjiectral wiihli of a distrib- 
uted feedback laser is much less than tJiat of a Fabry-Perol 
laser This greatly reduces the effect of I'lu omatic dispersion 
in fiber-optic systems, allowing for greater transmission 
bandwidtiis. 

The distributed feedback laser u.ses a grating, which is a 
series of corrugated ridges, just above the active layer of the 
semicotLductor (see Fig. 12). Rather than using .just the two 
reflecting surfaces at tlie ends of die diode, as a FalDry-Perot 
laser iloes. Ihe distributed feedback la.ser u.ses each ridge of 
the comigalioii as a reflective surface. At die resonant wave- 
length, all reflections from the ihffcrent ridges add in phase. 
Because of the miicli sniiUler spacings between the resona- 
lor elements compared lo Ihe Fabrj'-Perot laser, die po.ssible 
resonjuil Wiivelengtlis luv much farther upart in wavelenglli, 
and only one resoiiaiil vvuveleiiglh is in Ihe region of laser 
gain. This results in the single laser wavelength. 

The ends of thep-n diode still act as a resonator in the f)FB 
laser, producing lower-amplitude side modes. Ideally, the 
dimensions of the refleclive surfaces are selecied so thai Ihe 
end reflections add in jiliase with the grating rellections. In 
this case. Ihe main mode will occur at a wavelength halfway 
between Ihe two adjacent side modes; ajiy ileviation is 
calleii a cenler offset. Center offset is measured as the dif- 
ference lietween the niaiii-iiKDde wavelength and the average 
wa\'elengt)i of the two adjacent side modes. 

The amphtude of the largest side mode is typically between 
SO and 50 dB lower ilian the main spectral output of the laser 
Because .side modes are so close to die niiiin moiie (typically 
beiween (1.5 lun and 1 nni] the dynamic ranfje of an optical 
spectrum aiialyxer detenimies its ability In mea-sin*e them. 
Dynamic range is specified at offsets of 0.5 nni and 1.0 nm 
from a large response. The HP TUfil) and 71-1-tIA optical 
spectrum auLilyzers specify a dynamic range of -55 dBc at 
oH'sets of (un ;uiil gienler. and -liO dBc at offsets of 1 .0 nm 
and greater. Tliis indicates tlie amplitutle level of side modes 
that can he detected at tlie given offsets. 



66 IX-comlipr 1893 RewleU-Pflfkaftl Journal 

©Copr. 1949-1998 Hewlett-Packard Co. 







length (U 


Grrmfl 


















boo 


OripM 






-o-o- 









Peak 

^ ArnpiEluiH 



HeFlei^tive 
SuriacBS 



Fig. 12. .\ p-n jiifitlion reprpsPnIalion of a di!itjibutpd feedhaek 
laser which uses a series of reflecling ridges If> reduce the aiiiplit.iidf 
ofali huL one of liie -speclral [■otii|imients uflhe laser. 



Distributed Feedback Laser 
Advanced Measurtrnenl Program 

The distributed fcedbauk laser advanced measure me iii piri- 
grani automatically measures the parameters of the distrib- 
uted feedback laser at die end of eacii sweep. Like tlie Fabiy- 
Perot laser advanced measurement program, all of tJie 
measurements are based upon the detected modes of the 
laser, oi' Ijace peaks. Whal defines a peak is controlled by 
the peak excursion function. 

A stop band display function is supplied to verify that a 
pi oper peak excursion value is being used to delennine the 
con ed sloi.i bami modes. The stop band function, when 
enabled, displays a vertical line from Ihe bottom of the giid 
to earli of ( he selected sifle modes shown in Figs, 1 1 and 13. 

A side-mode suppression ratio display function is al.so sup- 
plied to verify that a proper peak excursirin value is being 
used to detemtine Ihe correal side mode. 

The other parameters provided by I he DFIJ measuremeni 
program inrliule: 

• Peak wavelenglli. This is Ihe wavelength ai which the iiuun 
spectral componenl of the DFB laser occurs. 

• Mode offset. This is Ihe wavelength separation fin naiuime- 
lei*s) between the main spectral com[ioneiil and file largest 
side mode. 

• Peal* amplitude. This is Ihe power level of Die iiiaiti .sjiectral 
component of the DFB laser. 

• Slop band. Tliis i.s tlie wavelength spacing bet ween I he upper 
and lower .side modes ai^jacent to tlie main mode. 



BandvnMi 




Side Mode Venicat line 



Slap Band 
Mean 

Fig. 13. The parameters provided by the distributed ft-ecibark laser 
aiilcunalic proEram, 

• Cent er offset. This parameter indicates how well the main 
mode is ce(ilere<l in the stoji liand. ThLs value ei|Lials the 
wavelengili of the main spectral comptjiient minus the mean 
of the upper and lower stop band component wa\ eleng(hs, 

• Bandwidtli, This p.uamcter pro\idcs a mcasiu-ement of the 
displayed bandwidth of die main ,'spcclral component of the 
DFB laser. The amplitude level, relative to Ihe peak, lhai is 
u,sed to niea.sure tin; bandwidth ciui be sel by Ihe user. The 
tiefaull amplitude level used is -20 dBc. Becatise of the nar- 
row line width oflaser.s. tlie result oflhis measuremeni For 
an viiunodulated iaser is strirl.l.v dejiendeni upon ihe resnlu- 
lion bandwidth Hiter ol' ihe optical spertnim analyzer. With 
iLioduialioii a[)plie(i, the resiihant waveform is a convolution 
of the analyzer's filter and the modulated laser's spectiTUn, 
causing die measmed bandwidth to increase, Tlie combina- 
tion of the modulated and immodidated readijigs can be 
used to determine Ihe l)andwidth of the modulated laser and 
tlie presence of diiip. 

Conclusion 

The Hi' 714'iOA and 71451 A are HP's iirst optical spectrum 
aiialy/ers. By leveraging our experti.se in user interlaces 
from the KK and microwave iiroducts, we were able to pro- 
vide a produci thai has liie familiar look and feel that users 
expect from HP. 

Acknwwledgments 

We would like lo thank Michael Levernier for his coiitribii- 
rions t.o Application Note 121H-1 "(Jpfical Specmmi Analysis 
Basics," some of which is used in this article. We would also 
hke to thank Loren Htokes for lus invaluable consuliing for 
the advanced ftiticHons of tJie optical spectrum analy/^er. 



© Copr. 1949-1998 Hewlett-Packard Co. 



DeraniburlWmii'wlPtt-l'aclianUiiuriial 67 



A Double-Pass Monochromator for 
Wavelength Selection in an Optical 
Spectrum Analyzer 

The wavelength-selection scheme used in the HP71450A and HP 71 451 A 
optical spectrum analyzers propagates the light from the device under test 
twice through the refraction and diffraction elements in the 
monochromator. 

by Kenneth R. WUdnauer and Zoltan Azary 



For many users of spectral ajialysis instmmonrs, measure- 
ment speed is of jirimary conrem, aiirl ha\i!ig a rlisplay of an 
optical speetniJTi in real line is liiglily desirabli'. Many users 
intert'steil in tiie purity of theii' soiure are also interested in 
being iible lo delecl low-le\'el signals tliat are very dose in 
wav eleniltli lo the priniaiy signal. Tlie ]-atio of the power of 
these low-level signals to llie main sijiual can be easily 
smaller than 10 "I (-40 dBc) al ulTset.s lesH ihaii one niuiome- 
ter away. The abliiiy ofiui iiismimeiil to resolve ur display 
these signals will he refened lo as close-in dynamic range in 
this article. 

Higher traasmission rates, better transmission titiality, and 
the longer iransnii.ssion dislances of today's fiber-optic 
transnufision sysleins have i.-reai.eci the need In measure and 
analyze ihese low-level optical signals. To measure low-level 
optical signals an instrument must be efficienl anri sensitive. 
To perforn^ these measure me nts quickly is an added chal- 
lenge. Also, many limes Ihe polarization state (i.e,, the 
orientalion ol' llie electric field) of the input signal is either 
not known or variable. Hence, ihe insliiiment measurements 
should be relatively insensitive to changes of the input 
polarization state. 

Because many applications find it useful to filter an input 
signal optically, an inslninieni that can produce optical out- 
put should also have variable optical baiidwidih. Willi ihe 
need for more precise optica! measuremeiits, the ir\sl ninient 
should be repeaiahle and accrirale iuid be atile to make these 
measurements in a standard insininieni enviroiinient — with- 
out tiie need tor an opiics table. Finally, it would be conve- 
fiieni if the uistrument that measiu'es these low-level signals 
could be small and rugged enough so tliat it can be moved 
arouml wiihoui any S[)ecial care. 

The HP T145IJA and 7145 lA optical spectrum analyxers 
pri.fvide tiie features mentioned abo\"e by using a spei'ially 
developed wavelenglh-sclection scheme — the doiible-]>ass 
monocliroinalor. A block iliagram of these analyzei's is 
shown in F\g. i . This article describes the operation and 
performance of the double-pass monochromator and the 
operation and characleri sties of the components in the data 
acquisition and processing system in Fig, I, 



Double. Pass ManDChiDmBUr 



Input 
Fibw 





Calliniiirlnii 






and 


Dift'flction 




fDCiuiiig 


Giallni] 




0|itlcs 


■ 



Oulput 
Fiber 



PhalDdelBDlor 



MuiiuE^ltruiiiHlDr 
Coniitil Bnnrds 



Dala AcquisiNDn 
and Processing 



► 



TiansimpedsncB 
AmplifiBt 



Analog-ln- 


1 


Data 


Oigrtal 




Acquiiiilon 




1 


Piocsssor 



Main 
PiOGCSSor 




Fig. 1. Black ilingrnni of Ltw m:yor components in liu' HP TUHit.A 
and T14olAupiic;ai speciruiii iitmlj'zers. 



Do utile -Pass Monochromator 

A doublc-paHs-uKinochrnmaror-based design was chosen for 
ihe HP 7145I)A ;md HP 71451A opHcal spectniiu analyv.ers 
rallnei' thiui a spectrometer-biised design for fwn reasons. 
First, a single photodetcetor (which is what the monuchro- 
nial.or uses) lias an inherent advaniage over tiie detector 
array of the spectrometer for close-hi dynamic range mea- 
surements. The dciecior array also costs more ihan a single 
detector. Second, a monoctironiator puts less demand on the 



68 Decraberiaiiailcwk-U-Pai'kardJoum^il © Copr. 1 949-1998 Hewlett-Packard Co. 



Apefture 




Fig. 2. Tlie elements of liip double-pass nionochromator showing 
the lighT beam as ll makes the two passes fhroligh tin- 0|.jlical 
cumponenis. 

Optical system for imaging a large span of spatially dis- 
perseil wavelengths. 'Hie cioiible-pass monochromator 
configiitalion fiirrher increases the close-in dynamic range 
of the instrument, essentially obtaining the range of [wo 
cascaded monochromators. 

A refractive opi ical systent was chosen to reduce the size 
require men Is of llie nionocfirontaUir so as to minimize the 
overall insmmieiil siKe. CaieFul design of the refractive ele- 
menLs has reduced the inlierent chrnrtiatie aijerratioius asso- 
ciated Willi such systems lo a tolerable level. The inherent 
disadvajitage of slower measurement sjieed because ol' scrni- 
ning with the diffiartion grating is reduced witli a direct-drive 
system and tlie properties of the double-pass configuration. 

Operation, The propagalion ol lighl Ihrou^'h Ihc system stalls 
wilh file light entering the munochrumalor from tlie device 
under lest (see Pig. 2 ). This light is then relayed by the input 
connector a.ssenibly (^ n in P'ig. 2). The user end of Ihis con- 
nector assembly is a nal-[>o!ish physical conlaci witli inlcr- 
chaiigeable adapters to allow coimecliiin lo slandard fiber 
inlorface.s, Tlie inotiocliromalor end is an angled iiiteiface lo 
air, Hoih interfaces niiiiiniixe reflections lo tlie user end, The 
li.ehl then propagates towards the lens ;ind is coUimated for 
ilhnuinalion of liie dilfmclion grating. The diffraction grat- 
ing is operated in very nearly a Lilirow condition* and can 
be rotated to tlie ilesired wavelenglh. The light is dispersed 
by the diffraction gratuig and reliinislhroiigh the same lens 
to be reflected by tlie fust plane mirror and imaged onto one 
of the apertures {m the rotatable ajierlure wheel - . By ne 
fating ilic aperture wheel, difl'erenl apeiture widths (slits) 
and hence resolution bandwidlhs ran be selected by the 
user. Once the light enters and leaves the apertiLre slit the 
fiis! pass of the douljle-pass monochromator is effectively 
complete. 

The second pass starts when llie liglil exits the aperture slit 
and is rellected by the second plane mirror mul [iropagatcs 
Lhrongh an achroniiitic half-wave plate -i'. The half-wave 

■ Sae ■'□iffracliun Gialing" niipagt '0 lui a dBScniiliDii ol Itraliltruivtondikm 



plaie is oriented so thai 11 causes a 90-degree rotation of the 
s and p polarization compoiienls as defined with respe<-l lo 
the Imes on the diffraction grating.** The beam is again 
colliniaied by the sanie lens and again illuminates the same 
diffraction grating. However. bccaiLse of the orientation of 
the first and second mirrors wilh respect to the dispersion 
direction of the diffraction gi'ating, on the second pass 
llirough the system the light is not dispersed any further l>y 
ihe diffraction grating hut is collapsed or recombined. creat- 
ing a filtered rephca of the input signal. This recombined 
beani ^ Ls llien imaged by the iens onto a fiber afler reflec- 
tion from a Third plane mirror near the fiber. This fiber, 
which is called Ihe output filler in Fig. 2. is a piece of multi- 
mode fiber that carries the light to the photodetecior for 
conversion inlo pliotocurrent for analysis and (iisplay. .An- 
other function of tliis oiitpiil fiber is to act as a second aper- 
ture in the system. As an option, lliis light can be directed to 
tlie front panel ofthe instnmiem pro\iduig an optical oittimt 
for Ihe user. In fronl of thus output fiber, there is also a tnask 
wheel that is coa.vial with tjie aperture sUt wheel and con- 
trolled by the same motor. To measure the dai'k current of 
the photo detect or, this mask wheel can be rotated such that 
the output signal is blocked. 

This double-pass monochromator system provides the high 
sensili\ily ryfiically found in a single-pass monochromator 
system and the high dynamic range typically foimil in a 
double monochioniator. Also, because ofthe half-wave 
plate, it pravides exceilent polarization insensitivity. 

Performance. Two niain facfoi-s affect the time it takes to 
make a swept nieasLLremeiit with an optical spectimn ana- 
lyzer. Tlie first is the abiliry to move the diffraction grating 
quickly, ;md the second is Uie signal-to-noisc ratio. The in- 
stnmient settings and the power of tlie user's input beam will 
iiffecl which of these two factons limits the mciisurement 
speed. For relatively niediuni to high powei' levels, which 
are greater than microwatts I >-t)Q dUni], the measurement 
speetl is usually limited by jnotor speed. For lueasurenLeiils 
in which the user is concemetl about power levels less than 
litis, the tneasuremeni speed is usually litiiilcil by signal-to- 
noise ratio, Tlic ability to move Ihe grating ijuickly. preci.sely, 
and reliably ui the HP 714.7il)A and TH.'ilA aiuil.vzei-s is pni- 
s'ideil by a direct-drive system. Tliis dire c't-^l live syslent of- 
fers signilicanl. improvemenl in meiisurement speed for 
cases iJi wliifh the meiLsurcmeni is limited by motor speed. 
Tills system is described in the article on piige 75. 

To minimize the measurement time for cases in which the 
signal-to-noise ratio is the limiting factor, it would be advanta- 
geous to mnxiniizp tlic signal-to-noise ratio by niavimizing the 
signal and muiuiiizing the noise. For a given input signal level 
the only way to maximize the signal without anijilificarioji is 
to minimize the loss through Ihe system. This is achieved 
through the use of higlily efficient and low-loss optics to 
collect as much ofthe user's input signal as possible. 

The two main components of noise are noise resulting froju 
scattered ligitt and electrical noise from the pliotodetector 
jmd its coniiionents. To tiiinltnize the scattered anil stray light 
in Ilic HP 71450A and TM.'ilA an;ilyKers, careful allejilion 

' SsB 'PnlarnstiDii Seri9itlv]ty'' anpnge?] lota desciiptioriDfs and ^pDlmailuii 



© Copr. 1949-1998 Hewlett-Packard Co. 



Oc'ti'iulJiT I'-liKi Hi'wlfU-I'jM'ltanl J.iiiriial 6fl 



Difiraction Grating 



A (frffiaction graimg is rnade up of an array ot equidisiant parallel slits (in the case 
of a Tfansmrssiue gratmgl or reflectois lin iha case of a reflective yialmgl Ihe 
spacing ot the slits or refleciors is on the oriler nl the waueteogtti o( the light for 
which ilie grating is inieniied to he used Tlie HP 7H50A ard optical 
spectrum analyzers use a reflectiiie grating 

TliB basic operatiun iif a diffraction grairig begins wtien light That sinkes ttie 
reflective lines nf the gtahng is fliflractsd For a given wavelength there will be a 
certain angle at which the diffracted wavelets will bs B«aclly one wauelengih oliI 
of phase with one another and will add consimctively in a paratlel wavefront (see 
Fig 1} 

The light nl a given wauBlenglh leaves the grating ai a specific angle, and light of 
Qiher wavBlengths leaves the grating at other angles. Although it is based an a 
different principle, the diffraction grating spatiallv separates the wavelengths of 
light that strike it much the way a prism separates the wavelengths of light that 
pass through it. 




Rg. l.Tlia Basic r)pBr3(itin of s diffraction flralma Incniring lighr slrifceslliB rellBriivfi gtsting 
praaucing dllfracled wavelets that ate sxaclly me taieiengxh dui nl phase with one analhei 
and add constniclivaly in a parallel wairBt'onl 



was paid tr.i creating the optics. Also, careful analysis and 
iillt'tilion was ijaici lo thi' absoiptiott ajici scattpiing of light 
Ijocaiise ofi't'nectiijtis from the input beam ajid th<' ilispefseri 
liglil I'min the statins leDet'linK offthe inteiTial surfaces of 
ihf inonoehrotiiatoi'. Tlie effecl of this stray light is reduced 
ihrnugli background light subtraction (described later). 

Tile noise from tiie photodctector and its associated elec- 
troiiic's ciui be lowered by reducing the eieclrical haiidwiillh 
(video hjiiidwidth I of the detci'tioti .systeiTi. Tliere is of coiu-se 
a penally in nie;istirement time because of (lie need to allow 
the video bandwidth filters to seltle. 

( )ne way to minimize the tiot.se power from Uie photodetec- 
tor wilhriul ri'duciiig the bandwidth is to use tlie smallest 
detector possible. An impiin;int feature of tiio double-pass 
coiinguration is the ability- to use a relatively small iletpctor. 
because on tlie second pass the beam is not dispersed liul 




Rfl. I The dlflraclmn Qranng in The 7t 45BA and HP 71 351 A oglrcal anectram analwfifs 
□pfirates in a tonfiguMiion calleB a litlrnw Hinaiiinn m wliich Uw wavelaiiglli nl interest 
travHls batk alona llM path o\ Ihe incidBnt baam. 

The general eqiraiion lur a diffraction grating is: 

n>.=2d(Bin(( + sin|ll, 

Where \ is the wavelength nf the hght, d is itie spai;ing of the lines on ifie grating. 
II IS the angle of the incident light lalalivs in the grating normal, [i is the angle at 
whichiightof wavelength J, leaves the graimg, and nis an Integer thai is called 
the orjer nl the spei:itiim 

When the wavelets ate each one wavelength out of phase the specfnim is celled 
a first-order spectrum At another angle where the wavelets ate all exactly two 
wavelengtfis nut of phase and will also add construchvel/, the spectrum is called 
a second-order spectrum Higher-order spectra may also be preset^. 

In the HP 714!iOA and 7145lAnpiical speMrum analysers Ihe diffraction grating is 
opEfaied in a special configuration called the Liiirow condition In this arrange- 
ment, the wavelength of interest leaves the diffraction grating and goes directly 
back along the path of the incident beam (see Fig 7]. Thus, in the grating equation 
u = (! I = Oi (see Fig 2] and the equation can de simplified to; 

nil = 2d sm H. 



recombined. In fact, the detector need only be as larpp as 
the oulijut image plus any watidcr or moi etnerl of the out- 
put image resulting from rtilaiion of lite dilTi aciioti grating 
during meiLsurement. in a symnietricu! .systetii willi no me- 
chiuiic al errors, there slioiild be no mo\'etiient of llie output 
image as the grating rotates. Because complete elimination 
of all ineciiiuiical errors is not possible, either the detector 
nntst be larger so that it does not jiiiss the image when the 
grating roiales and the image nio\"cs. or there Jtiust be some 
itieaiis for the detector to track Ihe mo\'emenl of the output 
image. To keep the detector small, w e ha\ c chosen to piT)\iiip 
a means for tracking Ibe output image ttiovoment in our atia- 
lyzers. This tracking tneehanism is described in the iirticle 
on jiage 80, Tliis stiialler detector getieraies less inlicrenl 
noise and thus allows the use of a wider \ideo bandwidth 
with the benefit of a faster measurement or sweep speed. 



70 IlerpmbiT IfHW Ilewlen-Pai-lfjirtl .liiiirtinl 

©Copr. 1949-199B Hewlett-Packard Co. 



Another benefit of the small ifeiectoral llie output is die 
abiliiy lu have a small output ajierlure. A small output aper- 
ture rouplwl «i!h tlie secoiiri pass ihmugh the s>-s!em in- 
ereases iJie close-in tijiianiic range. This is irue whether tJie 
serond pass is si-t up lo further ilisperse the li^t or to re- 
combine the liglil ( collapse the dispeisioii ). Besides allow- 
ing a small detector, there an? several other adv ajita^es to 
rccombining the liglit on the second pass rather than further 
dispeTBing the light. First, the output aijcrture does not af- 
fect the optical or resolution bandwidth of the system. TNs 
is solely (ietermined by the first pass. Second, the inlieren! 
lime dispersion of the firsl pass is canceled. Because of the 
v.a\'eleng!h dispersion of the first pass, there liat lo lie a 
corresponding lime dispersion. This can be seen l)y noting 
thai ihe grating is tilled with res|)eci to the wavefront illumi- 
nating the grating. Hence different parts of the wavefront 
see different path lengths iuid different time delays. By re- 
conibining tlie dispersed light on the second piLss, this lime 
dispersion is caiioeled because of the inverse path lengths 
across the wavefront of the beam on the second pass wil h 
respef/l to that of the first pass. Finally, re combining light on 
t.lie second pass results in an optical output that can be con- 
veniently jirovided on an opiical Tiber In the configuration 
mentioned above, tliis optical oiilpul ha,s aji optical resolu- 
tion bandwidth diat is variable and selectable l.>y the user 
and not limited by the oiilpul apetliire. Thus, tlie full set of 
resolution bandwidths provided liy the in.sirumetil are a\'ail- 
ablc. This allows the user to use Ihe inonofhroniator as a 
tunable, variable-bandwidth optical filter or preselector 

The efliciency of t!ie difffaclkin graling in the IIP 71 ioOA 
and 7145 lA optical specliimi analyzers is iiihereiilly depen- 
dent on the polaiization .stale of the light illuminating il. The 
grating is also the optical element with ihe largest loss in the 
system and the hirge.st change in loss wiili input polari/a- 
lioii. Because the input polarization stale c;m be different 
for different iLsers ;uul can change din-ing a me;isiii'enieni, il 
is desirable to compensate for the polarivfation ilepetulent 
efficiency. This is accomplished by rotating the s atid p 
polariKation components (See "Polaiization Sensilivlly," (Jiis 
page) by !)tl degiees between the first and second passes, A 
roUtion of ill) degrees will cause what was the s component 
on Ihe lir.si reflection off the grating (fn-st pass) lo become 
tiie |J ciiinpiinent on the second reilection off the grating 
(sec'inil pjiss). This s;une excliange happens for Ihe |i pnlar- 
izatioii. Thus the efficiency of the grating is always the prod- 
uct it{ the s and |j efficiencies of the grating for any input 
polarisation. This compensation scheme also )i;is the benefit 
of mainiaiTiing the same degree of polarization of the Ught. 
Therefore, liie light at Ihe optical output lias the same de- 
gree of jicilHrinijtion as at the input beam, but not Ihe same 
pnlai-izatioii. The ilO-dcgrce rotation is accom|jhsheil by the 
arhromalie half-wave jihile. 

Data Acquisition and Processing 

After the incoming light beam has been optically filtered by 
tlie double-pass monochromator, the foni]'oneiils In Ihe 
data aciiiiisilion ami processing section ul the III' 71 1'lOA 
and HP 7I451A optical speclriiin ajialy/.ers are respon.sible 
for detecting Ihe incoimng light ;md convening il to an elec- 
trical signal, which is then converted lo digital romial, for 
proi'essiiig and display. Fig. 3 shows romiiniieiits included 
in l.liis section uf the optical spectnini aiial.Vi'.ers. 



Polarization Sensiti\ity 

PolarizaiiQn ^sinvity results tjecause ifie 'ellection loss of iheiJiffiBciiBn giaiirg 
IS 3 function of the oolariratJon angle of ihs ligtit ifiat stiikra ii As the palarizaiion 
angle ot itie Irghi «Mes so rtoe$ tne loss "i Hie monoclwjmatcr Potanjat light 
can ts drvideil intn Iwu components Ihe comixinent patallel lo the difeelior of 
the lines on the diltiattm gtaimg is afxen labeled p pqlatizaiinn, ana the compo- 
nent perceridicula' 10 the direEtw oi Ihe ires on ttis Oiffraclion gfahng is nfien 
labeleO s colaiiiation Tlie loss at the diflfaclton gratrng drffeis fa' ihs two differ- 
ent DDlanralmnsait) escii toss vanes wll^ wsvelengih Ai each wauEieigth. ttie 
loss nt p polariEBd lig^il and ihe loss of s polauiEd Lglil tapresenl Ifie mrninium 
antl maximum losses possible toi linearly polarized light Al some wavelengths. 
The loss experienced l?v p polarized ligtii is greater ihan thai of s pataiiifld light, 
while al oihw wavelengths, the situation is rewised This polaiization sensiimiy 
results in an amplitude uncertainty lor /neasutemBnts of polanzed light and is 
specified as pDlanraTinn dependence 

To reduce polanzalion sensiiiviiy m the HP 71450A and71451A optical specffum 
analyzers, a half-wave plale is located lO the path of the optical signal between 
ttie first and second pass in the double-pass monochromaior Isee Fig 2 in accompa- 
nying aiiiclei This halt-wavB plate miaies Ihe componenls of polarization by 9D 
degrees. The lesull is thai ihe companenl of polaiijaliDn lhal receives the maxi- 
mum anenuahon on the t1rsi pass will receive the minimum attenuation on the 
second pass, and vice versa 



Pholodiode. As mentioned above, because of the mtinochro- 
mator's abihty to produce a small output image il is [lossible 
to use a photodiode with a correspondingly small acrive ai-ea 
for electrical detertioii. This phototliode litLS the aih antage nf 
having a very large shimt resistance (on tlie order of i (iQl, 
which allows detection of veiy low light levels. Sensiti\ity at 
Ihe photodiode is around -i-l'l dBni when ii.sing a lOO-MQ 
transinipedimce with a !1)-H^ video bandwidth. 

Tlie phol.oiUode output currenl is directly pi oportional to 
input optical power. Tlierefore, when converting tlie photo- 
diutle ciirreni into decibel iinit.s, the base-ltl logarithm is 
multiplied by HI instead of 20 as would be done with electri- 
cal signals. All electrical giiins referred to in this article are 
computeil litis way. For example, an amplilier with a voltage 
giiin iiT 100 is referred to as having a gain of 2n (IB, not 40 dB 
as is usually done. This convention causes all electrical 
giiins li> be \iewed in leniis of o|)fical power levels. 

Tratisimpedance Amplilier. The tratisiinpedance amplifier is 
an FET-in|int monoliiliie IC lliai has very low input liias cur- 
rent (less Ulan 1 pA iiiaxiniiim ai room leni[)eralure) and 
very low input noise nV rms, 10 Hz to 10 kllj:). The 
transimpet lance miipliner's gain {'an be selected from six 
discrete feedback ivsj.sior .sellings beiween 1 k£i anil 100 
M£2 in decade inerements, A 1I)0-M£3 feedback resislor is 
pemianentiy connected lo the junplilier. Five additional 
flower) gains are availalile by connecting other resist.otN in 
jiarailel with die first one. Shielded teed relays are ii,sed for 
this purjio.se and are located un Ihe Ihe luw-inipedimce (otit- 
put 1 side of the feedback jialh. By locating the relays here, 
l.lie effect-s of relay leakage eiitTents are ii^ininii/.ed and less 
expensive relays can hie used, 

Analog-to-Digital Conveiter, The data acquisition subsystem has 
a idUl] dynamic range of 11)11 dli, HO dB of which is available 
al any one lime, A lli-bil .MK' ninniiig til 27 kllK priivjdes2ri 
dl) of dynamic range wiih al>oiji I) i)2 dli <ir resoliitioti al I he 
low end. By switching in a 25-ilii aniplifler fan electrical 



© Copr. 1949-1998 Hewlett-Packard Co, 



ilBi'cniliiT Itniii IWIi'll-l'^iiliiirilJijiirnUl 71 




1BK 24 8<tE 1GK ' 24 Bits »K ' 24 Bits 



XRAM ■ VRAM ■ Progrnm 



Adilress IB 




Address & 



Dm>1B 



Peipphcrat Bus Id Ditlractian 
Molnr Cotilrol, a y Positionei 
and Other Parifherels 



1GK 8 Bits 



Memory iMpansicifl Pan 



Data ncqiiisiiiDn ProcesEor 



Hosi 

DiibI SbmbI Fori 
InleridGa Pdrl Interlace 



CalrbraliDn 



Address 3 



Data B 

k-.- — H 



Slale 
Machine 



18 

H— - Dale Qui 



Delia In } To Main Processor 
-¥ Control 



Fig. 3. A liluck diagram nf the comporetiM iii the data acqtiMHtHi ser lioii nl' lIih upiital sppclmm attaij^er. 



voltiiKe gain of 320: 1 ), tlic cffi?c'tivc inpul range of the ADC 
is incrt'aseti Lo 50 dB, A compai'ator decides whether tfte 
ADC should convert the oiitpul oftlie 25-(iB amplifier or the 
Lttiaiiiphfietl signal. If Ihe onlpiii from (lie iransinipedanfe 
ampliiler is < 30 m\' tlie ampliilf il sigiiaJ is vised. 

The range of transimpedance gams available (1 kJ3 to 100 
MQ) a(ld.s att adrlilional i50 ilB to thespeclnmi analyzer's 
ilynamic raiigp. Tlie finiiware takes advantage uC ihis by 
offering a mode of operation in which 1 raiisimpedanee gains 
arc automatically swiicheil during it iiiea-iurement lo keep 
the measured signal wiihin ratige. This mode is called aiitii- 
ranj,'ing. To gLtarantce adequate overlap between ratige.s, 
only yo (IB of Ihe dynamic range is available during any one 
nteasuretnetil. 

Detection and Sampling. The dala acquisition harilware is 
callable of either sampling or peak-deteciing the transim- 
pedance amplifier's output, Petik detection is useftil for 
catching narrow signals when ihe measurement s]jan is 
wide ciiniparcd to the resolution bandwidth. The firmware 
digitally extends peak detection to capture the peak of many 
ADC reailings taken for each trace dala point. 

When the hardware i.s set to satiipie deleclioti. llie rmnware 
passes the ADC readings llirough a single-pole IIR (infinite 
in]ntl response) digital filter. By fiitering .^C readings, 



system noise is reduced and lower optical power levels can 
be ttieasured. 

Grating Angle Measurement. Wlien a swept-wavelength mea- 
surement is taken, the motor control hardwiire is instructed 
to move tlie diffraction grating fi-om die start wa\ eleitgth 
aiiKlc lo the stop wavelength angle at a eonslanl angular 
velocity. This includes a certain amount of uvei'sweep ,so 
that the molor reaches full velocily before Ihe start wave- 
lengtli atid doesn't start decelerating until after the stop 
wavelenglli has been reached, 

Duriiig a measitremenl. Ihe ADC samples at a conslanl 
frequency and the data taken mu.sl lie correlated wilh ihe 
wa\'ek'ngtii. Tw i> factors make a simple lime correlation of 
ADC data wilh wa\'eleiigth impractical. First, altiiougli the 
command input to tlte gratitig motor controller (the des!re<i 
diffraction grating angle) can be made lo follow an accurate 
mid i)rediciable velocity profile, the problem is that lo tlie 
degree of accuracy we require, which is les.s than an arc- 
second, the a(^tual tr^eclory followed by the grating wi]l 
exiiibil substantial and unpredictable variations from The 
desired trajecloi^. Se<-ond, tlie relationship between the 
ciiffraction (;rallng angle and the wavelength is not linear. 
These two factors make a simple lime correlation of ADC 
data witii tlie wavelength unpossible- 



72 llpTpmhcr lt*93 IIPwIell-PHchanl .IfiiimBi 

©Copr. 1949-1998 Hewlett-Packard Co. 



A simple solulifin is iised to o%'crcome tliese problems in (he 
HP TI450A and 71451A, WTien the data afquisilion system 
lakes a sample, it also spiids a signal 10 the diffraction grat- 
ing motor (.-(uiirol iiardware. This signal oaiLses the at-tual 
angle of the grating ui lie siuird. Tlie fimivvare oati then 
read ihe angle and delennine the wavelength when Ihe ADC 
conversion is finished. 

The diffraction grating iiuiior system is described in the 
article on page 75. 

Data Represefitation, The digital signal processor processes 
data as 24-bit or 48-bil signed fixed-point fractions. The left- 
most bit (bit 23] is the sign bit and the radix point is as- 
sumed lo be jusi to the jell oft lie niosl-significaiil data bit 
(bit 22). With 24-bit data, the least -sign ifi can i bit is equal to 
2r^ and (he range of numbers thai can be represented is 
-1 to 1 - 2r'^'-K 

Amplitude data is represented by a 48-bit signed fraction. 
Tlic maximuTti AlJC reading at the kjwest-gain transimped- 
ance without using the 25-<iB )!aiii block is scaled lo 0.-5. 
Readings taken from the '15-i\B gain block or with other 
traiisitiipedanoes are scaled acccirdingiy. This data represen- 
tation has a dynaiiiir raiige of about 120 dB wilh 0.U2 dB of 
resolution ai the low ei^d. 

Zeroing and Chopping. Mosdy because of drift in the analog 
hanlwiirc such as op-iuiip itipul offset voltajjes and ciinents, 
the ADC reading corresponding lo zero optical input power 
changes wilh lime. To compensate for Ibis, the limiware 
periodically remeasures Ihe ;iero-inpu1 ADC reading. It. does 
tliLs by rotating tlie aperture wheel to a point halfway be- 
tween two apeitures (slit.s). This blocks liglil from making 
the second pass rlirough I he iiionociirnmator. It also blocks 
nearly all stray light fi om entering the outijut fiber because 
the output mask wheel has no hole at thi.s position. Nor- 
mally, this (iperal ion is carried out duiing retrace, and the 
time spent rezeroing varies with instrument setfings (Q-om 
less than 200 nis to several secondsj. 

The optical spectrum analyzer's dynamic range can tie liniiled 
by stray liglil in the nionorhromalor when large signals are 
present at other wavelengths. Stray light from lugh4evel 
signals can enter the monochromator output fiber and mask 
a low-level signal. It limi.s nut that stray light in die mono- 
chromator is uniformly dLsirihiili'd over any given small 
area. One of liiese areas is tJie small area surrounding the 
output beam. Because of (his fact the fintiwme can displace 
Ihe output fiber away from the outpiif beam and get an accu- 
nile estimate of the stray lighl level at tlie oui|)ut beam. Tlie 
si ray lighi level can then be subtiacted from measuremi'ms 
of the output l.ieani muplitude to inc'rease ovt'r.dl dynamic 
range. This is the liackgroimd light subtractinu techtiique 
mendotieri earlier. More details about tliis leclini[]ue are 
given ill the article on p^e 80. 

In our implemenlaiion, the iwo-axis micmposilioner, which 
positions the cjoi|nii fiber, is chopped between its normal 
posifion and a displaced posilioii. The ADC readings from 
these two fiber locations are sulnracted to arrive al a niea- 
surement that is much less sensilive lo stray iighl levels in- 
side Ihe monochmmator. Chopping runs al a 2tl-H/ rate and 
is automat iciilly enabled whenever at lea-sl one chop cyi le 
can be executed per trace bucket 



Digital Data Processing. The m^oritj' of processing performed 
on AI>C daia occtirs inside the ADC interrupt ser\ice routines 
nmning in the data acquisilion processor .^piiinde-versus- 
wa\e!englh (flatness) correrlions and conversion of linear 
aniphtudes (o dBni are Ihe only nvo opejations that do noi 
occur during intemipt servicing. All measurement data sent 
to Ihe maiii processor is in dBm units. Tlie main pri)cess<ir 
converts (he data back to linear imits when a lineax amplitude 
display is requested. 

The exact procf«sing performed on ADC data during inler- 
rup! servicing ^-aries depending on instrumcnl sellings, .Most 
of ihls variation is accommodated by providing iliffereni 
intemipt service routines to process ADC data for different 
modes of operation. This reduces processing time because 
the mlemipt service routine does not have to spend lime 
figuring out whai mode the instrumeni is in. Some iasini- 
ment settings are examined during interrupt servicing. For 
example, examining whether auioranging is enabled. These 
tests do not incur a large performance penally. 

The interrupt service routine also updates the output fiber 
micropositioner DACs as necessary during a measmemeni. 
This causes the output fiber to track the output beam's 
movement as the diffl:action grating rotates. 

Digital Signal Processing, The data acquisition section is con- 
triilled by a Motorola DSP5fiO00 digital signal processor It 
runs with a 20-MHz clock and topically exetmtcs instRic- 
rions at a 5- to lO-MKz rate Itwo-word instructions require 
200 ns to execute]. Many DSP50000 instructions are capable 
of performing m\ ASA^ operation and moving two words of 
data to or from different memory spaces in one instruction 
cycle. The processor also has a memory expai\sion port, a 
host interface port, which is used to commimicale with the 
main processor, and a dual serial jnlerface pori, which Ls 
used lo receive ADC data. 

The DSP.^OOOII has access to four differcnl memory spaces, 
one for progr.mi instructions (P), two for ilata storage (X and 
Y), and one for long word tlaia .storage (L). The I. memory 
space is nol acnially separate, l)ut consists of the X and V 
Sfiaces t'on catena led. The processor ftlso has a small imiouni 
of hiternal program anit data iiiemoiy Accesses lo intenial 
daia memory are faster than for exienial memory so mosi of 
I he i lala used by itUemipl. service routines is located there. 

The DSP-5G00I) Ls provided with a iull coniplcmenl ((i4K 
words''') of external progrfun RAM and KiK worIs of X and V 
data RAM, I/O is memory mapped at ihe upper end of Y 
memory and Ihe DSPfiOOOO provities low-overhead iiisinic- 
lions lo access this memory itrea, IfiK bytes of each X and Y 
memory space (jtovides access to Iwo KOMs conlaiiiing 
calibration data. One ROM is used for the monochromatoj' 
and tlic other for die data acquisition prinlerl cireuit board. 

To avoid excessive bus loading, the DSP560O0's memory 
expansion port is buffered to form a jieripheral bus lo which 
the calibration ROMs and all periiiherjd devices are con- 
nect ed. Peripherals connected lo this bus hiclude ihe <lif- 
fraction grating motor controller the aperture wheel motor 
controller, tiie fiber micropositioner DACs, the current 
source control registers, mid the ADC interface registers, 

' Onewurd \s 24 bits wide 



© Copr. 1949-1998 Hewlett-Packard Co. 



l)t(:pt»l)pr lBS;j HewlPlt-Pai'lonl .liMiriial 73 



Thi* DSPIiljOUO's host iiilcrface bus is ccinnoctecl to Ihc main 
prorpssiir lliniu^li I wri lii-bil buses and a stal<^ nuidiinc. 
ThL' statp niat'hiiiL' allows the main processor lo wril^ to Ihe 
DSPririOOO's host input i fgistiTS by placing S hits r)f dala and 
a 3-hit register numlwr onio a It'i-blt control bus (only 11 tif 
Ihe 1(1 bits are uspcj). When (lie DSPfjiiOOO has data lo st'nd 
lo ibe niain processor, the slate machine reads three 8-bil 
host registers and formats the 2'1-bit result as two l(i-hil data 
Irarisf'ers over tJie main processor's input bus. 

The DSP5G()CI0's firmware ts wrilten in C for most control 
fimctions and asseinhly iansSnaiie fur linie-crilical data pro- 
teasing liinclionH. Il is stored in com[>ressed format in the 
main processor's ROM. During power-up, the main proces- 
sor deronijJresses and downloads die fiiniwaj-c inio tlie 

Main Processor. The main processor's jirintcd circuit board is 
identical to (lie processor board used in Ihc IIP 70y(!IOI5 local 
oscillator module. Using this prijiled circuit buard alltiwctl us 
lo take advantage of the thousands of engineerijig hours that 
have gone into the the HP 70i)(K) nnjdular spc'clnim analyzer 
firmware. 

'Ilie main proce.ssor hoard conhiiiis an Mf'(nS(ia(l CPU ninning 
at 20 %\Hx. and an MCfifiSl^l lloaliug-poini coprocessor. The 
boiud al.so accepts a plug-iii <langh!er Imaid which is nu- 
renlly loaded witJi IM byte of ROM ajid IM byte of RAM. 

The inain processor board has two unidirecUonal Hi-bIt 
buses (one oul.pul, one input), which are used lo interface to 
other priiileii circuit hoards hi the HP 7090(IB local oscilla- 
tor module. During the investigation phase of this project, 
we determined that we could bnild a state maclihie to inter- 
face these two buses witit the OSP")(iO(IO's host interface. 
Since the HP 7<I90() controlled its measurement hardware 



over these two buses, we were able to substitute the optica) 
spectrum analyzer's hiirdware without major changes in the 
HP 70900 firmware. This arrangement alli.wft the DSP56000 
lo handle hardware control and data aciiui.sition tasks, and 
die HP 70000 liniiwaie to provide liigh-level iiseJ" interface 
functions. 

Originall.v. nearly all of the HP 70900 finnware access to 
extemal hardwaie went through a firmware module called 
the local osi'illalor slave. By replacinj; the L(.) slave module 
with our own slave module, we were able to u.sc the HP 
7n!)0l) luniware with suipri.singly little aileration. Some addi- 
tional work was required to add waveSenglh unil-s and other 
spectnim-analyzer-s|iecific functions lo the user interface. 

The HP 70y()0B local oscillator modulo and the HP 714S0A 
and 71451A optic'al specl.mm analyzers are now shijjped 
with identical main controller hoards and identical Orin- 
ware. During jiower-on, the llniiware checks lu .see which 
external hardware is present and configures il.seli as either a 
local oscillator or an optical spectrum analyzer. This has die 
additional advaiiljige in that featiires added l.o the HP 70900 
llniiwaj-e are available in lioth the local oscillator and tire 
optical .spectniiii aiialy/.ers. 

Acknowledgments 

The autlioi s would like to thank the other members of Ihe 
project team — Dave Bailey, Doug Kntglil, .Jim Stimple, Steve 
Warwick, and .Joe West — for their contributions and niaiiy 
beneficial discussions, .Mso, lh;inks go to memliers of the 
pholonics seclioii at Hewlett -Packiird Laboratories, in par- 
ticular Bill Chang, Brian Helfiier, Steve Newton, and Wayne 
Sorin, for their u.seful consul tation.s. Finally, thanks to Rory 
V'an Tuyl now of Hewleti-Packartl Laboratories who was 
involved during the early stages of the product design. 



74 Ih'i'fuih^s llpwlrtt-Pai-kartt .louraal 

© Copr. 1949-1998 Hewlett-Packard Co. 



A High-Resolution Direct-Drive 
Diffraction Grating Rotation System 



Creating a higti-resolution, high-speed positioning system that can provide 
over two miiiion data points per revolution of the diffraction grating 
required a design that is much different from the gear-reduction 
positioning systems typically used in optical spectrum analyzers. 

by Joseph N. West and J. Douglas Kiilght 



The wa\"elfng1 li liiiiiit!; of llic double-pjiss nifinocliromator 
iisfd in tilt' HP 714r)0A iirnl 71151A optical sppclrum analyz- 
ers is L'onlroileil by I he an);iihu' prisitiun ol llic tliffz ac'tion 
grating. After the input, beam is coOiniateti by Ihe lenis. it 
strikes tJie diffraction grating, where eatli wavelengtJi is dis- 
jjersed at a differpui angle. For each angle of the diffraction 
gralinfs a n ares ponding wavelength is j>a.ssed l)arl< ilirongh 
Ihf optics and fniused on Ihe center of the fii-st-jiass apcitiii-e 
l^.shi). Tbe width nf tiie slit det.eniiincs tlie resniiilion hajid- 
width of the wavelengllia that pass through thi* remainder of 
the system to the detector. Rotating the grating causes tiie 
dispersed wavelengths to sweep across the slit, iuaking the 
inonoch.ro mat or act as a tunable filter. Fig. 1 shows the 
housing thai contains the optical and cleclroniedianical 
components that make up the double-pass nionoclironiat.or 
assembly. 

The angular resolution requirements for the grating position- 
ing system in the monochi'omator can be determined by 
calculating the relationship between Ihc angular position rif 
llic diffraction grating relative to the coUiniated liglit and the 
spatial dispersion of the light at (lie resolution bandwidth slit. 

Using Ihe graiing eiiualion'^' for a scanning monochromator 
hke (lie one used in llie HI' 7I45(1A and Ti iri j A opl.iciil ,s|iec- 
tnmt analyzers, I'oughly 600 niicroradiatLs of diffraction grat- 
ing rotation per nanometer of optical di.spersion (at I'iOO nni) 
i-;ui be calculated. To represent naJTow signals, it is ilesirabli' 
to have at least sixteen data points acrfiss die narrowest reso- 
lution bandwidth of llie histninient (0.08 nm). Tliis translates 
to 20(1 data points per nanompler of dis|icrsioii. Di\iding (iOO 
micrnradians liy 20(1 points gives an angular resolution re- 
quirement of aboul Ihree microradiaiLs ("0.00017 degree) per 
jioint, or aboul 2,100,000 data points per revolution of the 
diffraction grating. 

Conventional Methods 

Till' tradiliiinal approach l.o building Much a high-rcsoiuriun 
positioning system is to use large amounts of gear reduction 
(see Fig. 2). This is the approach used in most older optical 
specimm iinalyzers. In (liese nvslems there are commonly 
iwo stages of gear reduction. The lirst stage might i-oiLsisr (if 
a j)l;inf'1ai7 gearhead with a reduction ofahoiil 20:1 which 
would be folhiwed by a wiinii drive with an additional re- 
du{'lion of about riO:! for a total gear redurtinii of about 

' Sae "DillracliDn Graling" on oaga la 



(il)0;l. The advantage of this approach is that it reduces the 
resolution j-equiremenLs of tile primaJ>' feedback dei'ice, often 
an ojjtical encoder. It is jiossible to get away with using a 
tairly low-lechnolog.v, 1,000-line TrL-outi.iut encoder, T.:Ookiiig 
at e\'ery zero crossing from Ihe two quadrature channels of 
Ihb encoder gives a resolution of 4,000 counLs per re\'olution 
of die encoder which c ombined with the (illOil gear reduction 
provides sufficient resolution to position a diSraction grating. 

llowe\'er, Ihe gear rcdiiciion ajiproach has several draw- 
backs. One of these is speed. With a (iO0;l gear reduction, 
ihe diffraction graiing is rotalmg at only i/tiOO the speed of 
the motor. Similarly, the acceleration of Uic diffraction grat- 
ing is only 1/CiflO the acceleration of tlie motor. Moving the 
diffrai'tiijn grating at any significant s]jced re(|uii'es that Die 
motor and gear triiin be accelcrale(i to vei> high speeds. 
Reversing the direction of the graluig requires decelerating 
the motor and gear train and accelerating 1 hem to high 
speeds in Ihe opposite direction, In the past ihis speeii pen- 
ally l anic to be ai'cepled ;is inevitable because no alternative 
w;ls generally aviiilahle for such a high-resoiiilioii system. 

A second ch'awhack to the gear reduction approach is back- 
lash. Backlash is Ihc ship cxhibitcri by a gear train when Ihe 
direction of l otalion changes antl die gears change from 
contact on one gear face l.o contact on the opposite gear 
fai'c, A number of techniques can be applied to mininiize 
liacklash in a system. These techniques generally consist of 
a method for (■ompliaiilly loading the gear train lo ensure 
that the same gciu- laces remain ui contact regiirdless of Uie 
direction of rotation. Fig. ;i .shows a simple spring used to 
reduce backlash in a s,ysleni that rotates over asimill angle. 
Obviously, in a riill-rol^ilion system a more coniplicaled 
.scheme is requirerl. These aiilihackla.sli t.ei-hniques help, but 
do not eiiininale backlash. Kven though ihc same ge;ir faces 
remain in contact, as the gear train reverses dirci-iioii lubri- 
cants are smeared in an opjiosile direction and Ihc bearings 
llial .siippoH the gears are dcOected in an opposite direction. 
For a high -resolution system, some degree of backlash will 
still be evident, 

A third drawback of geared systems is susceptibility l.o eiTora 
caused by weai' or by changes in environmental conditions, 
hi a ly[)ical geai-rediicl ion .-iystem, the angular position of 
ihe drive motor is monitored wilh an optical encoder. Tlie 
iuiguliu- position of the chffraction grating is nol mejiaiu-ed 



) Copr. 1949-1998 Hewlett-Packard Co. 



I)r?i'i'iiibpr Iflanilrwlcll-i'ai.'kard.luiimal 75 



Optical 
Encoder 



Gialing 
Rnlalor 
Ass e mil I) 



Output Fiber 




Fig. 1. The huiisiiig fur the optical ami GleLtruniecliamcal componenEs in the HP 7145QAaml T145i A optical gpectriim analyzprs. The 
dinieiAsicins for the hciusiiyi an~ 8(1 mm (3.16 in) high, 150 mm (6 in) wide, and 400 niiii ri5.75 in) kmfi. 



76 Dpofmber 1993 He wlelt- Packard .loiimal 

© Copr. 1949-1998 Hewlett-Packard Co. 




CiiartiHd 



DC 



20: T fteduclion 




Fig. 2. A wunii drive grating mtaliaii sysiati typically used in ofittcal 
s|ipclrii!ii LinalyKers. 



Fig. 3. BtirHjjsh is the slop c xliiliiinl Uy asi'ar rniiti whfJi the tlirpc- 
liiiTi ril riiuaioii ( liiitifii's ajid thf Hfwrs <'liiingp froni cimiai't r>ii imt 
fii-ai" Tacr- i'l i-'iiitai't. uji llip rjpposii.e gear Cii'p. A simple spring (;:in be 
iiswi ici rpdut-e backlash in a^'slcrti llial f.ilal.es over a small anglr. 



dirt'Ptly. lni( is infem'd from liic iniilor position and the gear 
rario. As goai-s wear oi' expand and coiitract wilh lonipera- 
ture. or as lubricant viscosities increase over lijne, the ac- 
tual position of the diffraction gratinj; relative to the motor 
piisilioii will chaiijic. Periodic recall tirat ion is needed to 
foiTecl these eri'oi's. 



Mirror 



Shu tt A no id Incndar 



Direct-Drive Gralinf; Rotation System 
Based on (he ilnmhueks mentioned almve. Ilie decision was 
made to use a direct -drive, dirw;l -readout grating rotation 
system in HP's dinil)lt*-pass sratuiing monochrcniiator. Fig. 4 
shows this system, A cirive motor and a rotary optical en- 
coder are directly attached to the sliafi that holds the dif- 
fraction grating. Tills system has tln' rollowing advantages: 

• Dtreci measiirenieiil of the angle of I he diffraction grating 

• No errors from backlash oi' "wind-up" ileflections in the 
gear train 

• No snsceplibility to wear ;uid much less sensitivity to 
(>nvironineiit:iI changes 

• Speed, corn pact 11 ess, and ruggedness. 

Intpleitieniiiig a high-resolution direct-drive, direcl-refidoiit 
sysletn placed siinie stringent reciuiremenis on the coinp<)- 
nents used and t.lie design process. Two tiiinss were re- 
quired. The first was a hifjli-torguc niofor for fast starting 
and slopping ;uid high-speed scanning. The motor we I'liose 
m a frameles.s, hm.slilcss de niolor. The pei inaneiil magnet 
rotor itses strung rme-eiU'lh magneis and tnnmii.'i direc tly in 
Uie tlifffdcl ion grating shaft. The slator iinnin;,s to the (ixed 
outer housitig where heal can be di.ssipated in a controlled 
manner. The tiiol.or is hrushless so then' will he no ilehris 



Fig. 4. 'I'lie dlri'i l-drln', din.'cl ri'iHhtiil gniting rulalniii .sysl.eni used 
in the d 1)11 tile -|iass ninriDi'hrnniHtiir 



© Copr. 1949-1998 Hewlatt- Packard Co. 



Dii'i'iiiliiT IliiCI lli'ivti'i[-J'urliiuil.li>iinnil 77 



gpnerali-d by liniHlicH wejirinji ;iikI tin maiiitriiaiii o fhrush 
reiilai-eiin'iil) uvpr ilif life of ihe irisfnimeiil. In ;tcl(iiii()n, 
linisliipsB do niolf.irs have an 3<lvajitage over nmvcntional 
l)nish-tyi>(> mulors in liiat (hero is no rriclioii hccaiise of 
riitibiiig Ijeiweeii ihe coiiiniuiatur ami llie Liriislics, Friction 
is a prol)leni in the control of higli-resoiution sysiems. 

A sL'i'imd, ajid more liifficiili rcc]uireMicnl is a iLigli-resfjliilitm, 
opticai-eni iider-l>aseil measure Mu*nt sysieni r.liat is able to 
resolve directly more iJian two million points per revolution 
of (he diff'radioii graliiig. Hiiilfliiig siicli a syslem involved 
searciiing for llie kitesl in opiical encoder lechiuilogy and 
ihen ajjplying considerable design effort to accomplish the 
necessary resolution goals. 

The enctider use<l In our syslem is a sine wa\e 'juipnl incre- 
mental rotary optical encoder witli S.OOfl lines on the rolal ing 
disk. In an incremental encoder a single light source, ly]jically 
a lighl-eniillmg liiiide. shines a iieaiii nfliglil liirougii a lotal- 
ing disk dial coniains radial sills wliicli allenialcly transniil 
or block llie ligiil. The liiiht pitssing lltrough the slits is tiv- 
tected by two set-s of phiHoileleclors thai convert tlie lijjlit 
into elect rieal signals. Before hitting ihe photodctectors, the 
light also jiasses through a phase plate containing two addi- 
tional pailenis of slits, Tticse two patterns are offset slightly 
relative to one aiioiher so llial the signals received by the 
I wo sets of photodetectors ai'e fit) degrees oiii of phase. The 
quadrature lelationshlp nflJie signals makes it |io.s.sible for 
the user to know the direction of rotation of the encoder by 
looking at which channel is leading by 90 degrees and w hich 
channel is lagging. There is also a liiird ciiannel that pro- 
vides an index pulse once per revolution for detemiinuig 
absolute position. The outputs of the main A and B channels 
are very clo.se to sinusoidal (see Fig, 5). 

Each zero crossing of the A and B channels increments or 
decrements a position comiter to provide co;ii'.se position 
information, depending on tlie relative phase of the A and B 
channels. To increase the resolution beyond the usual 4 x 
linecount value, a jiroccss called interpolation is used. Be- 
caii.se I lie signals are sinusoidal, there is additional analog 
iiifnrniaiiun between /.ero crossings which can be extracted. 
Conuiiercial circuiLs are available that perfonn the inieri'ola- 
don i'lmction, but they typically liave interiK)laIloii ratios dial 
are convenient numbers in the base 10 number sy. stem, such 
as or "ilK For a digital control scheme, it is more conve- 
nient to have an inl etiolation ratio that is an integer power 
of two. The IIP Tl lfjOA and 7145 lA opneal specli-um analyz- 
ers use an interpolation ratio of 64:1 factually 256:1 for data 
acquisition, but the two lowest-order bits aie not used for 





Fig. 6. A hlihck tliaflmin of the closed loop dtHraftiun grating syslom. 

control ). Tliis gives a resolution of ilOOO x 4 x M = 2,304,0(10 
counts per revohition detemiincd directly from the encoder. 

Inieipolation in thi.s design is achieved by amplifying the 
two sinusoidal ouiiuiLs of the encoder until tire minimum 
and inaxinunn values ;ue,jtist within the raiiHe of an mialog- 
lo-digilal converter (.MX"), The oiil|)Uts of the AUf are (hen 
a digitjil reiireseiilatioii of (he slue mid cosine .signals. The 
ratio of (he two digitized outputs is the taiigejit of (he migle. 
By looking up the ratio in an ;irclangenl table, tiie iiiigie that 
is the inleqiolated fractional position between the sine and 
cosine zero crossings cmi be found. The accuracy ol' (he 
inteipolalion is de[iendent upon the degree of distortion in 
the sine imd cosine signals, the phase migle between them, 
and the number of resolvable hits in the an;ilog-to-digital 
conversion process. 

One of the problems encountered in controlling high- 
resohilion sy.slems siicli as this is the cluuige in behavior of 
die friction in the system as the system changes from moving 
to a fixed position. This happens when the grating is either 
tuned l.o a fixed wav elength in zero span or it is momentarily 
stopped w hile changing directions at die beginning or end of 
a sweep. Tlie degree of resolution is so fine that the differ- 
ence lietween the static case and the dynamic case becomes 
readily ajipai'ent. 

Wliile the system is in motion, thai is. seivoing to a moving 
target position, tlie friction is a nicely behaved linear damp- 
ing icnu. The rotational inertia of the sy.slem inleraeling 
with the motor winding resistance forms a sinijile jiole. 
There is also a pole at zero freciuency since a consl;iiil input 
voltage to the motor gives a. steadily increasing angnlai' pnsi- 
1 ion. This is a fairly simple system to close a sen-o loop 
around (see Fig. li]. Fig. 7a shows the open-loop freijuency 
response of (he system while in motion. Tlie noisy measure- 
ment at frequencies below 1 Hz is because of a sig!ial-lo- 
noise ratio problem in this particular measurement and not 
an indication of the actual low-frequency response. 

Wlien Ihe system is servoing lo a fixed target position, static 
friction will lock the pieces together, Tlie elastic behavior of 
the pieces then changes the system behavior to thai ofa 
spiing-iiiass system with a cimijilex pole p;iir. Fig. 7b shows 
die measured loi.ip characteristics of tlie system when it is 
servoing to a fixed largei position. Again, the entire open- 
loop response is showii, iiot.jusi Ihe rotor characteristics. 



Fig, 5. Elei lrital oMIjiiit.s from llie upt.ical shaft eiicoiler 



78 HiTemherl!i9aiW[pH-Pni-tiart|.l"iini:iI 

©Copr. 1949-1998 Hewlett-Packard Co. 



40-D ; 










: ; ; ; 


' 1 










*-r- -'t't 




: : .; 




— r- 










s 


- ' ' i l 






















C 




















L 






Magnitnds 














L ; L L 




— 




III::, 


I;' 


— — * -'-^-l-l-* 




■ 1 ' ( 


;l ■ 





ft 
s 

£ 



t-ise 



050 



lb 



508 



1 



~4a.a 



- • 1 


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I i t ■ 
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pfiflEe \ ; ; ; ; 


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Fig. 7. Fr[-c]ue[icy respunse orUie diffraction grating roUtur (a] when 
tliR lat'gel jjosition is mnvitig anii (b) when the tarfp;t position is nxcd. 



The spring-fliass resonant'e in the fixed position is readily 
apparent from iJie peak in the magnitude response at -HI Hz. 

The torque required to break the system loose from a fixed 
position to rotation is not a well-known value. The liesi that 
can he done is to spccifj' the maximum breaJamay (nrque of 
the mechanical elemenls iHul ilien design ilie crtniTOl system 
so that it is able lo iteal with Ihe system breaking free at 
some lower lorque. The amount of breakaway torque will 
vary depending on llie positioii of the system, tiie current 
envirunnienial condilioits. and other variables. 

At lery low rotation rates, the system rapidly jumps l)eiweeii 
ser\'oing to a mo\iiig target position or tti a fixed target posi- 
tion. If Ilie ItHip is not ci)nipeiisate(l lo take this e)iange of 
behavior into accoimi, the result eaji be a system that is 
stable when moving but that (iscillaies when ii ser\os at a 
fixed position. 

!n our design we found a single set of loop rompensation 
values diat provide a stable response for cithei' operating 
mode, ensuring thai the system works well imder all 
conditions. 

Conclusion 

Direci-di ive technology has been apphed with great success 
in a nmnber of iiidnstiial applications ranging from phono- 
grapli turntables lo indiistrial robots and military giui luiTel.s, 
Applying fliese techniques to an optical spectruiri analyzer 
produces a system Ihal provides hisl, accural e, ami reliable 
rotation of Ihe diffracLion gralin^;, and wilh regard to motion 
i-oni.rol, brings Uie latest technology lo optiral spertitim 
analysis. 



© Copr. 1949-1998 Hewlett-Packard Co. 



Dopemhcr m:\ lk-wltlL-['acliarii,li,iimal 79 



A Two-Axis Micropositioner for 
Optical Fiber Alignment 



A positioning system with submicron resolution is used to keep the output 
fiber accurately aligned with the light coming out of the monochromator 
during movement of the diffraction grating, 

by J. Douglas Kiiight and Joseph N. West 



The iloiihle-pass mnnonhromaror ilesigii used in the HI' 
7145(1A Hiid T1451A ojitioal s[>fclnini analy/frs oOitk a iiuni- 
her (if iierfiiniiaiice adviuitages iivcr cojupetiiig riioiuiclim- 
malor <iesigiis, Several uf Uiese perfitntiance advantages 
eonif from the secondai'y fiiteriiiH effects of the npl.ieal fiber 
used a1 Ihc output of the seeuiul jjasH of tlie mciiiuchi onia- 
tor. The hiiiiled eross-secliojial area iuid hniilcd niiniei'ii ai 
apen.tirc of I he filler lielp rejeci stray hglil, giving giioil dy- 
naniif mnge perforiiiaiu-e imd good spiirioUH response ivjec- 
lion. t'oii])liiig ihe iighl into filier also allows the use of a 
small, low-noise plujtodeicclor whii h results in excelleni 
sensitivity even with rapi<l sweep speeds and minimal video 
fdleiing. hi addition, going into fiber allows the instrument 
In ha\'e an ojitical monochromator oulput that offers hoth 
fixed -wa\"elength and swept-wavelengtli modes of operation 
with a full range of resolution liandwidths selectable by tlie 
user. Tlieso advantages are signifitiinl, but tiesigiiing a sys- 
tem to keep Ihe outpul fiber accurately aligned witli the 
light coming out of Ihe monochromator during sweeps 
proved to i)e a consideralile design cluillenge. 

The Positionmg Problem 

Ideally, in a perfectiy synanetric di.uible-pass ntonoclu'onialor, 
Ihe spot of light al the ouipul of the second pass would not 
move. However, in reahly, as the dilTraetion gr;ilitig ioiat.es 
and the instnmient sweeps in wavelength, die spot (jf light 
at the outjiui of the monoclu omator does move slightly in 
two dimensions. It is therefore necessary to track (he mov- 
ing spot with the output fiber to capture the liglil completely 
and to realize the desired filleiing effects. 

In the dispersion direction (y-axis in our implementation) 
the movement of the outpul .spiil is Ihe result of asymmetry 
in the system. The sei-oiid pas.s is farther oil' the avis iif the 
lens than the first pass. This asymmetry is necessary to 
a\ oid picking up light frtmi die first pass with the photode- 
tecior. Tlie inovemenl in tlie y-a>ds is predicted by theory 
and is consistent from unil to utiit. 

hi the nondispersion direction (x-iuds). the movement of the 
outpul sfiot is the result of nianufacturtng tolerances ibal 
cause the lines of the difftactioti grating not to he perfeeUy 
parallel to Ihe axisofroialion of the diffraction gratuig (see 
Fig. 1). Parallelism and periJeiulicidarily of critical parts in 
Ihe grating rotator assembly lire aligned to \ eiy close toler- 
ances. Even so, tens of micro metei-s of beani movemeni still 
result. If the lines of the grating are not perfectly parallel to 



the axis of rotation, the lines of tlie grating will precess'i' 
aboul Ihe axis of rotation as the grating rulaies, causing tlie 
ouipul s[ioi to move in Ihe x-avis. P'or a given monochroma- 
tor this movemeni is very repealable anii tra<-kiiig is possible 
with a precise positioning device. 

TVaching the Output Spot 

Once it was determined thai a iiiicropositioiiing device was 
necessary to track Ihe ouipul spot, other advantages nf hav- 
ing such a device were envisioned. One julvaniage is noise 
and stray light cancellHtion. Most systems lhal attempt to do 
noise cajicellalion chop ihe optical sign;d wiih an aperture 
I hat allenialely passes or blocks the Uglit to the detector. 
Wiiei; llie Iighl to the detector b blocked it is passible to mea- 
sure the electrical noise of the detection .system which can 
be sulilracled froiu Ihe reaiiing obtained when the aperture 
passes light to the deleclrtn 

When lite aperture passes light to the detector, the output 
of the delecii<jn system re[)resenis sigiiiil + stray tight 
electi ical noise. Sulilract.iiig elecirical noise leaves signal + 
stray iiglit. 

' PTbuessiun rs the ivpa at mDllDti eiiperienca[lllyaBV'i^PBi^"W When a force is scpllBdsl 
right angles 10 the atis af rotalion 



Dispefsinn 
Dire Elian 




Fig. 1. A liUTrai'ti'.m grailng in which the lines nf rhc RriKtiig ;u'c ucil, 
Ijerl'i'i/ily jiaralli'l lo ilie fling's axis nf riiLatitm. This slight niis- 
iUigninenl wiUdBUse muvemenl of Uie output spot in the x-axis, 



80 lieirmli^r ISttO Ht-wlm-Pflckard .Joumal 

©Copr. 1949-1998 Hewlett-Packard Co. 



With a fasi and accurate micropositioner at ihe output of the 
monochromalor i( is possible to perform another kind of 
optica! chopping to remo\-e botii electrical noise and stray 
light. If we assume ihai (he stray lighl is relatively uniform 
in the region eiroimd (lie output spol as it niiglit l>e in the 
case of scatter from optical components and tlifftise reflec- 
lioiis from ihe inside of the nionochrornator ca\iiy. physi- 
callj' moving tiw output fiber laterally away Eriini the output 
beam wouk! allow a measurement io be made of stray lighl 
+ electrical noise. Alternately nio\Tng (he ontpni fiber into 
and out of the output beam allows the stray li^I + electrical 
noise lemi to be subtracted from the signal + stray light + 
electrical noise tenn leaving only tJie signal value, Tlie digital 
signal processor described in I he article on page tjS controls 
the micropositioner aJ id perfoims the .subtraction of stray 
light and electrical noise from the measurement. Tltis mode 
isaclivaierl automatically in the HP7U-WA and 714-t1.\ ana- 
lyzers w hen Ihe user requests a very sensitive setting tliat 
results in a sweep tinie greater than 40 seconds. 

Having an electrically actuated micropositioner at the output 
of the monochromator also eluuinates Ihe need for the user 
to make manual attjiistnients to the second-pass aperture 
(output fiber) relative to the first-pass ajienure (slii) to 
maintain signal symmetry or to ailjusi the uplical oiilpm of 
the HP TI4S1A for rLher-in/fiber-oiil nieasurements/" Most 
optical spectrum analyzers that have double uionochroma- 
tors require the user to at(just The optical output for maxi- 
niuni signal strength at a gi\'en wavelength with manual 
microniPters. With the HP optical spectnim analyzers, out- 
put coupling is automatically mamtaincd oi cr the entire 
wavelength range, If the inslmmenl is droppcti or cxiicri- 
ences significant changes in temperature, there is an AUTO 
ALIGN key on the front psmel of Ihe insinmieni thai llie user 
can [jusli to initiate an aligumeni routine to eiisine that 
optimum output coupling is reestablished. 

Implementation 

To couple lighl into the outpnl fiber efficiently, Ihe fiber 
must be accurately held in the output foc al platic of tiic 
monochromator while it is aligiu'il Io the spol within frac- 
tions of a micrometer. Tine oiitput fiber musi be able In track 
Ihe movement of the spot smoothly iLS the instniment 
sweeps. Thus, the micropositioner must have the following 
characlerisfics: 
Sul.miicrometer resoliLtion 

Smoolh operation (no roughness from rolling elements) 
No friction from shding members 
No screw or gear backlash 
Compact size 

Imnnitiity to changes in orientation relativ e Io gravity 
Some insensitivity Io vibralion and environniemal 
distiirbaiu'es 

Negligible movement in the z-axis while moving In Ihe 
X-axis aji(l tliey-axis 

Approximately ±176 juii of travel in each axis 
Remote activation 
Fast response lime. 

The micropositioner design used in our double-pass 
monochromator consists of a two-axis pimiar fiexurc plate 
acltiated by voice coil linear actuators with striiin gauges 
niounted on the flexing beams to sense the defiections of 

TtiB HP T'flMfl duBSfiT tiravjdE raistoiTiBr acceis !□ lha monadiromaKK rajtpiil (itier 



those beams and therefore the displacement of the fiber 
iPig. 2a). The strain gauges and the linear actuator a.ssoci- 
ated with each axis together fonn a closed-loop position 
servo sj-slem I Pig. 2b ). The riisplaceiiieni of each ;ixis of the 
micropositioner is delemiin(?d by the target position, which 
comes from the calibratic)n ROMs m the data acquisition 
unit described in the artitilc on jiage IJS, These HOMs contain 
factory cahbrsted x-y position values (described below) thai 
are correlated viith the angular position of the diffraction 
grating. To select a particular target position, the data acqui- 
silion unit computes the wavelength associated with the 
current position of the diffraction grating and uses tliis 
wavelength to iii'lex into the calibration ROMs to retrieve 
the x-y pair associated with the wavelength of interest. Each 
x-y pair is sent to the appropriate 12-bh digital-to-analog 
converter where It is convened to a voltage anil apphed as 
reference input to Ihe micropositioner servo system shown 
in Fig, 2b. Tlie voltage value from the con\'erter is propor- 
tional to the dis|ilaceinent each axis must make to keep Ihe 
outjiut fiber ahgned w itb the light coining out of the mono- 
chromator during movement of the diffraction grating. 

Flexure Plate. The fiexm'e plate used in the micropositioner 
consists of a frame- wit hin-a-lrame design (see Fig, 'S), Tiie 
oulpr frame mounts rigidly to the monoclinnuator housing 
shown in Pig. 1 on page 7lj. A four-har (parallelograui) Hex- 
ure linkage connects the inner frame to Ihe otiter ITaine aiifl 
guides the inner frame in x-axis motion. A second four-bar 
flexure linkage connects the fiber mounting collai-** to the 
inner frame and guides Ihe fiber moiiriling collai- iny-axis 
motion, .\clually Ihe inner fnmie ami the fiber moimling 
collar each move in an arc, but for small chsplacemonts the 
riinlion is close to linear. During (alibration at the factory the 
X and y locations of the uii en j positioner im^ recrirded for 
each angiiljir grating positiim. It is the aliilily of tlic iiosilioner 
TO repeal to tliesi' pri'set po,'iitiiins thai is imiiorlant and not 
the ability to reach absolute locations in Cartesit^i space. 

Hcxures of various t,vi)es have been used in designs for many 
years. The term fiexiire refers to an assembly ccmiposed of 
one or more flexible hinges maiie of an ela.slic material, 
which is lyiiically a metal that defiects within lis elastic 
range. Flexure assemblies are somi'finies conslniclc<.i from 
strii>s of riielal such as beryllium copjier or pbosjihor bronze. 
Flexures can also be made by machining material away 
from a solid metal plate in such a manner a.s to leave webs 
III ajipi opr iaie locations to act as flexure .joints. 

The micropositioner in the double-pass monochromator is a 
monolithic fiexiire of this second tyjie. Two lyT>es of nexmx- 
.joints are tised in Ihe micropositioner. The first type is called 
a transverse circular fiexiire joint. This type of joint consists 
of the web that remains when two holes are drilled and 
reamed near one itnother. This type of joint localines the 
bending Io a small region, winch gives good stiffness peipen- 
diciilar to Ihe ])lane of moliiitias well as good torsional stiff- 
ness. Because the bending is localized, this type of .joint sees 
relatively high stresses for a given angular dis|i]acement. 
The .sjiring constanl for a right circiiUu Ocxure hinge can be 
expressed by the equalion:' 



•■ The fiber iJiDLUlinD cotlai is wiiera the itiotiothiofiiatiir's nulpui libsr is trajUFited 



© Copr. 1949-1998 Hewlett-Packard Co. 



DpCEmlier tiH)3 Hcivlett-Packarcl Jniimiil 81 



Fiber Mounliny Cflllar 





T«rg«t ^ 
PosltrDn 



Digilal-tD-Analpg 
Canvartst 




Lnnp 
Conipeiisdliiin 
Rllei 



Pulsa 
Widlh 
Moduliilor 
□rivei 



'Cnnrains ihe Slrain Gauge Amplifier 
and Ihe SUdin Gaugfrs, 



(b| 

Fig. 2. (a) Tcip and side views of [lie x-y micnipfisiLlimer iiseil iii Ihe cInillile-iJas.s iiiiiiLrji liroMiiiliir (Ij) The sciti) loop s(:liemalit for one 
axis iiflhe positioner. 



where: 

M is ihe niumciii requiri'd ii> hi-mi llie llexnri.' Lhrough 
an angle t) 

E is the f ir iiioilulus nl' liii' Ot'xnre iiiiUerial 
b is [lie wiii[li ui' llic nt'MiriMf.iiiil 
t is the miiiinmni [hicknesH uf liie flexure 
R is llie radius of the right eirc ulai' rutoiit.s. 

Fig. 4 shows iJiese parameters. 

The niaxiniiini stress in the right ciruiiltir hinge can he 
expressed by the equation : 

, 4E['/^B 

Tlie second lyije of flexiire jomi used ui the niicToposi[i()ner 
is a tJiin rectangtiJai' beani flexure similar to the stiijis used 
in nonnionolilhic flexures. TiiLs type of flexure Joint thatrib- 
ules llie liending over a longer disianre and was chosen 



heeause it is suitable for mounting strain gauges lo sense 
deflec-non. ,Slaiid;ii'd reci;mgiil;u' beam equations Lan be used 
for caleiilalioii.s associated with this type of flexure joint. 

A discussion of flexures would not be complete whhoul 
mentioning llie topic of fatigue. The designer must constanlly 
calculate and recalculate the fatigue life of the tlexure Joints 
as llie design is modified lo ensure that fatigue failures will 
noi occur even afler many years of conliiiuous use. 

Linear Actuator, Each axis of the microposi doner is actuated 
by a viji<'e i riil linear actuator Tlie linear actuator consists of 
a nio\iiig coil in a fixed pem\aiient magnet structure. It is 
very nuicii like a voice coil used in a loudspeaker driver, but 
jieifoniiance is optimized for force I'ather Hum frequency 
le.sjiunse. Since there is no conlaci lielween Ihe coil and the 
magnet stnictiitv, ihe acluatiir does noi coniiihule any rougli- 
ness or sUding friction lo ihe operalion of the positioner. 



82 IJiTfiiiber ilPwlen-PnrkarU .liiiiniiil 

©Copr. 1949-199B Hewlett-Packard Co. 



Fibei 
Mounting 
CdIIbi 



Oirtpul Fibei 
GoGE Here 



trmet 
Frame 



thai Link the 
MoumiiTg Callar 
10 ihe Innei Frame 




aiMsr 
Frame 

Rexura Linkages thai Link HiB 
OutHrrD the fnner Frame 

FiR. 3. Till- ncxiiri' plate u^rd in Ihi- iiiir riiiiosiiiriner. 

Strain Gauges, The feedback sensors used in Ihe floseil-loop 
position servos are si rain gauges. Sli^n gauges were in- 
vented in the 1930s and are used primarily in force transduc- 
ers or load cells. They are essentially resistive elements 
whose resistance changes witii strain as they are stretched 
or compressed. Tliey ai-e honded to the flexintl beams of the 
positioner with special low-creep adhesive. As the beams 
bend, the outer surfaces of each beam experience tension 
and compress inn. Since the strain gauges are tightly honded 
in place, cat:h gauge experiences essentially Uie same strain 
as the surface upon which it is moimted. 

The -signals pniiinct^d by a si rain gauge are rather small. For 
the geometry of Ihe muToposilioner used in liie optical 
spectnmi analyzer, the change in resistance for each strain 
gauge is aboul ILfi milliohtn (oiil of H?iO ohms I for a one- 
micrometer position chmige in tlie llexure. To detect small 
changes such as this, balanced bridge circuits are the usual 




Fig. i. The lucatiniis t>n a tnmsvfise cirt'iilar ilfwiri' juiril for Iht- 
pafiiiiiPiers used in the equation l.ii compute Ihe spring mnslanl fm- 
Ihe cirt-iilar llexure. 




Strain Gauge 
an Tup ol 
Beam . 



Strain Gauge ^ 
on Bottum '> 
o1 Beam ^ 



Precision 
Matched 
Cunent 
Sources 



i 



SSkQ 



U 



► 



Td Summing 
Jui>ciLon in 
the Micropo- 
sitianEr's 

'fcuplifierl Ssrvo Loop 



5Dkn 
VA- 



-lOV 



i 



Fig. 5. The strain gauge ariiplilier used in tlie inicrnpnsitione.r. There 
is one aniplifiet for each asis. 

choice. In the case here, it is desirable to have a linear rela- 
tion Ijetween change in position and change ui reslslaiice. 
To achieve this, tJic usual Wheatstone bridge circuit must 
have two of its resistor elements replaced witli matched 
current soiu-ces (see Fig. 5). 

Tlie change in voltage thai results from tlie current IliiwlTig 
in the strain gauge resistors musi be jmiplified 10 usable 
levels. Tlie cuiTent Ls limited lo around Ti niA In limit healing 
of ihe flexure. The output from Ihe bridge eireuil i.s arimnd 
4,8 microvolts per micrometer of position shift. This signal 
is amplified to 0.7 millivolts per micromeler for use in (he 
position servo loop. 

(\n amplifier used In an application such as the double-pass 
mtmocliromaior niusi liave good input rharacl eristics, in- 
cluiliiig low iii[HiI i;iffsel voltage and low inpul offsel drift. 
Good ct)mnKin-mo<le rejection is also dcsinilile. The lunjili- 
fier used in our design (an OP 77) has very good itipu! char- 
acteristics, bill the common-mode rejei'tion is not adeciuate. 
The amplifier alone has a common-mode rejection ral io f)f 
one microvolt per volt or 120 dB. Tills can be degraded lo 
aboul 48 ilB by Ihe 0.1% resistors used in the rest of Ihe cir- 
cuit. The common-mode voltage in the original breadboard 
was aniund volts. This could produce an oulpul offsel 
of up lo ±|!) mV, which is the equivalent of 19 micrometers 
of movement in the flexure. Since submicromeler resolution 
was needed, this was clearly a problem. 

The scheme used ui our monochronialor Is to drive the 
strain gauges in such a way thai the common-mode voltage 
is sensed and servoed lo /.ero volts. In Fig. 5 amplifier 1 is 
the sirain gaupe amplifier, while amplifier 2 is used to sense 
the common-mode vnll.age iuid drive the .strain gauge volt- 
age source up or down until the common-mode voltage is 
zero at the sense point. 



© Copr. 1949-1998 Hewlett-Packard Co. 



Dteenilier 1H93 Hewlult-Packani.lnumaJ 83 



dB 



^^ dB/Di» 



-«.a 







• 1 


4 1 H 1 






> - * ■ 

4 - 1 • 


-TT-T- 






_j 


f i ii. ITT 
1 * *■><■» 


1 r-r-f- 

t Ik' 


< 1 1' 
. 1 1 


: I ; \ 


• ' » 

\i 1 • 


1 % 1 < I * 1 > 


I 1 . ■ 




] i :i 






• > f < 

-^^C— f \-\ \- 


1 1 < 
» 1 < 

lil 








1 ■ 1 


: — V? 

\ 111 
» III 


1 

i 
1 


1 


( 1 1 


■ * 1 
* * ■ 


1 












1 Hti 4 


' * 1 









































Fig. 6. nic IlL'Mirc freiiuericy response rmni liit- Hcliialui- drive tu 
I In- sirniri fflHifie iiutpuT., 

Performance 

The flexure Ireijiiency ref)ponsp from tlir acliialor drive (the 
in|iiil ) l(j I he strain gaiiRe ainiiiifier nut|.nil is shu^vn i[i Kij;. ti. 
TiiLs is Tor line x-axis; llie y-axis is ver>' similar, Thf resijonse 
is duiuiiiateii l>y liie resnnance at liU Hz. ITiis resonajn.-e is 
ilie naiural respfjuse "f Ifie spring-mass system formed by 
ilip flexure lieam sjirins rliaj aeleristic anil ilie mass of all 
the jiioviiig [larls suiipuned by liie flextire, Tlie resonance 
intrtiiliicps a 180-<legree phase shift al frequencies beyotid 
resonance. 

We wanted U> be able to chop tJie optii'al beam using the 
flexure al. around a 20-Hii rate. To do this with a sqtiare-wave 
position characterist.ir requires thai the control loop liave a 
bandwidth that is several times the chop rale. The loop 
iDajidwidth most inchnle the rhird hanimiiir- of ihe i hoji rale, 
and the fifth harmonic or higher is riesiiahle if the settling 
lime is lo be reasonably short. In adiUtion, the 29-Hz reso- 
n;mce is a manifestation of a sensitivity to \ihration. If the 
loop ba.ndwidtlt includes llie resonance then the loop will 
damp Ihe resonance. 

{'losing a feedback servo loop around a resonance m which 
Ihe resiill.ing open-loop i;aui al resonimce is greater tiian oiie 
is usually avoided bec.'aiise osciLalion is tlie typical (and 
iindesired) result. In our rase Ihe design required that we 
close the loop without o.scillation. If the :!!i-Hk resoiiaiiec 
were the only complex pole pair in the Hexure, Iheii flie de- 
sign of the feedback would have been straightforward. How- 
ever, there were also mechanical resonances at 950 Hz and 



at 2.2 kHz (2.7 kHz in Ihe y-axis) lhal pul .'in npper limit on 
what the loop banilviidth could be. In adililion, ihere was a 
delay mechanism because of the constniclion of Ihe voice 
coil actuator iJial added [>ha,sc shifl lo Ihe llcxiire res[ionse, 
cons I raining lire atnomit of phase margin dial could he ob- 
tained. Phase margin has a large unpart on tlie transient 
response of Ihe system. (Hii to fiO degrees of phase margin 
tunis out lo give a very nicely behaved Iransii'iil resjionse, 
while 4"i degrees is al)out as little as tan be allowed if there 
is aJiy concern over (lie transient response. J Since we were 
interested in a good transient response, the available phase 
margin became the delermininfi factor in selting the loop 
b:mdwidtli ai around 101) Hz, 

The target position for the servo loop is taken from the out- 
put of a 12-bit DAC. This is a .settability of one part in 11)9(3, 
Howev er, only a portion of the DAC range is usetl to drive 
the flexure. The remainder of the range is used to compen- 
sate for the met-hanieal and electrical tolerances that deter- 
mine the strain gauge amijlifier oul]3ul foi' the imdriven rest 
position of the flexure. 

Conclusion 

The micropositioner was one of a nimiher of key romponents 
ueedeil t<i be able io build a double-pass sc;inning nionoehro- 
maior. As described above, il provides a me:ms for liimslating 
the output optical fiber in a |ilane |)erpeudicul<u' to the out- 
put light beam to track flip output light beam diuing rotation 
of the diffraction grating. It is able lo move quickly and ac- 
cmately o\ er the necessary range of motion atid has proven 
to be a valuable asset in achieving our performance goals. 

Acknowled gments 

The authors wi.sh to acknowledge Ihe contributions of Jeff 
Hiimillou-Gahart who matle improvements lo ihe initial mc- 
chanieal design ajid caiiieil out calculations for fatigue and 
force requirements. Thanks also to Jim .Slim[)le and Kenn 
Wildnauer who provided techniciil consult.alion during Ihe 
projecl. Finally, ihe aiillioi-s would like Id ackiiowlerlge the 
efloris of Ken Lew, lJor<Jlhy Medeiros, Tom Herto, l^ariy 
Webb, and Ron Koo who developerl assembly and test 
proces.ses for manufaciming the microposit loners. 

Reference 

t. .1. M. Fai ii.s Hiid L. Weishord, "Hiiw to Design Flexure Hinges, " 
MacliiiiF Dt'sirpi. November 15, ISItS. pp. l.^l-lijfi. 



S.| [icrcmlier lltm HpwlPlt-I^rkarrt .roumal 

©Copr. 1949-199B Hewlett-Packard Co. 



A Standard Data Format for 
Instrument Data Interchange 



This standard format allows many HP analyzers to exchange data with 
each other and with applications software, Utilities provide data 
conversion, editing, viewing, and plotting and a function library provides 
access to SDF data from programs. 

by Michael L. HaU 



The Standard Data Fomiat (SDF] is a re cord-base (i liinaiy 
data file forma! that is used to store data fiom a variety of 
analyzrrs maiiulaciured by thf Ilewletl -Packard Lake Stevens 
Instmiiicn! Divisiun, Tlicsc ana]>"zcrs range from portable 
aci.nLslic analyzers and kiw-frecjucncj' FFT analjTicrs lo RF 
vector signal analysers (see Fig, 1). The SUFIlle format is 
flexible eiuiiigh to contain multiijle channels of data, multi- 
ple (tala results ir a single fiie. nuihiple scans or a data re- 
siill (walerf'all), and deeji cajiliiie or conliguoiis lime dala. 
The HP Sn4xxA vector signal analy/.er descrilied in Ihe ar- 
ticle on page (i uses SDF to store trace data (single result.sj, 
time capOirc data (up to one million time samples ), and 
waterfall data. 

Included with each instnimenl llial saves data files is the 
Standai'd Data Formal uiililies, a set of MS-DOS " programs 
that make it possible to ronveil flata from one roinial to 
anotheT', edit SDF records and data, graphically view data, 
andjdot (single or batch) data from SDF files. Fig, 2 lists The 
SDF utilities. 

Interchange ability 

Storing dala In SDF Ibniial allows many i?islruLTienl.s and 
apjiliraiifins lo interchange measnremenl data, lime captnre 
data, ajid waterfall or map dala (see Fig. 1). 

Since the amoimt of memoir varies from instrument to 
iiistrumeni . t.liere are restrictions on die aiiiounl of data 
tiiat each instnLnient can Lm|jort from ;iiiotlier .source. F'or 
exam]jl(|. die ill' Htl4xxA vector signal analyzer with (Jption 
AYS is restricted lo one million samples of time capture 
dala. tlther ui.'il.mnient.s, depending upon the amount of 
memory purchased, have oilier restriL'tions. 



□irecl Exchange 
ISuppons MS-DOS] 

HPBMiOA 
HP a3440A 
HP 35665A 
HP 35B70A 
HP 356BA 
HP3567A 



Eichange 
Lhjaugh Trans I a lot 

HP 

HP3B83A 

HP mwk 

HP 3560 A 
HP 356a A 
HP 3588A 
HP35B9A 
HP35B?S 



Supponed 

Appllcalion/Dala Fomals 

SprEatJshaets 

General ASCII Data 

MATLAB 

MATFIXi 

Gala Sal 5S 

Libraries 



Tlie HP 894xxA contains a 3.5-inch flexible disk drive that 
supports both MS-DOS and HP Logical Interchange Format 
(LIF) file systems, SDF dala stored on a DfJS file system can 
be directly interchimged between this analyzer and other 
instruments in the first colunm of Fig. I by exchanj^ng disks. 

Othei' instruments either do not store data directly in SDF 
format or do not supjioit MS-DOS flexible disks. The SDF 
ulihties include programs to translate instrument data from 
otiier formats to SDF (see "Instxmnent Translators" in Fi;;, 



IntBrlace 

LIF 

LIFDIAG 

DOWNLOAD 

HPIB63 



Logical Interchange Fonnal filer 
LIFiliagnosucs 

DnwnlQail HP 35Bi)A nr HP 35B9A file via RS-23Z 
Transfer HP 3562A or HP 3563A traces via HP-IB 



Tristrunienl Translalors 

B3T0SDF HP 356ZA or HP3563A to SDF 

SOFT0S3 SOF 10 H P 3562A or H P 3563A 

B60T0SDF HP 35660A to SDF 

BOTDSDF HPaSBOflioSDF 

B9T0SDF HP 35B9A lo SDF 

B8T0S0F HP35BBAtoSOF 

B9T0SDF HP35B9AtDSDF 
Apjilicalian ConvBilers 

SDFTDASC SQF to ASCII formfll 

SDFTOML SDF [o MATLAB matrii* formal 

SDlTOIkflX SDF to MATRIXn matrn formal 

SDFT05B SOF to Data Set SB 
ExaminiiiB Files 

VIEWDATA Grapliicallv view SDFdala 

REPEAT Pepeiitivelv eKecute aneiher prograjn la g., hstch plot) 

SDFPniNT TeWually viaw SDF haaders 

Changing Files 

ASCTOSDF 
SDFEDIT 
SDFTDSOF 



ASCII data to SOF 
Change SDF headers 
SplitSDFhlH 



Progtam Interface 

SDFUTIL SOF libraries 

FILTEFSDF MATLAB liliet Ume capture files 

Fig, 2. TIm' Sl.iimijirrI Daiii Fuiiiim iililiUes. 



Fig. 1. St.iuwIanI Daiii Tiiniiai siiiJ|n)rlral liistninn-nlB and njipliciUiaiis. 



© Copr. 1949-f998 Hewlett-Packard Co. 



Dti*mber tSffii HewlMt-Patkarf Jnuma] 85 



Foreign File Systems 

Smiie insliuiiK'ntt. do not support MS-DOS a^i a filf syslem, 
but do support LIF, including the HP ;S5(i2A. HP :^r)(;3A, IIP 
3rilJ(illA, HP :mSA, iuid HP '-imiA analyzers. To suiipoh data 
ijitt'rdiangc with Lli^st' iiistruniE-iits. the SDF iililiiy I.IF t an 
read and write on a LIF disk in tlie diniputpr's inl.ci-nal disk 
drive or an HP-iB connected L-xtemal disk driM-. The vitilily 
LIF rati: 

Irk^ntify HP-IB card location and any connecled external 
disk drives 

< 'ojiy files to and from LIF disks using optional wild-card 
file names 
Dclcic UF flies 
List ;i LIF dircclory 
Initialize a LIF disk. 

In addition, the SDF utility LIFDL^G performs LIF disk haek- 
iips even when a disk is damaged. It includes llie ability to 
read, modify, and write ijidividual disk sectors (iilvliyte 
blocks). 

Tlie HP 35fiOA imd HP 3569A analyzers are hatlei^-powered 
poriiilile instruments lhal conlaiii a nonvolatili' RAiM disk 
anti lia\ c an IiS-232 interface. The SDF utility U( )WNL(JAD 
can Iratisfer a file from the instmment to Ihe fiimpiiler via 
HS-2'i2, 111 addition, Ihe extended data transfer ulililics for 
Hie HP •■mUA conlJiin FILEOO, a filer lhal can liidireclioiially 
Inuisl'er groups of files via RS-232. 

SDF Format 

SI iF files contain binai*y records that describe various attri- 
bules of llie ilala (.see Fig, 3). All records contain a ciimliiiia- 
tioii "I'Ibe lolkiwing t>iies iiTdala: y-liil, ili-liil, and 3J-bil 
inlefitT, :i2-l)il and li-1-bii lloating-point, i;mcl null-lcniiinaled 
strings (C-style strings). Dirferent processors can interpret 
multibyte numljera in two ways: either with thf most .signifi- 
cant liyte IMSB) of the niiiiihei' appearing tlrst I lowest ad- 
tiress in memory) or with the MSB la.st. Numeric ([uanlities 
are stored in an SDF file with the MSB first. 

The first two bytes in an SDF file contain a fnniiat descrip- 
tor to identify this file as being in SDF t'orntat. Following the 
format descriptor are one or more records in ihe following 
fonnal : 



Field 

recortJType 
recordSize 

record dependent 



Size 

IG-bil integer 
.U2-bil integer 

record dependent 



The recDfilTypB (iefines the type of record and lite contents of 
Ihe record depeiideni section. The recordSize istliesixe of Ihe 
record in bytes and is used to liiid Ihe nexi record in l.iie file. 
All recorcis of the same type must be contiguous In the file 
(see Fig, 31- The SOF_FILE_HOfi is the firat record in the Hie 
and the SDF_MEAS_HOR is the second record in the file. 

The SDF_FILE_HDR record contains a pointer to the first record 
of each of the other record types (except SDF_MEAS_HDRI and 
a count of how many records of each type arc in die file, 
Tlie SDF_FILE_HDR also contains information identifying Ihe 
source of The data in the file and the date and time ihe data 
was created. 



n anil' n (} Ctiarsotml 


— SDF Fonnsi OescpUrr 


iUf m HUH 


1 Ftip HiiArli^r Rti^'iiril 

1 1 IPC llbu^DI r^driullr 


OHial ID and Cnunl nl 




SDF DltA HDH 




SDt VEC1DH HUH 




SDF CKHNNEl HOn 




SDF HDfl lUFIiailE tlDtll 




SOI SCAN STRUCT 




SDI SCAN BKiSM SCAN von 




SOf COMMFm HDD 




JDF HDRIi) 




Offset 10 SDI HDRIt) 




SOf wus noH 


-" — 1 Measurement Heailer HBcord 




SDt DATA.HDR 


■• — 1 OT More Data Headsr Reconts 


sat DATH.HDR 



SQE VECTOn HDR 



SOf_V£CTllR HDR 



SDF CIIANNEl HDH 
SIH_CUMNUJ<On 



SUf HDR IUni<|ugj 
SDF.HDfKUniqu*} 



SOf 5CAW STWUCI 
Sc^R Uariabis DMi 



SOF.SGAN.BIG 



SDF. SCAN. VftW 
Scan tfanabte Diiib 
SDF SCAN van 
Scan VpriaUtc Dm a 



5DF_CDMMtMt HDD 



Cmnmani Data 



SCF.COMMfW.MDfl 
t^manl Oats 



bbf_hdr:i.i 



X.Axis Oall 



V-Axit Data 
torliniSDF DATA. HDR 



loilasi SOf 0»ia HDR 



1 Di Maie Veaai Reader Hei:anls 

D or More Ctiannel Header Recants 

D or More Unique Records 

a er 1 Scan Structure Record 

Oar I Sean Big Record 

Q ur Mere Scan Variable Records 



D or Mare Cgmrnenl Records 



Doi1 X-AxisDala Record 



1 Y-AiisDala Record 



Fig. 3. Humdard l.latii l-'nrmal file stniclnre. 

The SDF_MEAS_HDR record contains hifomialion identifying 
the measurement tiiat created Iliis data, including the fre- 
quency anri average parameters. 

Tile SDF_DATA_HDR record contains infoniialion identifying 
each t>pe of data that is contained in the fde. incitiding Ihe 
liala's lyiie, lenplh, format, and nnmlier of logical clianneLs 
[ rows and colunms] of the resnll. Mo.s! insimineiils store 
only one logical channel of one data result in a given file. 

The SDF_VECTOR_HDR record contains infoniiafion relaiing a 
logical rliannel of a gi\-en data result lo a physical input 



Se f>p<Tiiilwrmillpv,-|(.ll-Purkanl.T™iniid 

©Copr. 1949-199B Hewlett-Packard Co. 



channel {e.g., specmim liaia) or a pair of channels (e.g., 
frequency response dala). There musl be one of these rec- 
ords for each logical channel [mw and coluriut) referenced 
in each SDF_DATA_HDH. 

The SDF^CHANNEL.HDR record contains information idenlift'- 
ing each physical daia inpnl channel in ihe inslmniem that 
is referenced liy any data in Ihe file. Most instnunents have 
only one or iw ti chajinels. This record includes infonnaiion 
on how the input is set up frange, coupling, input imped- 
ance). idcniiHes the filters ai)p)ied to the inpul data, atid 
indicates whether the inpul o\ erloaded while acquiring data 

Tile SDF_HDR lUntquel records describe information lhat is 
unique to a particular instrument and not used by oiher 
instnunents. Fur Ihe HP StWxxA. there is a imicjue record 
associated \v'Hh the data lhat. describes the format, of the 
displayed trace (coordinates and scidinj;! al Ihe time Ihe 
data is sa\ed. Tlie tlisplay is restored to this srate when the 
data is recalled and displayed within Ihe HP 894xxA, 

The SDF_SCAN_STRUCT record describes data results that are 
organized in scans. Scans are multiple measurements of Lite 
same result separated in time. Both waterfalls or maps and 
time capture results have scans. Tills record contains (he 
niQuber of scans and Ihe data for the scan v ariable. Kaeli 
scan has a value lhat de.scribes its lucaliun in lime: it may be 
a time offset from the start of the measuremenl., a shaft 
speed (r/niinj value [l.iie HP35(i70A lias a tachometer inputj, 
or just a scati number. 

The SDF_SCAN_BIG record describes dala results that are or- 
ganized in scans (similar lo SOF_SCAN_STRUCT). This record 
describes the lyiie ofsciui orientiilioji and cunlains the muit- 
ber of sciuis used for loiig(.>a27B7 scati.s} scan-orienfed 
data files. This record can serve as a replacement for the 
SDF_SCAN_STRUCT. 

The SDF_SCAN_VAR recoi-d describes one si;an variable for 
data results that aie organized in scans. Each scan has a 
value Ilial de.scribes its locarinn in lime. Ii may be a time 
offset from the stari of the meiism euieiit, a r/min value (the 
1IP35()70A has a tachometer inpul), or just a scan nimiber 
There can be multiple SOF_SCAN_VAR records in a file, .'^o llial 
multiple attributes can be described (e.g., lime ;uid r/min for 
each scan). 

The SDF_CDMMENT_HDR record describes any free-form lexl 
thai Ihe user may want tu as.sociale witii Ihe tlata file. It may 
contain information describing the particuku test setup with 
which Llie data was acquirett. it may contain anything the 
user waiUs. Tills record has a varialjle length so imy iunounl 
of text can he included in the data file. 

The SDF_HDRIxl record conlains the x-axis data for any data 
result lhat contains arbiir-arily spaced x daia (i.e., no! linearly 
or logarillimically spaceil). Fur ihe HP.SSWiA. for example, 
swept .sine frequency response datw is arbilrarily spaced. 

Tile SDF_HDR (v) record eontauis all Ihe y-axis data for all data 
results in the file, hifomiation in the SDF_DATA_HDR and the 
EDF_SCAN_STRUCT determine the location of the y-axIs data 
withiu (his record. The data is in llie same order lus the 
SDF_DATA_HDR records in the file. 



E xpandabili ty 

SDF is not a static standard. Since each record has a ^wi- 
fied size, it is possible lo add fields to the end of a record and 
increase its record size. This mechanism has been used to 
enhance the standard twice in Ihe past, Tlie first revision of 
SDF was releasee! wit h the firsi release of the HP 35fiBA and 
HP i567A analyzers. The second re\ision of SDF was re- 
ieiBed al ihe time of die HP 3r)li6-5A analyzer, later releases 
of the HP JofitiA and HP 35li7A analyzers, and the release of 
the SDF utilities. It included support for lime capture files, 
better support for waterfall (scan-based) daia and beHer 
accuracy for frequency parameters. The third release of SDF 
uicludes support for [nultiple sc an variables, comment 
records, and larger daia records. 

Exchanging Data with Applications 

Any iiarticiilar iustnmient is not always capable of perform- 
ing the analysis desired by Ihe user To addicss this need, 
the SDF Litililies contain programs to convert SDF data tiles 
to oilier formats that ai-e easily imported into computer- 
based analysis applications. 

Tlie most universal foniial used by many applications is 
ASCI! numbers. Spreadsheet programs can import ASCII 
nuniliers into columns or rows of a spreatlsheet. The SDF 
iilility SDFTO.\SC provides the cajjabiiity to conveit any 
portion of the data in an SDF file to a flexible ASCU format- 
The user can specify any C-styie print! formal specifier 

PC-MATI^B from The MathWorks, Inc., is a software pack- 
age for general digital signal processing and filtering. An 
opiional signal processing toolkit provides Ihe ability In 
perfonit digital filtering mid FfT opei-aii<ins on time data 
M^TLAB's basic data type is a matrix. The SDF ulihly 
SDFTOML converts any portion of an SDF data file to 
MATLAH matrix formal, allowing complex data to be 
unported directly inio MATbAB. 

MATRlXx, a product, of Integrated Systems, Inc., is a soft- 
ware pacliage for control system analy.sis. h i.s similar lo 
MATLAIS in ib;il Ihe elementary data type is a matrix. The 
SDF ulilily SDFTOMX converts any portion of an SDF data 
file to MATRIXx matrix foniiat. 

Data Set 58 is Ihe universal file format for mechanical apjili- 
calioiis. The SDF utility SDFTO^S converts any portion of an 
SDF data file lo Data Set 58 format as a matrix. 

Additional thiril-party converters are available lo ronveit 
SDF data to other formats. 

The SDF utility REPEAT makes it easier to deal with a group 
of dala fiie.s by auli.imat.iug hatches of operations on SDF 
files, such as file conversions. 

Examining SDF Files 

Somelimcs the user iloes not want to postprocess measure- 
ment data, bul wants to verify wliat dala is in a file, or wants 
lo make a graphical hard copy of Ihe data wiUi liie appropri- 
ate annotation. The SDF ulility VIKWDATA allows the user 
to \iew up to three traces of dala simuit.aneously in eillier a 



© Copr. 1949-1998 Hewlett-Packard Co. 



December I IWt llpwIfllPackardJounial S7 



stacked or overlaid foniial. Full x aini y aiuiolalion is jifii- 
vidL'ii, jiisl asif ilic tract- were hfiiiK disjiiayoi! by the Irisini- 
meiil. Tlie user has a full choice of coordinate systems In 
display ilie daia; liiicjiv. Ion. or (IB iiiiifiniUKle. real pari, 
iiiiaginai'y pail. wra|)|KMl or uiiwrajipefl jihase. Nichols (ilB 
versus phase), or polar (real versus imaginary). Full marker 
(and offset marker) rmiclioiialily is pro\ided with Ihe arrow 
keys or I he mouse used to control movenienl. Overlaid text 
arid imported ilP-l.lL Rraphics c;in be placed over llii' trace. 
Hard cojiy is provided to any HP-l.H, plotter or H.'I. prinier 
With the HDF utility HEPK.AT, V'IKWDATA ciui be used to 
hatch plot nuiltiph' .Slip' liles. Allhoiigli inatiy iiistninienCs 
call plot resiilt.s directly lo a hard-cu|)y device, the batch plot 
capability o^^^EWI^A^"A allows Ihe user lo awjid tying up 
expensive instrunienl tinie doing a slow operation such as 
plotting. 

There are many pieces of infomiation in an SDK file in addi- 
tion lo the circumstatvces of the data acquisition hi the .SDK 
data headers. The SUF utility SDFPUINT aUows ihe user to 
see litis iitTonnation in a textual fonuat. 

Changing SDF Files 

Up to this point, I ha\'e only described a one-way flow of 
data lieinf! cieaieil liy ;ui insiruineiil ;nid being iransfenvil lo 
another analysis tool. The SDF utilities jiiovide tools for 
creating and modifying SDF fdes so tiiey can he transfened 
hack into an instrument. You can use an analysis program to 
create time data to load mto the IIP Sf)4KxA analyzer's arhi- 
Iniry source or to create a userdeliiu'd input filler lo he 
used widi ilie HI'8!l4xxA's digital demodulalioii lueasure- 
ineni. The SI>F utility ASCTOSIlF creates a linuMlomaui or 
frequency -domain SDF file I'rotii AS( T1 data. To customize 
an SDF file, Ihe SDF utility SDFR!lITcan he used to change 
any header field in ;ui .SDF tile. Some in.stnmienis create an 
SDF file that has many resulTs in it (e.g.. ihe HI'^SfiTA), but 
most instnmienls will only use the first result in an SDF file 
(e.g., the IIP 8y4xxA). The SDF utility SDFTOSDF can be 
used to split a mulliresult SDF file into separate SDF files so 
thai an iiisirumeiil li.is access to all results. Some old instru- 
ments only support a certain munlier of IVeqiieiicy points in 
data results (e.g., the IIP DfiliSA suppoil-s only Sill poinl.sj. 
The SDF utility SDFTOSDF can be used to respace data to 
any number of frequency points. 

Accessing SDF from a Program 

Some users wain direct access I o SDFdala from apro- 
grammmg language. The SDF library jirovides that interface. 
Library tunctions alhtw access to ;ill .standard headers in ;iii 
SDF file and allow reading and writing of the SI IF data. The 
SDF libraiy scales the SDF fiata and ciiuveils Itie ilata to tlie 
format the user requests ( lU-bit or 32-l)iI integer or :G-bil or 
64-bit floating-point ). Tlie SDF library is also available ;is a 
Microsoft * Windows dynamic link librai>, jjt ovidinfS access 
to Wuidows-based programs such ^ls Microsoft \'isiial BASK'. 
MATLAB has the capability to call user-written functions 
(MEX files). The M.\TL.\B function FILTEBSOF is a^-ailable to 
digitally (I-TIi) filter mi SDF file. This is most useful wiih an 
SDF time capture file lo test the effects of a compensation 
filter upon measured data. 



Implementatiuii of SDF 

Different segments of ihe SUF code run on a variety of 
products: 

• Mi.tloi ola 1)8(1x0- bLised instrtiiTienis from the Lake Steveas 
instrument Division (e.g., IIP 894kx.'\, HP 35()70A) 

• hitel-based MS-DOS personal computers (SDF utilities) 

• IIP 93LX palmtop computer (extended data transfer utilities) 

• Microsoft Winrlows environment (SDF library dynamic link 
librarj') 

• HP yOUO Series 300 and 400 workstations (SDF utUities) 

• HP 9000 Series 700 PA-RISC workstations (SDF utilities ). 

The SDF utilities and (he SDF library ai-e written In ANSI- 
compliant ('. Three ditfereiit conipilers are used to generate 
code for the ililferent tai'gets: the HI" iillOO Series 700 V com- 
piler, tlie Gnu ( ■ compiler for the HP rtOOO .Series imd 400 
and tlie iiLstrument targets, iuitl the Microsoft C compiler for 
MS-DOS, HP f)5LX, and Microsoft Windows targets. 

hi general, the same code works with all lliree compilers. I 
only foiuid one code segment thai would not compile on all 
compilers. During development, 1 found (hat even with the 
strictest error repoilliig turned on, not ;ill l ompilers generate 
the same level of warnings lor an offending code segment. 
This iiuillicom|)iler approach turned out to be very useful 
for finding certain l.vpes of coding errors. 

The C compilers predefine some compile-time symbols, 
which can be tested by the C pi'eprocessor. The Microsoft. C 
compiler predefines Ihe symbol MSDOS iforllie MS-DOS. HP 
mi.\. auti Microsoft Windows largels) and the HP iHlOO 

Series 700 C compiler predefines the symbol hppa. These 

switches are used lo control compilation to resolve the 
various differences in the target systems. 

SDF files are in binary form with nmllibyte iitmibers stored 
with the most significmit byte (MSB) appealing first (lowest 
address in memory). Inie! 80x8(>based computers usenum- 
hej's with ihe MSB aptiearing last in the numlier This means 
that any .SDF uiilily ihai nuis on the.se compiUeis has to 
have tlie bytes in a number read from an SDF file reversed 
before it is used, The SDF library byte swaps every field in 
an SDF recoril after u is read from a file lo make the byte 
ordering lis tiiuisparent as possible to Ihe m;\joiiiy of Ihe 
software. Portions of Ihe soflware thai deal directly with 
data use a set of macros lo swap integers and noaling-point 
numbers. 

Tlie Microsoft V compiler tiefines the size of an integer as 16 
bits, whereas Ihe other V compilers define the sine of an 
integer to be 32 bits. For this reason, the SDF library does 
not use integers, but instead uses the data lyiie short to mean 
16-bit integer and long to mean Ii2-bil iiUeger. These data 
lilies are consistent witli all Ifie compilers. 

The Microsoft C compiler dethies the size of a pointer lo bo 
either l(i or ^2 bits de[»ending upon the memory model used, 
whereiis the other C compiler define the size of a pouiter to 
l)e 32 bit.s. Since the size of iui SDF file can be more tlian fi4K 
iiyles I the limit of a IC-bit iiointerl, pointeis to portions of the 
SDF file in memory miisi idwa.vs be forced to be '-il biLs. You 
can force pointers to be -U l)iis with Micrtisofl C by compiling 



SR PwrnilierTliaiUfWlctt-l'ackarrl.lniimEiT 

©Copr. 1949-199B Hewlett-Packard Co. 



with the laige memory mi>del (H2-liit program addresses and 
32-bif data poinfers). This works for MS-DOS programs, hul 
Microsoft Windows programs are generally compiled with 
the medium memory model (3--hit profiram addressee and 
IG-bit data piiintersj. Even in the medium memory mode. 
32-bil addresses can be coerced by explicitly delming the 
poinier as Jar (e it, char Jar *). All SItF address references in 
Uie SDF library use a define to foix'e .12-bii addres.'w^. 

There is a side issue to the use of 32-bit pointers with a 
lf>-b!l memorj" model. The C standard libraries for the me- 
fliuni memorj' model ail use iO-bil pointers (for example, the 
string access iimclions). Tliere are ^J2-bil versions of tnosi ()f 
tlie functions vi-ith a slightly different fimction name (e.g., 
Jstrlsn instead of strlen). This means more defines must be 
used for function names to allow either function to be used. 

Certain niles ai-e used in aligimient of variables within a C 
SI niel III P. For Microsoft C and Gnu L'. any integer or floal- 
ing-pnint \ ariable must stait on an even byte boundaij'. Only 
a char variable or array can have an odd byte size. To follow 
this structure aJigimieiU rule, ail chaiaefer arrays should be 
an even number of bytes in length. 

The porthig of the SDF utilities to the IIP !3000 Series 700 
fai'get occiined after SDF liad been defined and in use for 
several years. Tlie \ ariable alignment rules witiiin stiuctiires 
are different on the Series 700 Oian on the other targets. 
Siructiues on the Scries 700 are generally larger because 
ii2-bil micgers and y2-bil floaling-poinl variables nuist be 
aliijiied on four-byte boundaries and (i4-hii fl' ialing-point 
variables nuisi be aligned on eighl-by'e hoiindaries. The 
.solution is an SDK library fuiirlion dial unpacks an SDF 
header after reading if and packs an SDF header before 
writing it to a file. 

Both the byte .swapping and the packing and unpacking of 
an SDF header are handled with a I able for each SDF header 
liial describes die dala lype of each varialile in the sIrncMire 
along with its length for ;irrays. This Uible al.so conlains the 
ASCII name of each variable. If liie variable is an enum data 
ty))e, Ihen the table also contains a pointer to a talile describ- 
ing die ,A.,S('II iwnie lor each of ils defined values. Tltese addi- 
tiona! fields are used by the SDF utility SDiT'RINT In prim 
SDF headers, and by the SDF utility SDFEDIT to edit SDF 
headers. 

Care musl be taken when using C libraries lo make sure thai 
hmcHons really are standard ANSI C and as sneh will be 
p(3riiibie. Il ItuTis out diat caseinsensi five sli'ing coinpaiisons 
are uol stamlard. In Microsoft C. the function is .stncmp. hi 
HP ;H)(H1 Series 7(10 C, (lie funclion is strcasecmp. In (Inu V. no 
such fimction exists. Part of the SDF libraiy is a compattbility 
module to contain these nonstandard functions. 

The SDF libraiy is also compiled into a Microsoft Windows 
dynamic Unk libraiy (DLL], which places acicUlional restric- 
tions on the use of pointers. A Ifi-bil pointer should nol be 
used for Ihe address of anything on the stack (pjirameiers lo 
a fimcdon or automatic IocelI variables). To form a coniplele 
32-bit ad<i!'ess from a l(j-bit [jointer, the ciimpiler uses I he 
data segment register to reference global and sliilic viirial'les 
and tlte slack segmenl register lo reference local variables 



and parameters. For a normal C program, the stack segment 
register is set equal lo the data segment register so that a 
Iti-bit pomter can point to a \-a!ue on the stack or a static 
\ ariable. In a DLL. the data segment register points to the 
data in the DLL but ihe slack segment register points lo the 
slack of the calling fimriion. which is in a different program. 
Therefore, there is an anibigtiily in what a i6-bit address 
points to in a DLL. The code generated by the Microsoft C 
compiler assnmes that all lli-bit pointers in a DLL point to 
ihe data segment. The progranmier must be careful to make 
sure that ail 16-bit pointers always point to global or static 
variables. The altemati\-e is to use only 32-hit pointers, 
which is not always poiisible. 

Interfacing the SDF hbrarj- DLL to Microsoft Visual BASIC is 
straightforward. Stnirfurps arc supported as Visual! BASIC 
user types and the data types of l(i-bil and ^jf2-bil inlegers and 
32-bit and 64-bil floating-point mmibers are directly analo- 
gous to Visual BASIC data ty|)es, Tiie only data tyi)e not sup- 
ported by \'isual BASIC is C-siyle strijigs ( ntiU-lenninaied 
charactei- arrays). Strings hi Visual BASK ' are a dynamically 
allocated dala ly[>e, nol sialic in length. Since all the strings 
in an SDF file are of an even length, they can be defined as 
Ifi-bil inleger arrays in Visual BASIC. Arrays within struc- 
tures ar e suppoited starti^ig with version 2 of Visual BASIC. 
With ihe supplied BASIC fimction cZstring, the integer arrays 
are coin eried to Visual BASIC strings. 

SDF Revisions 

Tliere are ciineiiily ihree revisions of SDF files. The SDF 
library is written to handle forward and backwai"(.l compati- 
bility. When each SDF header is read, the library uses file 
size of the parricular SDF header strtictiire that is known at 
ccmpile time. If the acfiial size of Ihe header ( recordSizH 1 is 
largt t; the librmy ignores imy new fields (fiirwaid < oin|)ali- 
hility). If the actual size of (he SDF header read is smaller it 
assimii'K that the fiekb bcyoiul the read size are imuiitiaiized 
and then sets lliese fields lo default values. 

Conclusion 

The main fiirw linus of a measuremenl system are the rapture 
of dala and the presentation of the dala in a fonii tlial meets 
llie user''s needs. To lielp meet these needs, the Hewlett- 
Packard Lake Stevens Instrtimenl Diiision si ores all mea- 
suremenl data in a consislenl format and provides uliliries 
In manipulate Ihe data and lo ease the task ofimpoiting 
dala into a variety of applications. 

Acknowledgments 

I would like to ai knowledge Randy Sanin who originally 
wrote many of the translators/converters and Ken Blue who 
wrote SSTOSDF and SOTOSDF I would also like to ac- 
knowledge Phil llollenhorsi tor all his support in provithng 
resources to crcale Ibis ]iroduci. I would like to thank .lerry 
Daniels (ijmjeci manager for llie HP S!)4xxA) and Don 
Mat hiesen (project manager for the HP 3-5G65A) who pro- 
vided Ihe support needed to make Ihe SDF utilities a reality. 

IvtrnrasuK ann MS-nOS arE IJ S raEjiEleruil irailemaiks o( Micriwnft CD'Mreliod. 
PC-MAllAB IE i iradamark ot Tha MalhWaiKt, Inc 



© Copr. 1949-1998 Hewlett-Packard Co. 



ln-cfiiiljcr IW;llii'wleU-ra.-l!)ird.liniinnJ Sit 



North American Cellular CDMA 



Code division multiple access (CDMA) is a class of modulation tlial uses 
specialized codes to provide multiple communication channels in a 
designated segment of the electromagnetic spectrum. This article 
describes the implementation of CDMA that has been standardized by the 
Telecommunications Industry Association for the North American cellular 
telephone system. 



by Da\id P. Wliipple 



Tlip cdiulai telephone iiitlusli-> is linfd with llie [uobipm (if 
a customer base that is expanding while tlie aniounl. orthe 
t'lectroniKgnetic spectrum allocaled k.i celliiiiir service is 
fbiod. Capacity can be ijicre;ised liy inslalliu}; ailililioiiu) 
ceUs (subcliiidiiig), but the dei^ree or subdiusinn is limited 
because of the overhead needed to process liaiulolfH be- 
tween cells. In addition, property for cell sites is (iifficull to 
purchase in the areas where iraffic is the highest, 

Tlie ciuTpnt analog system divides the av ailable specirum 
into 3U-klIz-wide channels. This method ofchamielizatioii 
(division of tlie spectrum into miillipic cluiniiel^s) is com- 
monly called FD.MA, lor freijuency division iiiiillipie access 
(Fig. 1 ). Alternate niean.s of diaiinelizalion ;u e being devel- 
oped to allow more users in ilie same region of the sppc- 
Iniiii. TDM A. or lime ilivi.siun nuilliple aci-ess, uses Ihe same 
30-kH/ cliflJinels. but adds a time.sharing nf tJuee users on 
each frequency. All other factors being equal, this results in 
a threefold increase in capacity, CDMA, or code division 
multiple access, is a class of modulation that tises special- 
ized codes its the btisis of chiuiitelization, Tlie.'ie codes are 
shared by both the mobile station aj)d Uie base station. 

Wliile CDMA is a class of modtdalion, this paper foctises on 
the implementation of CDMA for Ihe Noilh Anierican cellu- 
lar market, wliich was initially developed by QUAl.COMM, 
Inc. anil has been slaiidarflizefl by tlie Telecommunications 
liidii.'sicy A.ssoi'ialion (TI.A), 

Interfere lice Effects 

The analog system needs attenuation of aboul 18 d8 foriii- 
lerlerence on ilie same channel to provide accepiable call 
iliiality. Tlie practical ramification of Ihis is Itial only a por- 
tion ol Ihe available speclmni can tie used; iiol ^ill cr the 
channels c.m be used in evei'y cell, A tieiiiieiicy reuse pat- 
tern of seven is cnnmionly useil to provide litis attenuation 
(see Fig, 2), In other words, only one seventh of all possible 
frequencies arc available in any one cell. In fact, sectored 
cells are tisuiiUy used when capacity i.'^ needed to allow^ the 
seven-cell repeal pattern to work. L'siiig three sectors per 
cell, only one out of every 21 available frecjuencies is used in 
each sector 

In CDMA, signals are received in tlie presence of higlt intei- 
ference. The practical limit depends on the ch;irmel condi- 
tions, but reception in the presence of interference that is IH 
dB laiger than the signal is possible. Typically, the system 



TTiw 




Frequencv 



Tim 




A 




Frequencv 

(El 



Fig. 1. I\'lliil;ir dianiielizaliuii tiielhuds, (iij FreqiHitii'y [iivisiuii 
miilti|)l[t avcess (FDMA), (t) "nme division multiple access (TDMAJ, 
((■) Ciide liivisMi miiltiplE! access (CDMi\), 

o|ierafes wilh better conditions, Tlie I'reQiiencies are reused 
m every secior of everv' cell, aaid appro xiniately half tlie in- 
terference on a given frequency is from outside cells. The 
other half is the user traffic front witiiin the same cell on the 
same frequency. 



90 tlwemlier 1993 Kewlett-Pafkani .luumal 

© Copr. 1949-1998 Hewlett-Packard Co. 




Fig. 2. Cellular frequeno' reuse paitenis. (a) KDMA n-ase, 
0>) CDMA reuse. 

Rg. ^ sliows a North .^meritaii cellular CDM.4 system. 
rDMA scarts wilii a basic da!a rate of tl600 bits/s. Tliis is 
then spread Ui a tratisniiTieil liil rate, or chip rate (the trans- 
mitted bits are calleti chips), of 1.2:i8S MHz. Spreading con- 
sists of appl.viiig digital codes In the data bits that increase 
the data rale while addiiig redimdancy to the system. The 
chips are iransniilled Lising a form of QPSK (quadrature 
phiLSf sliiri keying) modulation tliat has been filtered tu limif 
the bandwidth of the signal. This is added to the signal of all 
the other iLsers m that cell. When the sijpuii is received, ihe 
cofling Ls remov ed from the desired signal, returning it 10 a 
rate of 9600 bps. When the dceodiiif; is applied Id Ihe otlier 
users' codes, there is nu despre ailing; Ihe signals maintain 
the 1.22SS-MIIZ bandwidlii. The ralio orii-ansmilted hits or 
cliips to data bits is Oie coding gain. The coding gain for the 
North American CDMA system is 128, or 21 dB. 



An analogy to CDMA is a crowded party. You can maintain a 
conversation with another person because your brain ran 
irack the sound of iJial person s voice and extract tliat \ oine 
from the mterference of all ulher laJkers. Kthe oilier talkers 
were to talk in different languages, discerning liie desired 
spe€^ would he easier because the crosscorrelaiion be- 
tween the desired voice and the interference would be 
lower, Tlie CDiLA codes are designed to ha\ e veiy low 
crosscorrelation. 

CDMA Features 

Hie data niie of 91300 bits/s can be thought of as a modem. 
Tlie signaling and the services must share this fundamental 
data rate. The system is designed so that mtJtipIe service 
options can use the n\odem. Currently, service opticm i is 
speech, service option 2 is a liaia loo|iback mode used for 
lest puTiioses, and senice option is being defined as data 
services, which will siippoit l)oth fax mid asynchronous data 
(terminals), 

CDMA communication systems have many differences from 
analog systems: 
■ Multiple users share one carrier frequency. In a fully loaded 
CDMA system, Ihere are about 3^ users on each carrier fre- 
quency. (There are actually Iwo earner frequencies per 
channel. 4f> MHz away from each other One is foi' Ihe hase- 
lo-mobile link, which is called the forward dh-eclioiL, while 



^Jmm^ bw 



1.2288IMH1 BW 



9E0D Hz BW 




EDMflTransmittEr 



Baseband 




Encodinn and 


- -> 


WbMi Code 


OtiB 


liiteiteautiig 




SgiiBirlinB 


9.6 libils/s 




19.7 kbibis 




1ZZ8.B libll£/s 




CDMA RccBlvsr 



Walih Code 




Oicoding and 




Bamband 


Constator 




DsinledsBVing 




Oma 


I22S.8 libiRi/s 




19.2 kbits/s 


9.B l<blis/s 



-100 dB/Hi 



Sptitiuus Signats 



IZZSBIMHiBW 



1.22S8 mi BW 





BacligrDund Noise 



EHlernal Inlerleience Dtlier Cetl InlerfBrBnce 

tnferierenca Soutcbi 




Oilier UserNaiSB 



Fig. 3, Niirlii Aiucnfaii ri'lliiliir CDMA syBlem. 



© Copr. 1949-1998 Hewlett-Packard Co. 



Uecpmber IWJ Hcwlprl,.Papl(Bnt Juui'iial 91 



Mobile TelephDne Switching 0(1 ic a 



Cellular Technologies 

AMPS. Aduarceil Mobile Phnne System Tins is tfie curreni analog FM system in 
North America It uses 3D-kH; thartnels ard signaling is done iiiper audio, thai is, 
a! frequencies atiove ttte audio bandwidth for spescti, wtiich is 300 to 3000 Hz 

TACS. Tula! Access CammumcaiiDn Sysiem. Tfirs is ifie analog FM ^siam used m 
the United Kingdom and Japan It uses 25-liH? cfiannels and signaling is supetaiidio 

NMT Nordic Mobile Telephone Scandinavia led the worid m ceilulai systems. 
The latest system uses 3Q-kH; channels, and signaling is done using 1200-Hiand 
1800-H^ tones in moch the same way as a modam 

J-TACS. This IS a narrowhanif analog Ffvt system in use in Japan. Channels ate 
12 b-kH/ wide and signaling is subaudio, that is, at frequencies below the audio 
bandwidth for speech, which is 3O0 to 3000 H;. 

NAMPS. Narrow Analog Mobile Ptione System This is an anatag fM sysiem 
using 10-kH?-wide channels. Signaling is subaudio 

GSM. Global Sysiem for Mobile Communications This is the first digital cellular 
system to be used commeicialiy It has been ailDpied acioss Eurnpe and in many 
countries of the Pacific rim It uses 200-kH; channels with eight users per channel 
using TDMA, and has a vocoder rate of 13 kbiis/s. 

TDMA. Time Division MuliiptB Access This is the first digital system standatdi^ed 
in Nurih America. It uses 30-kHz channels, three users per chatinal using TDMA, 
and has a vocoder rate of 8 kbits/s, 

E-TDMA. Eiiended TDWA This sysiem uses the same 30-kHz channels as TDIvlA. 
but has sii users per channel The vocoder rale is cut to 4 kbiis/s, and the channels 
are dynamically assigned tiased on voice activity detection. This is being proposed 
as a to! low-on to TDMA 

COMA. Code Division Multiple Access This system uses 1 23-MHz-wide channel 
sets, with a variable numhar of users on each carrier frequency. The full vocoder 
rate is 8 55 kbits/s, hul voice activity detection and van able -rate coding can cut the 
data rale to 1200 bita/s. The effective data rate, determined empirically tor simu- 
lated conversations, is 370O bits/s Access is by code 



thp olhpf is for the mobile-to-base link, wiiich is called the 
reverse riiipction). 

The fhannel is derini'd Ijy a trjdp. There is a carrier fre- 
tiur'ncy assignnieiil , htil I he frequency band is 1.23 MHz 

wide. 

The capacity limil is soft. Addilinnal users arid more inler- 
ference to the sysl em, whirl) caii cause a liigher data error 
rate for all ii.sers, liiil this limit is not set by tJie number of 
physical channels. 

CDMA makes use orniiihiple fonns of diversity: spatial 
(hversily, Irequeiicy diversity, und time diversity. 

The traditional form of ^atial diversity — niulliple anteimas — 
is used fur the ccU site recci\"cr, i\nnther fomi of spatial di\ er- 
siiy is used during ibc process of handing off a call from one 
cell to the next. Called sqfl kandoff, it Is a niake-before- 
brealc system in which two cell sites niainlaiu a link wilh 
one mobile simullimeoiisly (Fig. A). The moliile sialion hius 
rniiiliple correlative receiver elements that are assignerl lo 
CiU-h incoming signal and can add these. There are at least 
four of (hese correlators — three that can be assigned In the 
link and one that searches foraltcniate paths. The cell sites 
.send the recei\ eii daia, along with a quality index, to the 
MTS( ) (moliile Iclepiione switching office) where a choice is 
made as to the betler of the two signals. 



Land 




Fig. 4. Sjiatial diversity iliiritigaofl. hanrion*. 

Frequency diversify is provided in the bandwidth of 1 he 
transmitted signal. A multipalh en^ronment will cause fad- 
ing, wliich looks hke a notch filler in tiie frequency domain 
fFig. 5), The width of the notch can vary, bill tyj.iically will 
be less than 300 kHz. While this notch is sufficient to impair 
ten analog channels, il only l emoves aboul 2-5',Hi of (he (_'I)M^ 
signal. 

Miiltipath signals are used to advantage, providing a form of 
time divereity. Tiie niiilti|jle correlati\ e receiver elements can 
be assigned to different . time delayed copies of the same 
signal. These can be (.■nmbined in what is called a RAKE 
receiver,' which hits multiple elements i-alli'd lingers (Fig, fi). 
Tlte term R.\KV. lefcis lo llie original block diagram oTllte 
receiver (Fig, 111)), wliicli includes a delay line wilh (iiiilHiJle 
laps. By wcighling the sipial at each tap in iiropoilion to its 
sirenglli. the lime-diverse sigiuds are combined in an opti- 
mal maimer. The j)icture resembles a gaiden rake, hence the 
name. 

.\nrilher form of time diversity is Ihe ii.se of forward error 
correcling codes followed by interlea\ing. Ixias of trans- 
mitted bils lends to be grouped in lime, while most eiTor 
correction schemes work best when I he liit errors ai*e uni- 
formly spread over time. Interleaving helps spread out em.irs 
and is common to most iJigital systems. 



1.22S8MHI BW 




Frequency 

Fig. 5. ODMA frequency clivcrsily, The wide spei'tnmi ccmibals fiid- 
ing causi'd by niiill.ipalli ir.uismlssion. Fading acts like n nnli h filter 
1(1 Ltie Hidf-Kpcctriiiii -sipial, T^'pically oidy ;i ^mnW jiiirl <>! die signal 

is llJBI, 



92 I>c emti.-r I f«);! Hpwiptt-Parkaid Jounml 

©Copr. 1949-199B Hewlett-Packard Co. 




VOrDrB 



Taps 









♦ 








1 J 


I I 


I I 


1. 


Weighls 




Ibl 



Ourpui 



Fig. fi. Till- RAKE refeivertakeHailvantageufiimli.ipal.il l.raiis- 
missKUi l.u realmi' a fiirm urtirue lilvt^rsily. (b) RAk'E receiver liloek 
illasraiii, 



Mobile Station Power Control 

Conlrol of The iiiol)ilf .^ilailuii yowrr is essfiiliiil lurCDMA Iti 
work. If one mobile slalion were to be received at the base 
station with loo muc'h power, il would jani the other users. 
The goal is to liave Ihe signal of all moliile stations arrive at 
the liase station with exactly the siirne ik»wit Tw() foniis of 
power control are usetl: open-loop and elosed-loop. 

Open-loop power (Control is based on the similarity of loss in 
the forward and reverse paths. The received power aC the 
mobile station is used as a reference. If it is low. tiie mobile 
station is presumed to be far from the liase station and 
traasuiits with hij;h |)ower. If it is liigh, the mohile station is 
assunied to he close and transmits with low power The 
product of the two powera. or the sum of the two powers 
measured in (IB, is a constant. This constant is -73 when the 
receive and IranMUil powers are measured in dBm. For 
example, if the reccivetl power is -85 dBni, llie transmitted 
power would be -t-iailBm. 



Closed-loop power control is used lo force the power from 
the mobile station to deviate from the open-loop selling. 
This is done by an actii'e feedback sj'steiii from the Yiase 
station to the mobile station. Power control bits are sent 
every 1.25 ms to direct the rnobLe station to increase or 
decrease its transmitted power by 1 dB, 

Because the CDMA mobile station transmits only eiiougli 
power to maintain a link, the average transmilteii p(>wpr is 
much lower than for an analog system. .An analog phone 
needs to transmit enough power to overcome a fade, even 
thoiigli a fade does not exist mast of the time. This ability lo 
transinit witli lower power has the potential of longer bat- 
tery life and smaller, lower-cost output amplifier design. 

Speech Encoding 

Tiie speech is encoded before transmission, 'llie piupose of 
encoding is to reduce die number of bits required lo repre- 
sent the speech, Tlie CDMA vokv coder (ivrail/'r. as it is 
called) has a data rate of 85-50 bits per second. After addi- 
tional bil-s ;iie adiled for en'or detection, the I'liaiuiel data 
rale is HtiOO bits/s. This is lowered, however, when the user 
is not speakhig. The vocoder delects voice activity, and will 
lower the data rale during quiet periods. The lowest ilata 
rate is 13.10 bil.s/s. Two inteniiediate rales of :i4U(l ;mii 
bits/s are also used for special puqioses. The :!40l) bils/s rate 
is used lo transinil iraiisients in the backgiound oobe. and 
the 4SI)n bitfi/s rat,e is used to mtv vocoded speech and signal- 
ing data {signaling consists of link-management messages 
between tlie base station and the mobile station ), In tliis last 
case, the channel data rate is 9tV)Q bits's, but half of tlie bits 
arc assigned to \'oice and the other half to the message. Tliis 
is called dim and buisl sii/iniliiig. 

The mobile station pulses its output power during periods of 
lower-rate data. The power is lumed on for 1/2, !/4, or 1/8 of 
the lime. The data rate is ftliOd bils/s when the power is on, 
so the average data rate is 4«(H), or 1200 bits/s. This 
lowers the average power and llie interference seen by 
other users. 

The base station uses a different metliod lo reduce power 
during quiet ijcriods, Il transmits with lOO'tfi iluty cyc le at 
9f)(ll) l>llJs/s, but uses only 1/2. or of full power and 
repeals the tviinsniiltcd data 2, 4. or 8 times. The mobile sta- 
tion achieves Ibe required signal-to-noisc ratio by combining 
llie nmltjplc transmissions. 

One imporlant aspect of the coding used in CDMA is Walsii 
codes, or Hariaraard codes,- These are based on the Walsli 
matrix, a squiuc matrix with iiinary elements that always 
has a dimension that is a power of I wo. It is generated liy 
seeding Walsh ( 1 ) = = 0 and expatiding as shown lielow 
and in Fig. 7: 



where n is the dinieiLsion of the matiix and the overscore 
denotes the logical NOT of Ihe bits in the matrix, 

Tlie Walsh matrix has Ibe proi)eri.y that every row is orthog- 
onal to every olher row and the logical NOT of evecy other 



© Copr. 1949-1998 Hewlett-Packard Co. 



Doceuiljer 1993 HpwIett-Pai:liHnl imiiviA 93 



0 a a 0 

0 10 1 

0 0 11 

0 110 



Fifi. 7. Wiilsh matrices. 

idw. OnliogonaJ means Ihal Uic dol priKliirl ofmiy Iwo rnw^i 
is zero. In siniiiler lenns, il means that in'tweeii any iwo 
rows exactly lialt' the hils mat.c li anil half the bits do imr 
rial ch. The CDMA system uses a ti4-by-f>4-bi1 Walsh matrix. 

Forward Link Eiiciidiiit; 

Walsh eiifiKiiiig is used in ihe rii™ai(l link (ijasc to mobile) 
as sliowii hi Fig. 8, Tlio fimdamental data rail? of the chaimcl 
is ! 11)1111 liits/s, Tlic d;ila is packi'tizcd iniii 2ll-ms blocks and 
has roi"ward t'lTor uorrtii'tion ajipUeii by list! of a con\'olii- 
tional encoder. This is done at half rate, wliic:h yields two 
bits out for e\'ery bit in. The data is tlien interleaved — a 
shiifflinn of the bits during the 2()-ms |)eiio(i. This is done tu 
bcller <listribntc bits lost during tnm.smission. h has been 
shnwTi tlial bit errors lend to tome iji groups rather than 
being si>r<'ad out in time, while forward etxor { orrection 
works best when the errors are distribnied imiformly over 
time. When ihe data is deinlerleaved, the lime-linkcd eiTors 
get spread over time. 

Fullowiiij^ tlie interleaver. the data is motiified by the use of 
a linisi rode, which senx's only iis a privacy mask. Tlie long 
code is generated by a ijseiidorandom binary sequence 
(I'liBS) that is generated by a •12-bit-long shift register (Fig, 
El). Tliis register is also used iis (he niiister clock of the sys- 
tem, and is syridironim! to the llniil of propagation delays 
atniing all b:ise stallions and mobile stations. A mask is ap- 
plieil to the PRBS generator thai selecl.s a combinalion of 
the available bits. These aie added modulo two liy way of 
ex elusive- OB gates to generate a single bit stream al 1.22SH 
Mllz. For tlic forward link, a data rate of only 19.2 kbits/s is 



needed, so only 1 of (U biLs gels usi-d. The long code gener- 
ated in Ibis way is XDRed wilii llie data from (he inierleaver. 

The resulting data Ls then encoded using the Walsh mal.rix. 
One row of tlie Walsh matiix Ls assigned to a mobile station 
during call setup. If a 0 is presented to ibe Wiilsh cover, then 
tlie G4 bits of tiie assigned row of ihe Walsb niatri.y aie sent. 
If a 1 is presented, then the NOT of the Walsh matrix row i.s 
sent. This ha.s llie effeft of raising Ibe data rate by a factor 
of ii'l. from l!i.2 kbil.s/s io I.22aS Mbiis/s. 

The last, stage in coding is to convert from a binary signal lo 
two liinary cbimnels in preparation for transmission u.sing 
QI'SK liiuadrature ph;ise shift ke.ying) modulation- The data 
is s|)Ii1 into I ami Q (in-pliase and ijnadraturel channels smd 
the data in each channel is XORed with a unique FiiBS sliurl 
code. The sliort codes are spreading seiinein es that ai^e gen- 
erated inucli like the lonK code, with linear feedback shift 
registers. In Ihe case of ihe shorl codes, there are Iwo shift 
registens, each 1-t bits long, with feedback ia[is Ihal define 
specific seijiiences. These run at l,22SH MH/.. The slion code 
sec[iiences, eacii 2'" hits long, are common to all CDMA ra- 
ihos. both mobile and ba-se. They are used asa rinal level of 
.spread big. 

After the data is XDBed wilh the Iwo short code sequences, 
the resuh is two chjuinels of data al 1.21288 Mbiis/s. Facb 
channel is low-pass ftltered digitally using an Flli {tuiite 
impulse response) fiher. The filler cutoff frequency is a|i- 
proximately til5 kHK. A typical FIR biter implemenlalinn 
might output 9-bit-wide words at -l.!ll:i2 Mil/., The resuhani I 
and Q -■signals ;u*e converted to analog signals and we sent to 
a linc;u' i/Q modulator. The llniU modulation is filtered 
QI'SK. 

Multiple channels in the base station are combined in Ihe I 
and Q signals lo supply the mulliple channels tran.sinilted by 
the cell iFig, 10). Because all user s shaic the composite .sig- 
nal rriim Ihe cell, a reference siHnal called Ihe pili^l is irans- 
milled- The pilot has all zero data and is as.signed Wai.sh row 
number 0, which consists of all Os. In olher words, the pilot 
is made up of only ihe .short spreading sequences. TVpically 



ZO-ms 
Blacks 





► 








ConnDlltliDIMl 

Encoder 




Idirts/s 


1 

19.2 
kbils/s 







IfllBrleffvoi 



Vo coded 
SpeeEh 




Wslsh 

Cover ^33S» 
MbHt/t 





1 19.2 
1 Uiits/s 








Long 




Cods 





Walsh 
Code 
Sequfncc 





■ 


FIR 




■ 


Filler 





> Q 



Fig. 8. 1.'DMA furward link physical layer. 



S-1 ri.TC[[ibf r \m\ 1 Irxlr'lt-Pcii'kanl .ir.iimiiil 

©Copr. 1949-199B Hewlett-Packard Co. 



Long Code 



A A 



1-1 



I 



T * 



- • 

SbiRREgistet 



CUB 



USE' 

Assigned 
Long Cade 
Mask 



Fig. 9. l/iiij? rwlp privacy ma^ 
fit-iic-ralicKi- 



20% of the tola] energy of a fell is rraaismitted in the pilot 
signal. The pilol signal fomi.s a rohereiit [iha.se leferejit e fur 
the mobile sialinns lo use in demociulatinH itie rral'fic dai^i. It 
is also tlie tiitiidg reference for the cotic ef>rrelati(>]i. The 
short sequences allow the CDMA system lo reuse all M Walsh 
codes in each a<1,iacpni cell. Each cell uses a different time 
offsel oti the short coiies and is thereby uniquely identified 
while being atile m reuse the Ci4 Wash codes. 

Reverse Link Encoding 

The mobile station canndl afford the power ofa pilot 
because it would then need Id iraiismil two signals. This 
makes the demodulation job more difficiili in I he base sta- 
tions. A different coding scheme is also used, as shown in 
Fig. II. 

For speech, the sanie vocoder is used in both direclions. 
T\M' data rate Ls again fl600 b|)s. A J/3-raie {'(invoUilional 
em*!jder is used, yielding an oul|iui rate ol'HS.S kliii.s/s. The 
out] ml iiflhis is InU'ili'aved and I ben laken sis bils ai a lime. 
A six-bit numliei' r;in range from 0 to and each group of 
six bils is used as a poiiitor lo onerowof iJie Wal.sh malrix. 
Every mobile .station can (ransmit any row of I lie Walsh 



matrix as needed. At ihis point, Ihe data rate is kbita's, 
liul (here is no iniique coding Tor cbaniielization. Tlie full- 
rare long code is then ajiplieii, raising ihe rate lo 1.2288 
Mbil,s/s. This final data si ream is sjilil inio I anil Q channels 
and spread with the same short sequences as in the base 
sfalioii. There is one more dilTerence: a time delay of 1/2 
rliijj is applied to the tj channel before the FIR filler. This 
results in offsel-QPSK modulation (Fig. 12), and is used to 
avoid the amphtude transients inlterenl in QFSK. Tliis makes 
the design of the output amplifier easier in the mobile station. 

The capacity is different in the I'orwaril luiil reverse links 
beraiise of tiie differences in modulation, nie forward link 
has llie pha.se reference — Ihe pilot signal — as well as orthog- 
onal codes. The rc'\'ei'se link signal is not ortliogiHial because 
tlie long codes are applied after the use of tlie WaLsii matrix. 
In (his case the signals are unconelated but not ordiogonal. 
Tlie base station has tlie advantage of multiple receive an- 
lemias (divej-sMy). .Ml I'acUirs taken together. Llie reverse 
link sets system capacity. 

Table I summarizes the CDMA ehannelization fimeiions. 



Walsh Code 0 



Pilol Channel 



AMDs 



Walsh t^d« 37 





CorlvoiTiD l/Q 
(nd Psaudo- 


1 Oala 


FIR Luw Pau 


1228,B kbib/s 




□ OaU 


Filler and 






wndom 
Spinniling 


Anslag 




Cflnveision 



Sync Channel 



t22S.8 kbhs/s 



4.B kbils/s 



Walsh Cades 1 lo 1 



Paging Channels ^^^122S.B kbils/s 

1 1Q 7 Channels *■ 

19.2 kbils/i 



Walsh Codes 8 lo 31. 
31IdG3 

I lo 55 Channels 

13.: kbiWs 



Canven lo VO 
and Pseud 0' 
rgiidom 
Spreading 



Cunviiii 10 l/Q 
and Pseuds 
rnndom 
Sp'tading 



Coiiveti lu I/O 
and PsnudD- 
landoni 
Spreading 



I Data 



QDaU 



IDalD 



OOata 



I Data 



aOala 



HR low- Past 
fittBiand 
0( glial 10 
Analog 

Convemon 



FIR Low-PkM 
Filler and 
DIgiUl 10- 

Anitog 
CDrvnmian 



Fin Low- Pan 
filtsr and 
Dlgil*l-io- 

Analng 
CDiivniiion 




Fin. Ill, I'DMA fiJiward link 
rliiiriin'l foniiiil, 



©Copr. 1949-1998 Hewlett-Packard Co. 



ni'i'i'[n)iBrllH):iHi?wli>tt-t'si^kunl.liiiimal 95 



IMS 

















t 

9,G 








VnEQited 
Speech 





m 



ConvatuliDnal 
Encodai 




Six -Bit 
Seleclot 




3072 
kbhs/s 



17288 
Mbrts/s 



Mbils/3 





1/2 Cfiip 
Delay 



Fig. 1 1. CDMA reverse Uiik (jliyaitiU layer. 
Call Scenario 

Til belter ilUisI rale how the t'DMA syslein operates, the 
system fum-lion will lie fiesi-ribeil in lemis of itiobiie stalion 
operation. 

When the mobile station first turns on, ii Icnows the assigned 
frpijuency for CtlMA servic'e in the lofal area. It will lime to 
lliai fiecineiuy and seairh for pilol sij^iials. h is likely thai 
multiple piiol signals will be foimd, each with a different 



Parameter 

Fret]iieiicy 



Table I 

CDMA Channelization Functions 



Walsh Codes 



Long Code 



Short Codes, 
also called 
the I iuid g 
s] J reading 
sequences 



Functian 

Di\ide,s IliL' spec- 
trum inlo several 
l,2;i-MH/. fre- 
quency alloca- 
tions. 

Sepaiales forward 
link users of the 
same ceQ. 



Separates reverse 
link users of t he 
same t-ell. 



Separates cell 
siles or sectors of 
celb. 



Notes 

Forward and reverse 
links are separated by 
45 MHz. 



Assigned liy eeil site. 
WaLsh code 0 is always 
the pilot chamiel. 
Walsh rode 32 is al- 
ways the sync channel. 

Depends on time and 
user ID. The long code 
is I'omposed of a 
42-bil-long PROS gen- 
erator and und a user- 
spec) fie ma.sk. 

The I and Q codes ai'p 
differen! bul are based 
on iri-hit-loiig I'RBS 
generators. Bolli codes 
repeat at 2l).66T-nis iti- 
ten'aLs. Ba.se stations 
are differential ed by 
time offsets of die 
short sequences. 



time offset. This time nflset is the means of di.ilinguisliing 
one base station from anodier. The mobile slafifin will pick 
the strongest jiilot, and establish n frt'fiiiem'y refereni'o and 
a time reference from lhat signal. It will then .start demodu- 
lation of Walsh nimiber ^1:^. wliicii is always ;issigned to the 
sync chamiel. Tlie syni' chamiel message contains die future 
contents of the 42-bit long code shift register. These are .J20 
ms early, so the mobile station has time to decode the mes- 
sage, load its ix'gister, and become sjiichronized with the 
base stalion'.s system time. Tlie mobile station may lie re- 
quired lo register, 'Hiis would be a power-cm regisi rat ion in 
ffdlich the mobile station tells the system thai h is available 
for calls and also tells the system where it is. U is anliri- 
paled that a sen ice aiea will l>e divided into xunes, and if 
the mobile station cros.ses from one /one to ;motlier while 
no call is m piogress, il will move it.s registration location by 
use of an idle state handoff. Tiie design of the zones is left to 
the service jjrovidci' and is chosen to minimize the support 
messages. Small zones result in efficient paging Imt a large 
number of idle slate liandoffs, Laige zones niininiiKe idle 
stale himdol'fs. bul l equire paging messages to be sent from 
a large number of cells in the zone. 

At tills point Ihe iLser makes a call by cnlerhig the digils on 
the mobile station kejpad mid hitting the send button. The 
mobile station will attempt to contact the base stjition with 





Fig. 12. ('tin.'ilellat.iiin diiigi'ams for ('DMA mmluliilion Ciirmiil!-. 

The base siation tmnsinitTiT aaes filtoi'i'ii tJl'.SK |b) T1ll> iniibile 
Blalloii transniiller Uses filtered uffsel-QP.SK. 



96 Dtremlier 1093 newj('IT-Piii.kar<!.Triiima] 

©Copr. 1949-199B Hewlett-Packard Co. 



an ac(*ss probe. A long code mask is used thai is based on 
cell site paraniptere. It is possible thai nrnlliple mobile sta- 
tions may atiempt a link on the access chaiuiel sirauita- 
neousiy. so cdliisions can occur. If the base station does not 
acknowledge (on ihe paging channeij llie access attempt, 
the mobile station will wail a random time and Uy again. 
After making coniaci, Ihe base station will assign a traffic 
chaiinei Willi il.s WaLsh tiuniber. Ai this |ioint. ihe mobile 
station clianges its long rode nia^k H> one based on its serial 
mimlier. recet\'es on Ihe assigned Walsh number, and starts 
the conversation mode. 

It is common for a mobile stalion romniiiricaiiiig with one 
cell to deled anolherrell's pilot tiiat is strong enough to be 
used. Tlie mobile .station will then rRjuesi soft handoff Wlien 
tliis is set up. the mohile station will he assigned different 
Walsh numbers and jiilot timing ami use these in different 
correlative rerei\'ing elements. It is capable of combining 
the signals from both cells. 

Evejitually. the signal from the first cell will diminish and 
the mobile station will request from (he second cell that soft 
handoff be tenninated. 

Al the end of the call, the clianneis will he freed. When the 
mobile station is turned off. it Hill generate a power-down 
registration signal that tells the system that it is no longer 
availalile for incoming calls. 

Testing 

The complexity of the CDMA system raises substantia] test 
issues. What tieeiis to he tested, and what environment Ls 
needed for testing? To test the mobile station, die l est equip- 
ment musi emulate a base station. The tester needs to pro- 
vide t)ie pilot, sync, paging, and traffic chaimols. It miisl 
provide another signal lliat uses ortliogonal Walsh symboLs 
that represent the inieiference generated by otJier users of 
the same cell, and il must ijro\'ide additive noise that sinni- 
lales the combination of CDMA signids from other cells and 



background noise. Bit error rate is not a nieamngful measure, 
since substantial errors are expected at the chip rate and 
these arc not availtible for test. The bit.s ai the 9ti()ll-bitS''s 
rate are the only bits available for test, and these will either 
be all correel as a resuli of error correction or will liave sub- 
stantial errors. What is used instead is the frame em>r rale, 
a check of Ihe received bits and the associated CRC [cyclic 
rertundaiii'y code | in each 20-ms block. 

To test the transmitter, a new r*sl has been defined: icave- 
fnrm quality. This is based on the cros,scon'elalion of the 
actual iraiisniiiieii signal lo the ideal signal trdn.smitling the 
same daia, TiiLs is important to tlie system because the 
CUMA receixers are correlators. In faci. they correlate the 
received signal with the ideal signal. If a signal deviates sub- 
sianlially from the ideal, tlic correiale<l ponion of that signal 
will be used to make the Imk and the uticon elated portion 
will act as adihtive inlerfen?nce, Ciosed-loop power control 
will mauitain the correlated power at the needed level, and 
excess power will be iransmilled. The specification Ls that 
tlie radios shall transmit with a waveform ([ualiry tiiat limits 
the excess power to less tliaii II. 2,^ dB, Oilier transmitter mea- 
surements include frequency and power control operation. 

Conclusion 

CD\L\ pi-o\'ides an advjinccd technology for cellular applica- 
tions. It provides high-quality service to a large number of 
users. It is a system that has been extensively tested and it 
will be deployed later this year in precommercial ajiplica- 
tions. Commercial service is scheduled tu begin in UI94. 

References 

] . R, Price and P.E. Green, .Ir, "A Commuriirntioii Technique for 
Mullipati! nmwi'is,'' Prorepdhigs of Ihe IRE, Vol, 40, Marcti WfiH, 
pp. hTi7y-T>l{\. 

2..\.ii. Proakis, Digilal Cuiiiniiiiiifiilitms. Samiil Edilitm. 
Mcdraw-ilill Buok Co., 198U. 



©Copr. 1949-1998 Hewlett-Packard Co. 



llecember lOM Hewli-Li-l'ack:u'il Jiiiiiiiiil !)7 



DECT Measurements with a 
Microwave Spectrum Analyzer 

An HP 8590 E-Series spectrum analyzer with DECT source, demodulator, 
and measurement personality can be used to provide a cost-effective 
solution to development, manufacturing, and pre-type-approval testing for 
compliance with the Digital European Cordless Telecommunications 
standard. 

by Mark A. Elo 



The HP 8590 E-Series microwave spectrum analyzers provide 
a portable, niggeri, aiifl vei*fiatile general tesi solulion. An 
iLciiled advatilage oflliese analyzera is llieir iiesigne<l-in flexi- 
bility, which allows theni to be configured for specific mea- 
surement needs. Tlie analyzers can accept a i-ange of op- 
tions in their hiiilr-in caidcage and iheir ROM-caid reader 
allows the use of cuslomizeri dowiiloadable programs. The 
domiloadable programs available for die HP 85(J0 E-Sories 
analyzers includL- CT2. GSM. NADC, JDC, and now DECT. ''^ 

The HP 8572:1A DECT measurement personality' is an HP 
application-specific downloadable program that gives the 
analyzer measurement capabihties rotiuircd for tesLuig to 
the Digital European Cordless Tclecommiinications pliysical 
layci' siandaifl. The nteasm^ement of DECT RF characteris- 
tics can present the engineer with many complex measure- 
ment requiremenl.s. The ability to reconfigiire a standard 
speclnini aniilyzer with a dciwriloaiiable pnjjn'ani and some 
cxira hai-dware allows lliese difncidl DKCT lesis lu be 
made ai the press of a button. Exteiirliiig and enhancing (he 
fimctionality of readily available test equipment in this way 
provides a highly cost-effective solution. 

This article aims to explain the DECT jihysiral layer defini- 
tion and explore some of the exiension.'s mid enhaticements 
appUed to the HP 8590 E-Series s|ieclTiim analyzer tliat trans- 
forms it mto a DEC T test tool. After an inlroduclion lu the 
pliysical layer stai^dai d and some basic definitions, tlie soft- 
ware tecluiiquos ;md hardware requirements of the spcctnmt 
analyzer configured for DECT jDhysical layer measurements 
are discussed. 

WTiat Is DECT? 

DECT which stands tor Digital Eurojieaji Coiilless Telecriiu- 
munications. is a licw stajidartl ihiLi presents many new mar- 
ket oppoitunilies in electronic i.ommimi cations equipment. 
It has been a common misconecplion lliat DECT is just a 
cordless telephone system. However, the DECT standard 
ha.s pro\ision.s for more .services than .just telephony, witli 
pos.sible applicaikiiis ranging fjom pacing to cordless L.\N. 
DPXT's versaiiliiy can he ailribuled in its protocol simcrurc. 
wliich is deilved fr'oin tlie OSI {Opm Systems Interconiiec- 
tion) seven-layer model of the Inl.emational Standards Orga- 
nization (ISO). Tlie OSI mode! sinicliires a piece of equip- 
ment mto specific parts allowing for inodulai- conipatibilily 



between difl'erent pieces of communication equipment. This 
allows DECT to provide for nol jiisl European user compati- 
bility but also worldwide clt'Cl ronic compatibility, offering a 
cordless link between most pieces of electronic communica- 
tions equiiinicnl. The top layer of Ihc model, layer 7, corre- 
s]3onds (11 the user interface, for example, the mkrophiine, 
speaker, or key].iad of a l.flf[ilioiie. The bnUoiu layer, the 
physical layer, corresponds In llie I rails mission meihum. 
which for DECT is a radio link. The OSI model as originally 
conceived had no provisioji for a radio physical link. This 
was .solved by redefining Ihe lower layeis of rhe DECT pixi- 
tociil model. The bottOLU loin' layers of the DECT model 
correspond lo ilie iower three layers of the OSI model. Doth 
of Ihe network layere eorrespond exactly, while the data link 
control, medium access control, and physical layers of 
DECT have no OSI equivalent (see Fig. Ij. 

ETSI 

Who defined tlie reqiurements for DECT? ETSI, tlie European 
Telecommunications Standards Institute, foniied a nimiber of 



OS! Lasers 



7 


Apjili call nil 
Lavs I 






6 


Piassntalisn 






5 


Session Layer 




□ ECT Layers 


4 


Transpon liyef 




Netwoik L^ysi 


4 


3 


MetWD ill Layer 




Dala Link 
CoiitrDl Lsyot 


3 


1 


Dsia Link layer 




Mediitm Access 
Control Inynr 


Z 


1 


Phyiical Layei 




Physical Layer 


1 



Fig. 1. Till? DECT protticol structure is based on the Iiik'nuitional 
Sltuidanls (Irgaiiization'ssevpn layer Open Systems Intrn'iinnettiun 
niodrl. Kei'anse the DE{T ])hysifal layer ig a railiti link, Ihe [niHlel 
h;id til be muUiEei!, The lower ftiiir layers of the DEf.'Tmmlel cirre- 
sponJ I (J t.lte luvs'er lliree layers nf Llie ISO OSI Ji lot lei. 



98 Detembpr lilH.3 Hewlelt-t^-kaid .loiinHil 

©Copr. 1949-199B Hewlett-Packard Co. 



working grwiips to lieijt (lefiiie European <'ompatible rDninui- 
nicaiiijns sj'stems. The Radio Equipment and SpecifLcatiotis 
Group (RES J has tJie task ofiTeatingrt orking subgroups mn- 
sisting of delegates from relevani equipmeni manufacturers 
to help define standards such as maritime nioliile. electro- 
magnetic compatibility' (EMC), and now DEC"T. BES3 is re- 
sponsible for DEtTaiui Ls fiinJier sriUi inio fi\f groups, each 
with specific esperiise. Tht-se ^miis cover lest methods for 
Ijlie approval. Oie s>-stem physical layer characterisiies, the 
protocol, system operation, and audio parameters. 

Physical Layer Specification 

t'EPT. Conference Europeen lies Arfniinistratioiis des Postes 
et des Teleconmiiinications. has allocated a frequency band 
of ISSO MHz to mm MHz for DECT, thus making it manda- 
tory to have this frequency band available throughout 
Eiu-ope. The DECT specification' defines len carriers in this 
band nidi a carrier spacuig of 1.728 MHz. Each carrier band 
has a chamiel ititinljer from U to 9. Tlie cliannel 9 carrier fre- 
quency is 1S81,T92 MHz and the channel f) carriei' frequency 
is 1897,3+4 MH/., Kacli of Ihe len ciirriers spaced across tJiis 
Irequency hand i.'i used in a lime division multiple access, 
time division duplex ( TDMjVi'DU ] scheme. Each carrier can 
be turned on up to 24 times in a period of JO ms. Two-way 
commimicarion i.s achieveil by using (he fir^t 12 instances as 
the tran.sinit time Mid Ihe second 11 as the receive time. The 
data is modulated onTo Ihe pulsed KK carrier using (iuussian 
niinimiun shift keyijig |GMSK|. Tliis method lie viales the car- 
rier frequency by ±288 kHz, with eacli deviation representing 
a one or zero, respectively. 

TheGMSK modiilared data is ()rgaiiized in ihe form of pack- 
ets, the lengtii of each packet cori'esponding lo the on time 
of the RF burst, A.s shown in Fig. '2a. a DECT jiackei can i>e 
splil inIo three fields: ' Ihe synchronization field, die data 
field, and an oplioiuil error correction field. These are abhre- 
viateci as the S, U, and Z fields, respectively. The firs! 1 R bits 
of Ihe S fiekl contain a preamble of ahemaling Is and Os, 
either 1010, . . or the inverse depending on vvliellier it is a 
fixed part, transmit packcl oi' a portable pari. Iransmit 
packet. This is uscil for clock recovery. Tlie second Hi bh.s 
of the S field is die synchronization word, which again has 
one form if it is a fixed pan transmit packet ;md the mverse 
if it is a poriable part transmit packet. 

The D field can splh inlo a furilicr two fields: the A ;md the 
B field. The A field is Ii4 bits long and corilaius DECT signal- 
ing data. The B field i.s the p;ul of the [)ackel that coMl;iiiis ihe 
information that needs to be Iran.smhted, although die fi field 
can also be used for signaling data in B-fiekl-only .systenis. 
The Z field is optional and is four iiits in lengt li. It is a copy 
of the last fruu" bits of the B HeUi and is u.sed for delecting 
time collisions from other noasynchronized DECT systems. 

Pour packet sizes with varying B field lengths aic defined 
lor DhXT: short., basic, low-capacity, and high-capacity (see 
Fig. 'ih). Tlie short physical packet lias no B field; it trdiisfers 
signaling data only and can be used for such apiilications as 
paging. The basic DE( 'T physical [(acket foi sjieech has :i24 
biLs in the B field mid is 421) bits long allngetlier With a hil 
rate of 1152 kbits/s this equates to a packet length of Ijfrl.l.i 
(is. The iow-capacily packet has a liata field snutllcr than the 
hiisic ]iackel and the high-capaciiy [lackd'sdata field is 
larger. There is no set applicaiiun for the low-capacily 




(■I 



Short 
Packet 




Basic 






B Reld I 


Packet 


B4Biu 




1 










Law-Capacitf 


Afield 


B 




PBCkBI 




M 


1 



Hrgli-Capacity 
Pacliel 



lb) 




Fig, 2, (a) DECT dutii patket. (b) Tlie D field is split into twii flplds: 
A and B. The IciiBl.lis ofthe A and B fields depend on the DECT 
iip7ili calkin, 

liacket, but the high-ca]iacity packet can be used for applica- 
tions tliat require low overhead, such as cordless LAN. For 
Ihe purposes of this article only the definition of liie basic 
physical packet will be considered. 

It has already been mentioned tlial each jiackei of datii Ls 
modulated onto an active carrier within a 10-ms frame. The 
frame I'lintmns 1 1.F>2fi liit.s, which aie equally divided inlo 21 
hill slots cir4H(l hits. A fidi slot is 4Ui.7 as long. The short 
physical jiackel ;md basic phy.sical packet both occupy a fiill 
slot, while the low-capacily packet uses only half a slot iUid 
the high-capacity packet occupies two full slots. 

Two other definitions important to the UECT physical layer 
{■haract eri.si ics are ilie loopback field luid the position of bit 

When Ihe equipment under lest is placed In l(io|iback 
mode il iniisl relransinil llu- relevant I > field data received 
I'rom the lesler. The loopback pait <if tiie l> lielil is generally 
Ihe data contained in ihe B field except forthe A-fie!d-only 
short packet, for which the A field is looped hack. IH'A 'T bit 
Pli i.s the liming reference [)oinl that defines the beginning of 
the packet, iLs jiosilion is bi liil periods iielbre the bit inuisi- 
tion lhaf oc<airs between the preamble and the syiichroniza- 
l.ioii word. To meet the type-approval lest specidcalion, it Is 
important that a reiation.sbip between the position of pu 
and the triggeiiiig time of tiie measuring instnmient be 
established. 

Testing UECT RF Characteristics 

One-button measurements available with tiie l[P8r>7i1A 
DEl^'T pi-rsonality arc: carrier power, powcr-versus-time 
template measuremcnls. spurious emis,si()ns, intemiodulalion 



©Copr. 1949-1998 Hewlett-Packard Co. 



bi-cpiiibcr 1B93 ilewlptt-Packimi .luumsil 99 



allenuation, acDacent tliaiuicl power due lo moduIiiTion aiicl 
transients, aril frequenry innciulatirai tests such as rieviation 
aiirt carrier accuracy. These tests require more llimijust a 
staniiaici HP 85!)0 E-Spries microwave si)i'ctniTTi atial.VKor. 
The optidnal fast .sweep aiid lime gale are requireii, plii.s a 
Dt^CT-specillr demodulal.or option. A built-in DECT source 
can be added for RV coniponeni tesliiiH and receiver sensi- 
tivity api)liraiinn.s. The analjv.er's aliilily lo receive and op- 
tionally iraJisniii Rive.s the user a sinsle-ljux DKCT test tool 
that can efl'eciively double as a DECT transceiver. 

The DECT Transceiver 

Ati Uiulerylaudin!; of how the HP 8i5SK) E-Series analyzer is 
coiiHgured as a DECT u iea.su renieni Iran.sceiver can be 
gained l)y cotialdering how a pacifei of iJIOCT data Hows 
Ihrougii the analyzer, referring lo Fig. 3. The analyzer can be 
split into two jjarts: ihe receiver and the transmiller. Initially 
the data niiisi be transniilled lo the anil under !esl from tlie 
DECT source or transmiller. Two considerations nuist be 
tal<eji hilo account when transmitting a digital signal in a 
TDMA system. The Tirst is lhat the data must be modulated 
onlo ihe RF carrier in such a way that il wiU cause mminuJ 
spectral occupajicy. Secondly, tiie c^arrier must be swilclicd 



on in a correct DECT time slot. The backpajiel of the ana- 
l.v^er has iwn BNC TTL mpuls called m Date In iuid Time Slot 
In. The TTL data input signal is initially passed Un oiigh s (1,5 
OMSK illter (as defined in the DECT staiidiu-d] to smooth Ihe 
edges of Ilie digital signal. At the same lime the earlier is 
switctied on for the data paclii'f's dm-ation via tlte RF switch 
which is activated by a TTL signal input corresponding to 
Ihe lenglh of the packet al llie time slot input connector 
Data and signaling from an external source iiri.- modulated 
onio Ihe tracking generator signal and transmilled from tire 
DECT source oihput on the front panel. 

Tlie equiiDment UJider test receives tlie RF signal, demodu- 
lates and decodes it, and assuming thai Ijie transmitted signal 
contains the required protocol, retratismits a response. The 
HP S5!J0 E-.Series microwa\e speclnim analyze!' receiver 
corn erts the high-freguency KF signal response from the 
equipment imder test into a low-frequency signal that can he 
cjisily processed. This di>wn-con\ened or fF signal is ampli- 
Fied and fed into one of the analyzers analog-to-digilal con- 
verters for ]Drocessing and disjjiay on the CRT. A parallel path 
to the analog-to-digitai converter is llu'ough an FM discrimi- 
nator, which demodulates the signal for FTVl measurements. 



Raceiver 



DECT DeiiioiIulslDr 



III TDMA t^ontrol 
Signal Ironi Equipmsnt 
Under TesI 




TransniiUer 



Squiring 
Ci'cuir 



TTL Data In 





Trackiiig 


1^1 


GenetlW 




Fig. 3. Block diagram of an HP S5!tO E-Seiies spprtrum analr/.erwitli Optiuii IJ12 nFXTEnurce npfi.ia 1!:; DECT dcmoduktor Other 
Mfiliuiis needed for DEt'T mea-iurpments are Opnon OM prf'cisirjii frPqiiency refon-'iice. Option 101 fast time-domain swt'pp, and Optiuu 
11)5 time gated spectrum anoljisis. 



lOU Ucfcmher tlftl3 RpwIett-Paritio-il .Iminiiil 

©Copr. 1949-199B Hewlett-Packard Co. 



The demodulated data also passes through a squaring rircuil 
and is converted to a TTL level, which is output via the Dmb 
Oui connector un the rear paiiel. 

To enable the TI)MA signal to be displayed correcUy the 
display ilriver circiiitrj' must be supplied with a suitable trig- 
ger. TTie trigger signal path has an important rote in Uiis ap- 
phcation since the analy"ipr niiisl know the position in time 
when the moilulated carrier is received. A trigger can be 
provided in iwo ways: from a detection de^ici? that outputs a 
positive-going edge when ilie RF switches on. or from a 
Ifigic .signal indicating the slot liming, derived from the con- 
trol circuiuy of the eqiiipnient under I.esl. The trigger signal 
is initially fed into the input of the gate card. Tlie gate card 
has TWO functiojis. The first is to provide a prognirnniable 
delayed outpiil «'luch is then conneclec! ro tlie external trig- 
ger input. pro\i(ling a post-trigger function. The second is 
the ability to switch the video ))atii of the analyzer, allowing 
only selected lime intervals of the signal to be measured. 

Emissions Due to Modulation 

The first test to examine is the dieasuretnent of emissions 
due to modulation. This is an iniponaiit measurement in any 
communications system and indicates 1o what tiegree die 
transmitting channel of interest it^ierferes willi its acUacenI 
channels. Adjacent chaimel power pan be ilefliierl as a leak- 
age ratio, thai is, the ratio of the power Iraiisniitled (by leak- 
^e) into an actjacent channel to the total power transmitted 
by the trajismitter. Tlie primary requirement for the riieasiire- 
meni is lo measure Ihe itital power in a deiined pa.ss!iand. 
The receiver bandwidth lor DECT is 1 .\lliz. Tlierefore, the 
ratio of ilie power itulie transmit band to tlie power mea- 
sured in a 1-MH?. passband centered on any other DECT 
channel will give Ihe atljacenl channel power ratio. The 
eniLssion.s due tu modulation meiisureineni on Ilie DECT 
system is a measure of how much power leakage into 
a(ljaceni channels is caused speciriciilly by I.IMSK modu- 
lation prtKhicts. Two factors must be considered in this 
measmc me nt tec h 1 1 i 1] u e: 

The transient prodm ls caused by the RF switching of a 
TDMA/TDIJ system must be elimhuileil from the nieasure- 
menl and cliaJ-arleriKed separately. (Hherwise. they will 
dominate the residt. 

Tlie measuring device, in i bis case a swept spccl.nim ana- 
lyzer, must be aliie to measure the sum of a mimber of 
powers, which will incliKie noise as well as sine waves, in a 
defined band. 

To meet Ihe first requiremeni Ihe lutie-selective spectnim 
analyzer option card must be used. To oitiit all speetral coni- 
jDonenls resulting frtmi time division swilcliing of the RF 
signal the analyzer must only make a measurement when 
the carrier is switched on (iuring the on ponioii of thf bursi, 
thus making the analyzer's sampling hiirrlware think the sig- 
nal is present all of ihe time. Ttiere are many places in Ihe IF 
path a time-selective switch can be placed. For optimuni 
results I he HP 8690 E-Series microwave spectrum analyzei- 
uses tlie l ime gate option card lo perform a video time gate. 
This peniiiis i.lie video signal to reach the sampluig hai d- 
ware only ihiring the selecled lime uiterval. giving the aj)- 
pearance of a CW signal that is purely digilally modulated. 

The imal.yKer musi be provided witli a sigiial to infonii Ihe 
video gate wlieii to open and close. The reciuiretl sigriid is a 




IV In 



FP = fned Part [Base Staiionl 
PP= PonaMe Pan (e.g.. a Handselj 

Fig. 4. Fur siJiEir iuf<)sur(.'Mii'iiis t)ie analyzer miust lii- provided with 
n sisiiii! 1,(1 tfll the vlilwi gali> wiiant'i open and ••liise. Tliii- HP 
HfiiilKA burst rarrier trigfier delpftfir i.ircuilry produces n ptisilive- 
going TTL pilge when thp RF hilr.'il sw-ilches oti. 

TTL signal from the equipment imder test to iiidicate Ihe 
switching on of the RF burst (TDIVLA. signal in Fig. ;)J. This 
can be achieved directly by taking a TI L signal from the 
control circuiliy of the equipment undei' test or indirectly by 
using the HP 85y02A burst carrier trigger as siiown in Fig. 4. 
This signal is connected to [he time gate card input. The 
card also needs two pi^ogran unable parameters: gate delay 
and gate length. The gate delay is a time delay between re- 
ceiving the gale card input .signal iuul sw ilcliing the video 
gate on, and the gate length tells how long the video lime 
gate switch remains closed. 

Time-s elective spectrum analysis can be a useful tool for 
measiu'ing TDMA systems. However, the video time gate 
ci.implicates the RF palh in Ihe spectrum analyzer Spectnim 
analywr settings such sls resolution bmidwidlh, video b;uid- 
widlh, and sweep time can be diflicuit to optimize for a 
gated measuremenl. 

A number of rules were followed in imiilemenling a lime- 
selective measuiemenl. in the HP 85723A personality to en- 
sure that the frequency and iuiipliUide results are coirecl. 
Tlie screen dis|jlays 401 data poirils from the ADC. So that 
the spectnini mialyzer can ciiiTeclly sample pulsed tnodu- 
latcd signals, Ilie sweep lime musI be long enough to ensure 
that each ilata point contains some burst information. This 
is guaranteed if the sweep time is set to 401 times the pulse 
repetition inteival, which fur DECT is 10 ms, 'Hiis equates To 
a 4-to-5-second sweep time Por eaoli sample, Ihe time pe- 
riod between ihe RF's switching on and the video gale's 
switching allowing the signal to be aiiaiyzed must, be suffi- 
ciently long to let the resolution bandwidth tiller charge or 
sel up. This is reh'rred to as the .setup lime, Tile video band- 
width niter must also have suflicient lime hi liiarge. an<l 
because tlie video bandwidth titter is po.sldeieetion, its 
charge time is dependent on tlie gate length. The ETSI speci- 
flcalion dictates thai a tiO% [loition of the burs! canier must 
lie anal.yzeil. which sliould begin ai least 2^i% of the total 
leuglli of the biicst away from the .stari. Compaiing this to 
tlie timing chaiacleris[i<'s ofa basic physical packel shows 
that the observation window or gate lengtli should be 218 jis 



© Copr. 1949-1998 Hewlett-Packard Co. 



t)ei'i>ml)er im HcwleU-ParkHnl Jmirnut 101 



Imiih sfMi-lingOl [IS away from start uniie biirisl or the 
[iDsiiioji 'il'bil In this case Iheii, ihe.sftui) lime is approx- 
imaiely 90 us. Tht* resoliilion brindwittlli must hi> set to a 
value grrari^r lhaii 2 dvpv the sctiiii time, iir 22 kH/., aiui t.h:> 
video Ijaiuiwidlli iiiiist be greater than I over Die gale lengtli 
orl/218ns = 4,rikH/. 

To nieel tlie second reqiiiri'ment abovi'. a noisc^lyiii' measiirc- 
mciil must he maile. but it. is iii >t simply a case (if placiufi IJip 
mai ker on the noise floor, more c'om[jlex nioasiLre merit con- 
sideraliiins must be laken into account. AdjacL'nt channel 
power measiu ement.s have traditionally lieen carried nut wilh 
a ini'asiLring receiver, which mcasiues the sum of powers 
through a specified filter. However, (he ETSI l,y|)e- approval 
speeificatioi! is mmc suited l(t a speelnim analyner. 'IVn key 
jminls can he taken from the .s|)eci heal ion. The rii*sl is that a 
lUD-klb. resolution ljiindwi<ltli filter shouUI be swi'pl over » 
l-MIlz range, and the second is IJial arluid power is the aver- 
age or the powers uieasmed in all of the IDd-kllz trace data 
V>oinls, Uliy should it be flone this way? Wliat is reijiiired is 
the noise power in a specified band, wilh the noise leakage 
from other chajinels ke|)t at ;m absolute minimum. Tlie roll- 
ofr" characteristic of a KIO-kHz filter is substantially better 
than that of Ji l-MIiz filter Therefore, if the power in each 
lOll-kllz trace data point is measm"eri, and the sum of the 
mciLsiircd powers is divided by the number of measure- 
MU'iU.s litken, the total noise power within a l-MHz biuidpass 
filter is resolved with the noise response of a lOO-kHK filter. 
The result is dien norTiwIixeii with a scaliiif! fact or to correcl 
tor using a IdO-kH;^ filler insiead of a l-MH/ resoliilion liaiul- 
width filler. This ine^LsiireineiU is repealed on all DEf'T 
channels relative In the transniit ( hannel. IIP 8iif*ll E-Series 
analyzers' ability to perl'onn math on displayed traces is 
ideal for this type of noise measuremeni application. 

Emissiaiis Due to Intermodulation 

The emissions due lo inlennodulation lesl follows all the 
same rules as The emissions due to modulation t#sl. The 
lirimaiy rest requirement is lhat two caj-riers are set up on 
different cbanni'ls but using the same time slot, ThLs enables 
the time gating fruiflion lo capture only the on jieriorl of the 
burst carriers and integrate power in a l-MIIz band. Fig, 5 
shows the effecl of removing Ihe TDMA u.siiig time gat ed 
spect.nim analysis. 

Transmission Burst Timing Parameters 

The ETSI ry|je-a|ipro\'al specification requires that the switch- 
ing characteristics of the RF burst fall within the limits of a 
specified template. The NTP, or normally transmitted power, 
is Ihe a\'erage jiower o\'er Ihe burst duration. Tile rise and 
fall slope times nmsl be less lhan ifl its ajid the power of Ihe 
hursi must he less l.hiin -47 dBm after 27 |is on each side of 
(he burst. A apectmm aiialy/er is ideally suited lo perform- 
ing this mea.suremenl, having tlie abihty to generate liiiiil 
Unes represenling the specified template that aulomalically 
deieel an incoirect signal. Elowever, some complex mea- 
surement rules' must be observed and accurate liniit lines 
must, be created for every size of DECT packet. Tiiis again is 
a set of tests that can be performed easily witli Ihe aid of a 
measuremeni personality. The measurement per sonahty 
chooses the optinnan resolution and video bandwitiths, 
selects the rele\'ant sweep lime, and calculates the correct 
timing parameters, providini; a suitable trigger. 



rii .• alt ini m at 





FiB- 5. Time Hated sppct.riin'i analysis cnn displny interninfliilalJim 
PIT nil 11 -Is thai would iKiniially In- iiklden in I In' ,'\M splatter of a 
'IIJMA sy.stem. (a) Time gale iiii (h) Time gale cjrf. 

To view the rising or falling edges of a pulsed RF signal wilh 
good resolution, it is vital that the trigger timing he correct. 
For the rise time measuremeni a prelri^ger funciioii must 
also be included, A i)rel rigger fimction in zero sjian is be- 
yond Ihe fimclionalily of most siicclnim aiialy/,ers, bul a 
posMrigger signal can he created by using the flmetionality 
of the time gale card. As in the ]irevioiis measuremeni, Ihe 
e'iui]iiiienl musi provide the gale ran I Willi a TTI. signal inili- 
ratiiig lhal liie RF luii^st is switching on. The gate delay and 
length can then i)e set using the ap|)ropriate function. In this 
case, the signal thai would normally swilcli the video signal 
on or off is diverted to tlie gate oiitpul IJNf ' on I be back 
p;uiel of ihe analyzer (see Fig. '■^). Wbal lliLs now provides is 
a signal thai can be delayed in lime un iiplioiial anioiml. If 
this .signal is then conneded lo Ihe external irigger inpul of 
ihi' analyzer, an effective trigger delay can be created, thus 
oflselling tlie juialyzer di.splay hi the time domain. This gives 
a jiosl -trigger, which is not enlirely whal is required for this 
set of lesLs. However, it is quile easy to | pr ovide a pseudo- 
preirigger. The easiest way to do this (see Fig. ti) is to trigger 
off burst a and monitor burst (n -t- 1). If. for example, a trigger 
delay lime of l.iO ^rs is required, the gale delay of Ihesiiec- 
irum analyzer should be set lo the pulse repetition interval 
( 10 nis for DECT) minus tlie pretrigger lime {30 (w). 

The analyzer settings must aLso be optimized for' pulse mea- 
sui enienl perforTnance,'' Two measuremeni criteria aie inr- 
portanl: the first is an accurate representation of tlie pulse 
shape and the second is good timing resolution allowing fast 
events lo be obserxed, hi the c:ise of DECT, the coned 
choice of re.soiution bandwidth will give a good jnilse re- 
sponse. If Ihe resoliilion iiimdwiiltb is loo narrow lor a digi- 
tally modulated burst, the frequency excursions resulting 



102 r>Mi-ml.(i IWMllPwlpti-Pnc-lianlJtiumnl 

©Copr. 1949-199B Hewlett-Packard Co. 



TnTime 
Slot Signal 



I Trigs" 
Poinl 











G«e Card lp 


Gats 








Gate Card Dul 










EltBrnal 






Trigger In 










1 




Gale Dslsf 


Trigger 
Time 













RF Burst 



Display 

Fig. B. I'dsr-inggi'r fuiiclionalily tan be used lo provide a pseuilit 
(ireirigger b> irigg'^riii)! tin burst n Hnd moniroring burst (n + 1). 

from digital modulation will move ttie carrier down along 
UiP filter skill. This will i-aiisf FM-lo-AM rciin'ersion un llic 
stable part of ihc burst. A wide resolution baJidwidlli will 
avoid the conversion process. For GMSK fonnats such as 
DECT, the resolution bandwidth must be gieater than 1.5 
times Ihc bit rate. A uan'ow video bandwidth riegi-ades (he 
pulse response available from the resolution bandwidlii, so 
Ihc %'ideo haiidwidlh must be greaier than or equal to lhai of 
rJie resolitlion bandwidth filter Any sweep time can be cho- 
sen to view (he desired linic. with the best time resolution 
being a 2f)-ii.s swecj) lime wilh I us of jitter 

One HP 8590 Ei-Scries advantage used in the power-versiis- 
lime mea.suremcnis is the added functionality of 1 lO-dB 
dynamic iiuif^e. Tliis is achiev ed by using the speclruni ana- 
lyzer's buill-in ability lo manipulate the display malhemaii- 
cally. Tlie spccl.nini analyzer's display can be stored in inter- 
na! lra<'C Idcalions or lefjiHtcrs. The mea.surcment uses this 
as fiillciWM: I wn mciusuiemenls arc laken with Iho vertical 
axis sel lo lU dD per divi.sion, One measurement is taken 
wilJi (he reference level and attenuator set lo suitalde values 
for a noiiiinally 2fl-dHm iriiiul signal and is stored in a Irace 
regiKlcr. Tlieri a second lueasiirciiicnl is laken with Ihc al- 
temia(or sel lo 1(1 dli and (hi' reference level to -10 dB and 
is sloreti In anoiher Icace rcfjisier li the trace datii from l.he 
two measiirpmenlN is llien merged and displayed with Uie 
vertical axis set lo 15 dfi per division (lie result is an effec- 
tive 1 ll)-(iH nuige. [This gives rise lo some conecni regard- 
ing overdiiving the mixer into eompressiim. However, the 
intermodulal ion effects would iiol tie seen because I he mea- 
suremeiil Is zcrcvspan. Th<' atnplKudc effects would also not 
he seen because (he mixer is iinly In compression in Ihe 
second measurcmcnl, when the bottom pan of tiie Ijursl is 
being analyzed.) 

Demodulation 

The tests (hat a spectnim analyzer can [lerlnrm lhat rei|tilre 
deniudulaiion are cmrier accuracy and IVciiucncy deviation 
measiu'ements. Both meiisurements are carried out in the B 
field. This applies (o all |>aeke( tyjies apart from the short 
physical packet whose [''M characteristics are lesled in the 
A field, for Ihc frequency accuracy le.sts Ihc B field dnta is a 
sueccssiuii of four (Is and four Is and fur the frei^uency devi- 
ation test t.lie loopback data begins wilh 128 bits oscillating 



- 4 Ones iniilBm 

juiJirinjijmrLnjiJi^^ 

Frequency Acciuacf 
IMBits r- — G40nes>ndHZenti — -j 64BfB 



Frequenct DnrialioD 

Fig. 7. Tiie Irxipliack field for IJic basic DECT iiflfkei Is ilie H field 
Giled witli set dalu pailenisas di-nitrd in the ETS 30tVlTfi DKI'T 
s[»?<;(flcatl<ifi 

betweeji 1 and 0, tlien 64 ones. 64 zeros, aiid finally Hi bits 
of 101010,.. (see Fig. 7).^ 

DECT has a ma-ximum hit rale of 1 1.52 kliils's. so 1011)10... 
cquaies (o a frequency of approximately 570 kHz, TJiis im- 
plies that a wiclehaiid de(nc)dalatioii device is required lo 
resolve ;uid display DI-X'Tdaia, Op(imi«:i[igihespecinmi 
analyzer for this rneasnremeni involves liodi hardwju"? and 
software eonsidei-a(ions. The s(aiiilard demodnlaOir hoard 
for tlie HP 8500 E-Seiies analyzers needs to i>e reciiiilignreii 
to provide addetl functionality in the form of a wider receiver 
bandwidth. Tlie analyzer sampling time mid data analysis 
must also be considered so thai DECT data can he resolved. 
Tlie spcctnmi analyzer in FM demodulation mode using the 
Option 112 DECT demodulation card displays the signal on 
the screen with die y axis representing frequency and the x 
axis representing time, gi\ing I he effect of a fretiucncy oscil- 
loscope, where ainpliliide is eqiilvaleiil lo frequency deviation 
and iJie rvnlerofihe screen con'esponds toihe caiiier center 
frequency (Fig, 8), 

In the RF signal palh { Fig. ). Jiftcr d own-con vc'rsion and 
mnplificalion Ihc signal niiisl piLss through the demodulator 
board. The display also needs lo be (rigHered eonccdy. The 
swileimn time of I lie fJF buisi nm.si again be cxiracled Irorii 
Ihe input signal In determine ihe appioximale [Hisition of hit 
P(i and the star( of Ihc da(ii jiackel. The dcmodulalion lest is 
iui iiieal apiilieation for the added post-trigger liaiclionalily 
pKiviilcd by the gate card. For a basic physical packet the 
devialiim and i-enler frequency accuracy are calculated 
l'r<im Ihe way ilal.a deviates in frequency only in llie B field. 
The trigger signal path in Pig. H (lows inio Ihe gale card in- 
put. The gate c;ird is again sei to divert the trigger signal 
fiiini Ihe video gate switch to the gat^ card oiilpiil, which is 



»EF -iH.O tie* 

t nv 

D£VM 

111 



■ itl iB M 




•veil 1 n"; 



Fig. 8. Tlif ffcjuwicy-versus-IiriK.' ■fistiliiy iif (he dcitKidiilalimi nii-a- 
Mirciiiriii biliiiw.i fri-QUfiicj [ii'via(iiiii anniiul die ''I'liier rri'iiiicnei' as 
■A fanclioiuiriiiiii-. Tltc INSPECT PACKET liincLlon allows tlif \\m\ loslfip 
l.liriiiish llic packcl ill H0-|is ini'rt'iiir'nls, 



© Copr. 1949-1998 Hewlett-Packard Co. 



UecpmbiT ll«i;i Jli wli-tr r<u k!ml.lmirEial WA 



401 BytDS 



Fast Analng-lo-DigiUl 
Convener Card 




Hst ADC 
Mcmary 
16K 




Analvie' 
CPU tnil 

Orrver 



Fig. 9, Till.' r^l a!ialrjg-tri-cUsital cfwivrrlcr v.ard has an onbtiartl 
liiK-lali' RAM, iiJInwiiig T.lii' arialvK'T In eapliirc n h'IioIi' ["JEIT [mr'kel 
Willi eiiQiLgli resolutifin lo see raaximuiti-hil-rato rial a I riuisilions. 

connected to the external trigger input. The fi.viifhninization 
field and t!ii> A field ai e approximately &3 fis iji length, su (Jie 
gate delay is set to this time, forcing tlie measureineni and 
the CRT display to ignore llie S field. 

Fur the demodiilalioii test the DEt'T bit rate and the ana- 
lyzei 's sweep linio must be taken into consideiatiim. The 
sweep lirne and display resolution are fiinctions of the iip- 
diile lime of the analyzer's an iilog- to-digital converter Wliat 
must, he considered is the time it lakes for a Dl.CT bit-to-bit 
transttiim. A DECT data packet has a worst-cjise bit-lo-bit 
tranHitiiin <jf 571) kHx or one bit ciiange per 8158 ns, Tu dis- 
play this signal the aiialyzer liuisI update its a;ialijg-l(j-di!^lal 
converter at least twice during this oscillatiDu. T)ie sample 
time of the converter is 50 ns and the screen widlii consists 
of 401 data points. A biust is 3G4 us in duration, sri to disjilay 
the whole hiirsi in zero s|ian the sweep time would he set to 
364 |ia. However, 3fJ4 (is/4Ul means liiat each measurement 
point woidd represent 907 ns. wliich is not enough to re- 
solve a piece of DECT data at its maximum rate. The sample 
time of the fasl mialog-lo-digital coinerter is adtxjuaLc to 
resoh e llie data eiwiiy. Tlierelbre the challenge is l.u capture 
one whole single packet of data aivi display it with enough 
resolution. Choosing a faster sweep lime helps, hut then 
how can the whole packet be ins|jecred? One feature of the 
fast analog-to-(UgiIal com erter option card is tliat it contains 
its own ItiK-bytc memory block. As shown in Fig. 9. the ana- 
lyzer's display has 40 i horizontal measurement points, and 
on each sweej) of the analyzer l(),;i84 measurement points 
are collected. Therefore, with an 80-[is disi.ilayed sweep time 
the actual total data capture time is 3.2 nis. An 80-ns sweep 



time was chosen because at least one DECT data Iransition 
wifli the retjuireri test dahi patlem.s can iie seen, The ana- 
lyzer can capture a whole DECT packet and siiii'e it in die 
fasl anaiog-to-digitiil meinoi^. Once a DECT" data packet has 
been capturi'd ilie measurement can be performed, and even 
with ihc RF ui|)ut removed tlie user can slep up and down 
the packet in Hl)-|js segments to inspect tlie received data. 

The DECT Source 

To complement the dowrifoadabie prograni and provide a 
more nimiileie solution in one box, the standard tracking 
generator of Ihe Hi' 85fKJ E-Series microwave analyzers was 
modified to provide a DECT source. This source combined 
with die receiver capabihty of the spectrimi analyzer pro- 
\ides DECT physical layer tunctioiialil,v. Pig. 10 is a simpU- 
fied block diagrmn of the tracking generator moili!ie(f in ihi.s 
way. The user supplies two uipuls: the data and a signal to 
switch the carrier on and off. The data input accepts a TTL 
level signal which is fed directly into a 0.5 Gaussian fillen as 
specified liy the standard. Tlie smoothed eiafa is used to fre- 
ijuency modulate a GOO-Milz V'CO, which is leverage<l from 
the HP 8920 RF conmiimications test set , producing a signal 
at 600 MHz ±288 kHz, Tliis signal is then fed uito the track- 
mg generator and mixed with a 14-Mllz signal. The Vitri- 
able ilG-tmied oscillator is mixed with the resnltani signal 
to produce the rctjuircd 1 'ECT rre(|uency output. The oitt.pul, 
frecjueiicy can be set from I he downloailable program by 
either selling a channel or eiilering a specific freciuency. The 
kF switch rise and fall times can also be iiser-ilefined by 
setting Ihe swiicli current input on the back panel of Ihe 
analyzer. 

The dynamic range tif the source is bi'twecn -20 dBni and 
-8li dBm. Tliis range was chosen to comply with the needs of 
sensitivity lesling, as defined iti DECT standard KTS 300-176. 

Applications to Sensitivity Ttests 
The sensitivity tests or bit eiTor rate tests for DECT are 
delineil requirements.'' Ttie standard dictates that the eqiiip- 
inenl under test must be set in a certain mode. To achieve 
this mode the test eiiiiipmenl has to communicate wilh the 
eiiuipmeni under test, respomi to Ihe eriuipmenl under test's 
reply, and tlien make a measurement on only a specified 
piece of data (this has already been seen in die FM tests ). So 
that tlie ty])e-approva! tests can be met fully, the medium 
access control (MAC | protocol layer and the physical layer 



0 



6011-MHz VCO 



□ECT Source 



VIGOsGillator 



Dbu In 




Traching Generator 

Q) M.H mi 



6O0MH1 
3 288kHi 




RFSwhch 



RF 
Dul 



Time Slot 
Pulse In 




RF Switch 








»■ 


Driver 







Fig. 10. Block diagram of the DECT source tbrHPasail E-Series speclruiii analyzers. 



104 Deicnilier iiW Hcwlelt-PafkBrrl .Iriiirnal 



ICopr. 1949-199B Hewlett-Packard Co. 





1 i 
T . 


Luapback 
Data 


PrDMCOl Irleilao 




Time Slot 
Pulse In 


DEcrm 

Data In 


TTL Data 
Out 




EquipniBUI 
Under 
lesl 



Fig. 1 1 . For sensitivity tests, the DECT lypp-approvaJ spprifi rati 011 
requires that the niediiiiTi arfpss coritrnl m\d physii'al protocol layers 
be implemeiiLed as part of the test, system. A protocol interface pro- 
vided by Lhe DECT equipinenL manol'attiirer is used for this purpose. 
The ec|iiipriienr luider test is sf t into a ioopback test mtnle so that it 
retrananiits the received data. A bit error rate tester compares the 
relransniittRri liiopbatk iteici with the data [irifiinally transmltt.ed. 

must be implementpd iLS part, of the test systpni. Tlie IIP 
85EI0 E-Seiips sppctnim aiialyzpr opfiniizpcl for DECT nipa- 
suremeiits can traasmit TTL data packpts frDiii an extemaJ 
protocol generator and rpcpivp them Irom the pqiiipmont 
under test. Thus, with some hardware and software addi- 
tions, lhe sppctmm analyzer can form a significant pari of a 
sensitivity test set. 

In brief, tiie l.ype-approval specitiealion says that lhe hit 
error rat* musi tie below a certain ratio for rpceived signals 
of less lliati -73 dBm or -fi-S dBni depending on the lest. The 
bit error rate lest can lie done in the B fielii of the DECT 
basic phy.sical packet, A psendoraiKkmi hiiiaiy sequence 
(PRBH) niusi l.ie iri.serleti into the lesl.er's tvansmii fipld or 
loophack (lala field. In loophack modp. tlie eiiuipntent under 
test will reit'iin.sniit its t'eceived loophack data hack to the 
tester. The receiver part of the tester must then recover the 
ckn'k. strip tiff llie .syitchrnnization field, signaling data, and 
erroi' coiTection Held, and then comfiiu-p the leceived B field 
FRBS Willi the oi iginally transmitted PRES. If the DECT 
inannfacl iirer can provide an interface lhal generates the 
required levels of protocol a sensitivity lest can be carried 
out.. Tlie HP 3784A bit ciTor rale tester can generate a PRBS 
to iiiseri liitri the loophack fielil anil perform a bit error rate 
test oti the received loophack data. Fig. 1 i shows how this 
can be acliieved. 



Manufac taring Remote Control 
With lhe added analyzer fiincrionality of the Option 021 
HP-IB (IEEE 488. EC 625| or Option fi23 RS-232 interfaces, 
all the functions of the HP S-5723A measurement personality 
can be accessed remotely. The IIP S5723A measurement 
personality has an extensive set of prograntmiiig commands 
allowing all fronl^anel coniinands to be executed in an au- 
tomated test sysieiii. Tlierefnre. lhe same tool that was used 
in de\-eIopmen[ and pre-1yt)e approval can also be of use in a 
manufacturing emTTonment. 

Fig. 12 shows the analyzer set up as a manuracturing trans- 
ceiver, nils type of system would uicJude the DECT equip- 
ment manufacturer's owii protocol generator. 3 PC controller, 
and the HP 85ft02A l.iursi carrier trigger The test suite this 
system would typically perfonn consists of an absolute 
power measureiiieni. tJie power-versus-dme template mea- 
-Siirements. the frequency error/deviation test, and a sensitiv- 
ity test- The first thi-ee test.s can be carried out automatically 
with Just a com roller and tlie HP 8;)9l) E-Series spectrum 
analyzer with the HP 85723A measurement personahty. The 
PC or coittroUer calls all of tlie RF ineasiu'einent routines 
through an HP-IB or RS-232 interface, and the measuremeni 
personality returns ah of the results once the measturment 
has been completed. 

The HP 85723A software has been designed for versatility. 
For example, flie main manufacturing need for test equip- 
ment is speed. For maxumini speed, Uie personality can be 
programmed to perform the tests oplimally ral.her than by 
type-approval specification. Tliere are two paths the manu- 
facluiing engineer can laJ<e when programraing a suite of 
DECT RF tests: tlie direct route and the block route. 

Outlined below is a typical programming example for lhe 
emissions due to modulation lest. The HP liistruiiiejil BASIC 
language is used a.s a plalfonn for accessing lhe HP-IB, al- 
though other languages can be used as well lo coniro! lhe 
personality. To set tJie ajialy/er lo measure willi the rpfprence 



Proiocol 
GflnQralar 




Eifiiiiimant 
Uhdei 



Fig. 12. Spi'i inim analyzer sel up for niniiufac Luring l.ransteWi^r 
(j]ierulitiri iii an aiilninnti'd tesi syfilem. 



© Copr. 1949-1998 Hewlett-Packard Co. 



DoceitllttT lBB3HL'wletl-I'at'kardJouriinl 105 



on DEt'T channel 9 for the tesl the rwllowing codt is 
needed: 



10 OUTPUT yiS/'.CHNg-" 

ZD OUTPUT 71 B;"_ACPMOD;' 

30 REPEAT 

40 ENTER 71B;Done_flaB 

50 UNTIL DDneJlag=1 



lljne 10 OECT channel 9 
!Do the lesi lo type-approusl 
Isweep time reqjiremems 
ILoop Lntil tlie analyier has 
ICompleted the maasureniEnt 



If, however, some analyser setup parameter neods lo be 
rliaiigeri. for exarnple llic sweep lime, iheri ACPMQD caji fie 
split inui rwii parts or bli>cks, _ACPS and _ACPM. The tlrsl is 
I he juudvzer setup for the measiu-einent and ilic second is 
the ac'liia! measurement. For example: 



10 OUTPUT 71B;"_CHN 9;" 

20 OUTPUT 71B:'_ACPS;" 

30 OUTPUT 718:'ST 5;" 

40 OUTPUT 718;"_ACPM:" 

50 REPEAT 

60 ENTER 718;DoneJlag 

70 UNTIL Done. flag=l 



ITiine to DECT channel 3 
ISeiLip 

lOhange sweep time to 5 seconds 
[Do ilie measurement 
ILoop iimil the analyier has 
ICompleted the measurement 



Tins is agoiid example of how ihe II1'S.572:JA nieasiiretnent 
personality ean i)e r eiiiiilely projitariinied ki perfur ni a lesi 
more oplim^illy. The MKCT lype-apj)n)VHl spei iricatioji de- 
fines a resohilioii baiidwidili lllll kH>;. a video liandwidtii 
greater tlian the resolution liaiidwidlh, and a sweep time 
grenler lhan 12 seconds. This arliele has already dLseussed 
I he IIP HHEin E-Series ;uuilyzer settings for sueh u measure- 
nienl, Tiie I iP S-^EtO E-Series microwave siieetnim analyzer- 
eaji make the mcasiu-ement just as ateurately with a sweep 
time of 5 seconds. 

Summary 

DECT's growTh is expected lo meet rir exceed that uf GSM, 
making it the definitive cordless lelecommmiicalions stan- 
dard in Europe and in other parts of the world. Wilh such 
high jirojei'ted ii.^ie especially with potlablc hanrlsets, the 
type-approval speeiru alion iiuisl cnsur-e lhal I'KCT does mil 
uuerfere with <iiher radio sytsieiiis. The HF spei'lnim in Eu- 
rope is already crowded with many other commiuiicaiionK 



sysienis giving manufactmers a challenge lo ensure llial. 
iheir RF ennuminication equiiiment meels lire required slan- 
daiti. Tlie lype-ajiproval standard ensures that IlECT equiii- 
ment will work freely wilh other radio syslem.s. The stan- 
dard is Hearing final verifrcation. hecotning European law, 
and eleclTotiic communications eqiiipmeiK nianufaetiirers 
will need lo invest in test etjuipmenl lo verify their products. 
In Llie futiui', Tesl equipmenl cirffriiig iriori' complele solu- 
tions will he ref)uir<'d. bul today s research and develoji- 
menl, pre-Iy(ie-approval. ajiil manufacturing needs are well- 
served hy the HP .SlitHJ E-Series microwave spectrum 
analyzer with its associated options to configure it for DECT 
meiLsirtenu-nis. 

Acknowledgments 

The authoi' would like tt) thank and acknowledge lire hard- 
wai'e engineers on tliis project: Rik Smith, Ihe DECT demod- 
idator hoard designer aiul Dave Dumu', ihe DECT modulator 
board designer, pltLS extra iJianks to Dave foi' his researeli 
aaid input lo this arliele for the concept of receiver sensitiv- 
ity cesfing. Hoy Mai-Nairghtun, Tom Walls, and l^ry Nirtting 
should al.so he mentioned for Iheir help and coimsel. l-'inully 
Ihe author would like lo Ihiuik the markeiuig, tjuahly, and 
manufaef uring teams at Queensferry Microwave Division for 
nijiking Ibis product possible. 

References 

1, L, Nulling, "Celluku-iunl PCSTDMA TYansmitUTn.'stjiig Willi a 

S[ii-itnnM .^jitil.v/.er," f'mrmlhujx nfllir Fiful Atimiiil Wire/ess 

Sj/iiiiiiisiiiiii. San Jusi'. f^iilifumiii, January 191): I. 

2-H. Chen, 'Time Division Multiple Access (TDMAl Traiismilter; 

Characterizing Power. TlminH, ^lul Mediilaljini Act uraey." IIP l!if)3 

WiifU'fix C'liiiniiuiiciiliiiii^: Syiiiii'Mhiiii. 

:!, ETSI ET.S mi 17R-1 fliwriil DtXT DivrriiilUni. 

-I. ETJJI ETS:J(M) 17:,-2 riu/.sirril l.iiii<'r [)<'fiiuniiii. 

5. H, Clifti, Tiliie-Selettive Specimiii Aiinlysis on IIRFC Sign;Ua," 

HP NFCniii'iiiinirnlUDis Syiitpnsiiim. 
11 T'BH Oli/ETS ;iOII-17l) Ti/iw-Appniral Trul Dwiiiiit'iil. 



106 ppi-pmber I3!l,'l Hpwlrn-Psck.ird .liMini^il 

©Copr. 1949-199B Hewlett-Packard Co. 



HEWLETT - PACKARD 

JOURNAL 

INDEX 



Volume44 January 1993 thraughDMember 1993 



YrtoHawa-Hewiett-Packard ua , Si^ivami-Ku Tokyo 168 



Part 1: Chronological Index 



February 1993 

nir>i(micTpi hiii)I'tg.v for lightwave Cmnrnnnicatiiinfi Ti-sl Ai)plic;i- 

tioii.s, Wiii/inh S. Ifihnk. Kent 11' lUiinni. Slpn'ii A. Ni^wliiii, titnl 
Willidin R. Ti-iiliiii. Jr. 

Timablp Ijiser yources for Uptical Aniplifipr Testing, Btfiid 
Mitisi:iilMrher. Edgar Li-rkpl. Robi'rl Jiiliii. /mil Miiiiad Fott 

EbiTf mal-( ■a\'iT>' Laser Ucsign and Wavelength ( all b rati on. Emmm-ick 
Mi'iUer, Wiiljyiiiiy Rt'irhi'rI. Ch'iiii-iix HiicA", iiii'l Riid' Siri iim' 
Exlomat-Cavity Laser Temperatiure Stabilization and Power ControL 
HfirsI Si'hwi-iknrill mill Edgal La:ki.'l 

Dual-Output Laser Module Cor a Tunable Laser Source. Rui/Er L. 
Jungennan, Ditvitt M. Bmuli. uiid Kan K. Snl'iman 

Researfh on External -Cavity Lasers, Witliain R. Tnihiii. Jr. mill 
I'liiil Ziii'iilit'iUiiii 

Design of a i'rei.'ision Dplical L'.iw-<.'i)herenec Renecloinett'i i D. 
Niiiriinl BtiDxti i; H'lirii Clioti. Mirlnwl G. Hin t, Slri't'ii J. Mifniifl, 
mill Rrillm F. Riinison 

.Avfiaging Mfjisurenienls in Impriav Siiusitivity 

FalirK.uhoii oflliffiised Lliodes for HI' LigliLwave Applicatioiis, 
PalriHi' A. Bi-ck 

High -Resolution and Iligh-Sensitivity Opiital Reflection Mca.'iiire- 
iiii'iUs I siriH Whili- Liglu InLerferiimeir.v, Hiiitij CIiiui mill 
Will/Ill' V .s'i<iv» 

A Mudidar All-Haul (.)plkal Tinie-Doinairi Ri'llecliinii'ler for Cliarai'- 
ti?ri/irig Fiber Links, Jiwcf BvUit niiit WHfrh'd Plfss 

h HiHh-iV'ifoinianiv Signal Pmcesslrig System fw Iht HP RUliA 
Uplical Tiiiie-Doiiuun Rpllet-totnt'ter. Jnsi'J' Bclli-r 

Improving SNR by Averaging 

Design Considerations for the HP MI46A OTDR Receiver, Fmiik 
Mil 

User Interface Design for the HP ai lGAUTDH, Rolii.ii Jiiliii mid 
flmiilil Stvyer 

Analyzing ' )TDR Traces on a VC with a Windows User Ittterface 

High-Peifortiiaiii K' Ojiiinil Return Loss Meaaiirenieiit, Hieijmii.r 
Sell III iill 

lligh-SiJi'c'd Tiiiii'-Duiiiairi Lightwave IVleciiirs, Raiidnll Kiny, 
I 'II rid M. Hniiiii, .S'irplirii IV! Iliiich. mid Kmi Shiilieii 

InlVUiliiuWInP P-l-N Phiiiuclrtpfturs for Higli-Speed Lightwave 
Detettdrs 

Calibriitiijii <i( Liglilwave Di'levti>rs lo (!Hk, Diii'IiI ■/, MrQiiiiif, 
Kok Wtii ('liiiiifi. mid Cliristiijihur J. Miiddi-n 

April 1993 

A New FLiiiuly uf Mi<Towave Sigri;il Gerienilors for the lUSOs, 
Vidliam W. Ik'-lm. Himidd E I'nill, mid I'rln- II. Fi.slicr 



Brtiadband Fundamental Frequency Syiillii*sis from 2 to 20 GHz, 

Brinii R. Short, Tlioinns L. Charll, mill Kdirmrl t;. Crislnl 

A Ni'w High-performance 0.1)1-1 o--2(l-GHi: Syiuhe.timl Sij^riiil 
Henerar or Microwave (.'hain. Will io in D. Biiiii'iijiir'H' i: .Frdiii S. 
Hrfniu'iniiii, Jiihii L. Impei-aiUt DiiugiiwA. Larson, Hirontu de 
Mello Pcreijriiio. and Greyury A. Toylnr 

Interna! Pulse Generator 

ConcuiTent Signal Generator Engineering and Manufactiuliig, 
Ciii-istapber J. Btistak, CiniwJaS. Kolneih. and Kpvin G. Smilh 

A Design for Manufacturahilily, lU'.sign for Testability Checklist 

A New Generation of Microwave SwceptTs, Aluii R. Bluum, Jason 
A. ChodoTa, mid James R ZkU^is 

Third-Order Curvi-Fil Algoriihiii 

A DigiLiUly Ciirrecled Fraclional-N Synthesiser 

Microcircuits for the HP 837-50 Stories Sweppets.fli'ic V.V. llri/mnn. 
Rirk R. Jiinii'.s. mid Rourr R. Cini-lier 

A PrograiLiniatile Pulse Generatiir, llnns-Jiiiyen Wagner 

Pulse/Daia Chatmel Extends Programmable Pulse Generator 
Applications, ChnnlDph KnIkllM 

Hestgn of a M'Ah. Pulse Ijeiierator, Peter Srhimcl, Aiiilirus Pfuff, 
'llhonam Dippon, Tliomu.-i FUdier, iiiuiAllmi R. .\riiii^hiiii!! 

Cnoliiigol'rhe Frequency Divider IC 

A Multirate Bank of Digital Bandpiiss Rlters for AcuiistJf Applica- 
tions, Jew m IV! Wiiitr 

Conlimious Monitoring ofReniole .Networks: The RMON MIB. 
MnllliPivJ liiinlirk 

The HP li4TI)(l Kmbeddeil Lieliiig KnvlroiuiienL: A New E'iin.tdigm for 
liuibedded Sysl.'m hitegration atid Ilebugguig. Ruber! I), liiimliiiiil, 
Riclmiit A. Ni/giiai-d Jr.. and Jiilui T, RaspLf 

The Value iiriisahUily 

The 111 'bug Environmeiil Cimneclion lo HPSoftBench 

A Rcal-riiiie Openiling System Measurement Tool 

A New Perspeclive on EiiuilHIion Hardware Modularity 

Software Perfonnaiice Analysis of Real-'lliiK- Knihedded Hysiems. 
Aiidmr J. Blii~<iriiik. Dunkl !.. Nciiiier, and Armlii S. Bergrr 

Juiif 1993 

ORC'A: Optimized Robot for C'lieniiral Atialy.sis, 6Vijv; Ji, (liiiiinri. 
Ji-sfjili (' Ruiiik. iinil Arthur Si-ldr'iffr 

■|"he MP DRCA, System Outside ihe AiuilylicalLaboratoiy 

flravity-Setistiig Joy Stick 

Absolute Digital Encoder 



© Copr. 1949-1998 Hewlett-Packard Co. 



ni'ceiMluT llMi Hfwlfii-Pm'karrl ,lciufiiiil 107 



HP OpC'tidDB: An nhjcfi-Oricntod Database MaiiHHpriienl System 
Tor CoiiiiiR-rrial Applications. UnJ'Uil Alinii nml Tu-Tiiiii Chmn 

The HP nira VGA Graphics Board, Myron R. Tulilc, Kenneth M. 
Wilson, Samuel H. Clinii, ami Yung Deng 

POSIX hiterTace ftir MPtyiX. Riijpsh Lalwani 

A Procoss for Prevt-nting Soflwaii:' Hazartls.flivini Cuvrinlly 

C'onflgiiralion Maiiagenienl for Software Tests. Ij^mmrd T. Stkmalh 

Iiujilenienliiig and Siislainiiig a RolYwarc Inspccrion Piogram in an 
R&U Ewmmwwt, -I HI II M. Miirl-rr,/! 

Tlie Usp 'jfToiai Qiialily Control Tfiliiiirjiii's lo Impmvc the Suit- 
ware Localizitlioii Process, -lalui W. Gmiliii)u\ i'hidiv A. Hammond. 
Willinm A. Koppes, John ./. Krieger. D. Kris Roi'eU-RUK. and 
Sniidni ■!. Wtirner 

Tools Tor (he l^piage TVanslatioii Process 

A TVansaction Approach to Error Handling, Bvure A. Rafnet 

Emir rieflriition 

L'ser Interlace Management System for HP-LIX System Aditiinistralion 
Applications. Murk H. Noless 

SAM versus Manual Administration 
August 1993 

iligh-tllTiciejicy Aluminum Indium Gallium Phosphide Light-Emitting 
Dio<les. RfAipvi M. Flrlchci: Chihpiiig Kuo, Timolhy D. Oficiiliiwuki. 
JUinn Gu'tj V'li, mid Virgiiiki M. RuhbiK^ 

The Structure of LEDs: Homojunctions and Helerojunrtiona 

HP Task Broker ATool for Distribviting Computational Tasks, 
TcirfiiccP. Gin/; Rtnalo G. Asshii. John M. Lewis. EdwiirdJ. 
Sharp'', ■Inme^.I. Turner, iiiul Mhliarl ('. Wiitil 

HP Tiisk Broker and Computational Clusters 

Tiisk Broker and DCE Interoperabili^ 

HP lask Broker Version 1. 1 

The HP-RT Real-Ume Operating System, Keoin D. Morgan 
An th'en,1ew of Threads 

Managing PA-RISC' Machines for Real-Time Systems, Gmrgr A. 
An::ingpr 

Context Swilching hi HP-RT 
Protecting Shared Dala SLructiues 
The Shadow Register Enrironment 
C Envii onment 

The HP Tsutsuji Logic Synthesis System, W. Brnrc Ci-lbr'rlson. 
Tosliik i Osiimi::. y/jsliisiil.i' Olsiiri'. J. Bnnn Sliinklt^furd. inid 
MuUio T'liiiik'i 

Designing a Scanner with Color \ision, K. DimgUis Gciniellmi nnd 
MlchrieJ J. Slcinlf 

Mechanical Considerations for an Industrial Workstation, Hni<l 
ni-iiii'ii In 

Online CD? l-iser Beani Beai-Hnie Control Algorithm for Orthopedic 
.Surgical Applications, Fninrn A, Caiii'siri 

(Jnline Defect Management via a CI i en t.'Seiver Relational Database 
Management System. Briiiti E. HoJXmnnn, Do rid A. Kcefer. and 
Dotiglas K. Howell 

Client/Server Database Architecture 

Realizing Productivity Gains with C+-i-, Timalhy C. O'Konski 
I ;iossajy 

Uriiiging the between Structured Anidyais and Structured Design 
for Real-Time Systems, Joseph M. Luszv: and Daniel G. Mdier 

Structured Ajialysis ami Structured Design Relreslier 



October 1993 

An H-liigasfiniple-pPr-Second Modular Digitizing t>scilloHCope System, 
John A. Sch'irn'r 

An H-r,iy;LSiirni)le-per-Scci)nri. H-Bil Data At ijiiisition System for a 
Sanipling Digital Oscilloscope, Mieluwl T. MrTiyue and Palrick J. 

A Difiiti/.ingOscillosco|)pTinK' Ras(= and Trisgpr System Optiinized 
for Tliniiighput and Low .lirtiT, Diii'iil l>. E.'iki'Maon, Reginaltl 
Kelliiin, iiinl Diiindd A. Whilfntnn 

A Rugged 2, [>-G Hz Active Oscilloac ope Probe, Tlioiniis F. Uhling 
and Jotiv R. Slfitwr 

Accuracy in Interleaved ADC Systems. AU.e)i Mont.ijo and Kenneth 
Bush 

Dither and Bits 

Filler Design for Interpolation 

A SLudy of Pulse Pariitnetpr Accuracy in Real-Time Digitizing (Oscil- 
loscope Mea.suremeiils, Kenneth Rush 

Architectural Design foraMiKltdar Oscilloscope System, Daivi L. 
Johnson and ChrislojikcrJ, Miignii.wn 

A Survey of Processes Used in (he Dei'i'loj.iment uf Firmware for a 
Multiprocessor Embetldeil System, Diivi'l W. !.on<i nnd C/f c; ,!(((/;/( ci' 

De\'eloj)ing Extensible nniiwaie 

Mechanical Design of a j^ew Oscilloscojie Mainframe for Optimum 
Perfomuuice, -ytj(i«i W. Cainiibell. Kenneth W..lohnson. WnyneF. 
Hclyolh. tllid Willirnn II, FseoriU 

A Probe Fbdun? for Wafer Testing High-Performaiwze Data Acquisll ion 
Iniegratpd Circuits, /Jnnip/ T. Mamling 

A High-Pc(fomiancp l.fi-CiHz Vector Metwork and SpectriLcn Analyzer, 
Shi'/er" Kdicnbntfi and Akirn Nnkii/aniii 

Receiver Design for a Combined RF Network and Spednuii Analyzer, 
Yashiyi'ki Ynnagimolo 

DSP Techniques for Digital IF 

A Fast-Switching, High -Isolation Multiplexer, Yoshiyiikl Yiiwigimoln 

A |[J-Megasainple-per-Sccond Analog-to-Digilal {'onverlcr v.M]\ Filler 
and Memor>', llowoid E. Hillon 

A KI-MHz Analog- to-Digital Converter with 1 10-dB Linearity, 
lloiiaiil E. Hdtoii 

Derember 1993 

Vector Signal Analyzers for Difficult Measurements on Tune-Varying 
and Comiik'S Modulated Signals, Kpiiiielh ,/. Blue, Roberl T. Culler. 
Dennis P. O'Hrien. Douglas R. Wngnei; and Benjamin R. Za.iiingo 

The Resampling Process 

Applications for Demodulation 

A Firmware Ari'hiteclurc for Multiple High-Performance Me-asure- 
menls. Den II IK P. O'Brien 

Rmi-Tiine-Coiifigiirable Hardware Drivers 

Remote Debugging 

Baseband Vector Signal Analyzer Hardware Design, Mnfifreri Rnm, 
Kcilh A. Bo i/piii. Joseph R. Dirdei-irlis, and David F. Kell^y 

ADC Bits. Dislorlion, and Dynamic Range 

What Is DilheriniC? 

RF Vector Signal Aiialy;(er Hardware Design. Rfd erf T. Culler. 
William J. Ginder, Timolhy L. HilLtlmin. Kevin L. Johnson. 
Roy L Maaon. and James Pielseh 

Microwa\'e Plate Assembly 

A Versal ile TVacking and Arbitrary Source 



108 Ufcfiiibcr tim HewlpU-Patkanl .lounial 

© Copr. 1949-1998 Hewlett-Packard Co. 



Vector Measu^nients beyond L6 GIIz 

Optica] Spertnim Analj-zcR with Hijii Ehtianiii- Range and Exrellfint 
inimt SeliHiUvity. i>'H'ii/.4, Bniley iiiut Jaima R. Sliiiiiile 

Optical f^jectnuii Atial.vsis 

A Double-Pass HoniK-Iiruniator for Wavelength Seletiion in an Optical 
Spet'lnini ,\iial>'z«'r, Kfiiiirlh R. tt'ililiiaui'i- uml Xiillaii Azary 

Ui&aclion Grating 

Polarization Si-iisilivity 

A H^-Resolutaun Direct-Drive Diflraction Giadi^ H«ation Hyaem, 
Jvxeph .V. West and J. Douglas Knighl 



A TXvo-Axis -Micmpijsilioner for t^cal Fiber Alignmpnl. J- Douglas 
Knight unit Joxcfll ,V- W'fw/ 

A Standard I'ata Formal for InstnimenI Data Interdiange. Michael 
L. Hall 

North American Cpllular CDMA, Dari'l P. Wliipjile 
( 'ellular Technnlosyts 

DECT Meastirements with a Microwave Spectnim Analyzer. 
Mark A. Elo 



Part 2: Subject Index 



Subject Page/Month 
A 

Abstract data types 85/Aug. 

Access control definition 4S/June 

Access cunTrol, HP OppjiODB 28/.lune 

Aci'iyac:y, iiscillostopp 8, 38, i7JOcl. 

Acoustics. 

industrial workstation 6B/Ang. 

AequisiiJoii system, 

oscilloscope 9, 11. ISyOcl. 

Active rncasuremcnt mode 2'3/Dec. 

Active probe y|/t)ct. 

Activity nieasurenwiils llHVApr, 

ADARTS ( Ada-l)Hseri iJesifpi 

Apjiriiacli to Real-Tinic Systems) , HO/Aug. 

ADC, lO-MSa/s 100/OcI, 

ADC hils, distortion. 

dyjianiic range 38/De(.', 

ADCcWps 14/Ocl, 

ADC, large-scale dithered .'iti/Dec, 

ADl' system, 8-GSy/s VJ/Ocl. 

Address ali^ment Ill/A|)r, 

Affinity value IT/Aur. 

.Agent. RMON 85/Apr. 

Aggregate ohjed 24i'June 

Airflow nianagemeni, 

indiisuial workstation 6S/Ai]g. 

.Mgorithm, laser surgoiy (iS/Aug. 

Algoritlim, third-nrder curve fit ... il/Apr. 

AilriUal' LEDs tyAiig. 

All-haul ojitical linie-douiain 
refli-Llunietry 6()/Fi'b, 

Aiii|ililier, iiulpul 24, OS/Apr., :i&T)pc, 

Aiiiiilifier, shaping IW/Apr, 

Amplilier. .switchable gain 35/Dec. 

Airi|ilirier, 

traiisiinpedance ■ia'l'''eb„ 7l/Dec. 



Atnplini?r, Inivpling-wave 22/.\pr. 

Amplitude modulation 25/Apr. 

Analytical robot Syime 

Analyzer, real-time fnKjuencry 73/Apr. 

Analyzer. 

software performance 98, 1(l7/Apr. 

Analyzers, vector signal (VDec. 

.Anti-.ilias filtering .. .il. lUlAJct., HfVDec. 

.^1 tire fie tUon coating 20. 32/Feb. 

Aperture wheel fi9, 76/Dec. 

.Application ilevelopnienl, robot . . . liV-Jmif 

j\rbitriitiotA, nuJtiproccss ftrvApr. 

Architecture, multiprocess 94' Apr. 

Architecture, oscilloscope 51/Oct. 

ASIC design -syslfira 3a/Aug. 

Assault handling a7/Dec, 

Atomic objet'l y4/.lune 

Atlenuator, optic-al U/Fel). 

Attemiatots 34, 4i)/Dec. 

Ailemaators, t'ET 22/Apr. 

B 

liaseband vettur analyzer 31/Dec. 

Batch .Sl/Aug- 

Bijifiiar sampler 17/0c(. 

BislahiUty tHWeb. 

Block diagram design entry 43/Aug. 

Bone characteristics fB/Aiig, 

Bricks , lifVAug- 

Bright LEDs G/Aiig. 

Butst timing. DECT KKJ/Uec. 

c 

C++ .....avAug, 

C/I'-+ delitiggcr ffif/Apr. 



C environment 35/Aug. 

Calibralioji, ADC aWOrt. 

Calibration, ADC residue gain . . . lt)WOcL 

Calibration, DAC llW/OcL 

Calibration, lightwave detectors . , ST/Feb. 

tlalibrarion, pulse generator 6.WApr. 

CalibiaCion, tunable laser 25, 30/Feb. 

Calibration, 

vector signal analyzer 54/Dec 

CaU stack Ill/Apr. 

Callee-saves 

;md caller-siives regislers Ufi/Aug. 

C^arrier frequency determinalloii .. l^VDec, 

( '{'D detector 53, SsVAug. 

("DMA, Nortli American cellular . . SWDec, 

CeUular CDMA system WVDec. 

Cellular lechnologifs !K!/Dec, 

Cerainic sul.isirate lireahage 34/Oi.'t, 

CGA 31/Juiie 

Chief moderator 61/Jtme 

Chromatic dispersion SfVFeb. 

Client/setvPi' iu'cliilecture 77/Aug. 

Client/server database 

architectiirt' ^S/June. "S'Aug. 

Clocks, oscilloscope 2!t/0ct. 

Coating, antlreflection 20, a2/Peb, 

Collective computing 15. IB'Aug. 

( !olor Grapliics Adapter 31/JuilG 

t'olormatcliitig 54/Aug, 

Color science 52/Aug, 

Color separdlioii, scatmer 54, oS/Aug. 

Complex objei'l.-i , , , 211/Jiiiie 

(lomponent interface 

speciRcalions 9S/Aug. 

Component versions 54/June 

Ciimputational clusters 1(5/Aug. 



© Copr. 1949-1998 Hewlett-Packard Co. 



IJi'fUliilier liJOa iltwlelt-l'ackaril Juiuiuil IU9 



CumiJUlet! ruiiclioii 27/Jimp 

C'omrete dala types 85/ Aug, 

{'Qiifiguralioii flle, 

HP Task Broker L6/Aug. 

t'litifijwraiidn iiianaRpmBiit Ki/Jiiiic 

CivLLiieflivily, iiiUviKlriul 

workstation li:.VAii);. 

Conlaui EtriiJ, 

osfillosi.'0|je taljijit't fiS/Uct. 

Context indejit-ndenl 

error codes T3/Jiine 

Context switching --32/Aug. 

Control, measurement 23/Dec, 

Converter, analog-to-di^tal sei' ADC 

CdoliiiR, U* lil/Apr 

CiJtTi'i'lion of time data lO/Dec. 

Cori'elDlor, ADi:' a9/I5fc, 

Cost, of a variable , 46/Allg, 

Counter circuit, UHF 52/Dec. 

Coujilci^ Ipveling 2(VApr, 

roiipliiif;, lim'-tioiso 2(i/()ct, 

Cuivc-HI ;ilgorillini , 4]./A])r. 

Customizai.ion, 

mcasiirr'nii-'nt - . . 22. 26/Dec, 

CViiiiiili? estiT 4S/Dec, 

D 

Daemon, eimJation system QS/Ajjr 

Data in terci large format ......... 85/Def. 

Data narrowing 87/Aug. 

Data vpctor aiThitecture 18/Dec. 

l)al*i vii'wiT 47/Aug, 

DatabasB, oli.iect-oriented 20/Jiine 

nCK inlwopc'rabilily ly/Aug, 

Dc'ljujj envininment, 

cmbi'dded system 9fJ/Apr 

Debugger macros lOw/Apr 

Debugging, remote 2SI/Dec. 

De<-inmtton, HP BMBA ........... 64/Fel). 

Dt'cimariijn fillcring 102/Oct.. 41/Dec, 

Df cituHlioii. sampli' rale Tfi/Apr, 

DECT as/Dec, 

Defect causes (j2/June 

Defect management system 

(DMS) - 73/Aug, 

Defect prevention Gi)/.iii)ic 

Defect sharing 77/Aug, 

Defect tracking system 74'Aug, 

Delay loop a8/Apr. 

I I' kiv. switched 62/Apr 

Lii'liiy. variable 132/Apr 

Df niodulatioji. AM, FM. PM , . . 11, I2/Dec. 

Demodiilatiim. DP:iT lO^S/Uec, 

ncrivi'ii ftirii-ikiii ar/.lune 

Design for maimfacturability 3(VApr. 



Dewclors. liglitwave 83/Feli. 

Development cnvirimmiml, 

JIP-RT -li/Aug. 

DilTraction graling ... 12, 20/Feb., 70/Dec. 

Diffraction grating rotator ?7/Dec. 

DiffiLSi'il iMoilcs W/FetJ. 

Diffusion bacricr !t()/Fi'1). 

Digil.il sign^il 

pniccHsing 10. 7'1/A|)r.. 8, "-IWec. 

Digilizmg oscilloscopes 6/Ocl. 

Dim imd hursl signaling (KVDec. 

Direct -diive diffracnoii grating 

sysleJll 7-yDei-, 

Directory slniclure. MPE/iX 41/Jime 

Display diivei-, HP Cltra VGA . . a2. :WJune 

Dtstonioii, ,AI)I' 106. in7/0ct. 

Distributed feedback lascis fili/Dfic. 

Dither, ADC 41, 42, 44/Ocl.. 

Dilhcnng, laige-scale 36, 44/Dec, 

Diviclpra, 

binaiy frequency 18. 24, 58/Apr. 

Diiuiilc-balanccd mixer SG/Ocl, 

Dciublc-pa'is iiionocliroraator fi8/Dec. 

Diiwii-conveitpr, \'cctor analyzer . . SH/lJec. 

DriviT.'i, nm-tiMHK'oMnguralile aiVDec. 

DSP techniques WOrt. 

Dii;0 YIG oscillator tli/Apr 

Duration measurements l()9/,\pr. 

Duroid 17, Ifl/Apr. 

Dynamic typing 21/Juiie 

E 

EDKAs 11/Feb. 

EGA 31/June 

Emiieilded system debugging 90/Apr 

P:;MC design 7U/Apr. 

Emissions due to modulation .... lOiyDec. 

Eiiiiiliiliiri'aiialyzcr 92/Apr 

Encoder, alwtilute digital 14/Jmie 

Eiiliaiiced Grapliica Adapter gUlmie 

Ergonoiuics, HP Ultra VGA 3Mune 

Error definition , 72/Junp 

Error handling 71/Juiie 

Errors, time base 24/Oct. 

Etalon 13, 21/Feb. 

ETS! 9Snifc. 

Event tree analystis oO/Jtinf 

Exceptions SS/Aug. 

Evtemal-cavity 

lasi-rs T, U, 2U. 28, 32, Sa/Feb. 

External fuiiL'tioti 27/Juiie 

F 

Fabry-Perol lasers 65/Dec. 



Failure modes 

and cffecla atialyBis 49/,:une 

Fault Iree analysis 49/Jiiiio 

FFT in analyzers R/Dec, 

Field solver program . 19/Apr 

File access ajid control, MPE/IX . . 44/Jime 

FilciiiuuinB, MPE/UC 43/Jiini- 

Filler, aiiii-:iiiiLsing ... II. IDl/Oct.. Uli/Dec. 

Filler, baiidwidlJi limil . 4-l/C)ct. 

Filter. IF 19/Oct 

Filter, interpolation 45/Oct 

FUtering. zoom and decimation . . ll)2/0cl. 

Filters, decimation 41/Dcc, 

Filters, hamiojiic 25'Apr. 

niters, liiw-|)!LSSswitcbeiI 2i.!/Apr 

Fillers. luiiliirate digiial 7:1/ Apr- 
Finesse 2.'!/Feb, 

Firmware development, 

oscillosco|)e S9, ()4/0c[. 

Firmware, optical source 15/Feb. 

Firmware, signal generator 10/Apr. 

Firmware, ^ ecl or signal analyzer . . 17/Der-. 

I''ishbone diagram Uli/Juiic 

FISO memoiy 14/0c1, 

Flash AD<' 14/Oci, 

Flexure plate 81/Dec. 

Flow grdph optimisation 7S'Apr. 

Form factor - 6!3/Aug. 

Forward link encoding Si4/Dec. 

Fractional-N 

phase-locked loiip 14, 42, 44/Apr. 

Free apectrdi rdngc 2tVFeb. 

Frequency cotilrol IW/Apr. 

FrequeiK'y conversion 49, 50, HUDec. 

Frequency diversity 92,11ec. 

Frequency measurement SVApr. 

Frequency niodulal.ion 13/Apr. 

Frequency selective analysis - d/Vec. 

Freiiuency syTitliesis, 

LiruailbiURl fundamental 12, 3K/Apr 

Fr&juency synthesis 

subsystem - 12, 42i'Apr. 

Full widlli at liidf maximimi 

(FWHM) - b'i/Feh.. iWDec. 

Fmictioti duration 

measvirements ll(l/Apr 

Func lioiUi. HPDpenUDB 2(VJiiiie 

G 

Gate array, data -57/Apr. 

Gear b;tcklash 77/Dec. 

Ghost elimination 75/Feb, 

Graphits en^e, HP Ultra VGA . . . 32/June 



110 DL-iTiiiber 1993 Hcwlelt-Packanl Joiirnul 

© Copr. 1949-1998 Hewlett-Packard Co. 



Grating, 

dilTraiilon 12. avFeh.. 68. 70 Dt-c. 

Gnuiiy-iM'flsuig jtiy stick 12/Juiic 

H 

Handle classes. tv+ 8T/Aiifl 

Hwil door 38. 4(W)«- 

Hamionk ilrive mJufUiBi LL'June 

Hiutaid avitiilafii'e proL-ess 4SQune 

Hffiiiles Grdpliics Carii -31'Jiine 

HcliTJxtyiie (land niicrofircuit .... 49/.\pr. 

Helerxigeneil.v 15, 79/Aiig. 

Heterojunctiofi LEUs , , S/Aur. 

HGC 31/Junr 

High-isolation shielding 9a'()ft. 

HiSh-specd nitiilipU'xtT !^5/Oct, 

lioriiojvuictiiiii LEI)s &Aiig. 

IIPCMOS. HPlTltraVGA SS/June 

HPUVGA iiliDty .... a7/.Iu:ie 

Hybriii ADC rhannel !4/()ct. 

I 

[Frfeleclion 88/Ocl. 

Impulse response, photoreteiver . . SS/Feli. 

Induslriai workslalion 62/Aug. 

Input, analog 32/Dec. 

Input sensitivily. HP 71450A/51A , . 62/Det'. 

Input trip 34/Di"f. 

Inlegration, (iiarrete 7S/Apr. 

inlfHril.v and sifurity, DMK ?!)/Au(;. 

[iiTurciiiiru'ci, pliiK-iii lih/Oci, 

tnffTTjiPC classes, C-i-t SiVAug, 

liilerlravp(l.\IK'systi'm l-i,3S/Oi-l, 

Interleaving (10' H14nA] a4/Feh, 

lMler]'fiiaLioli, wavefonTi 4fi/(Zli'l. 

Interpolator, time base 27/()fi. 

Interrupt liantiiiug rili/Aug. 

In temipl alii lily 

intersymbo) Interference , . . - 4iJ/i.>i't 

Iniertxisk (■(inimunication 94/Aiik. 

Inlerviil duration 

rneiisijtenienl.s 109/A|)r. 

I/U arcliit^'cture 33/Aus. 

ISA Inis 3a/Juni> 

Isolators, nptiiral 7/Fclj, 

J 

Jitter, useillosiope 25/Oel. 

Joint spapp, rrihnt. ll/,Iune 

.loint sen'os |;i/Jiirie 

.Iiiy Slick 12/Juni' 

K 

kCodp arVDcc. 



Kernel semaphcirRi 27/Aus. 

Kernel soRwmi-. HP RT 24. 25'.\ug. 

Kinematjcs pnifessor ll'June 

L 

Lalmrmorj' nitnit fVJiino 

Language muislulion liS'Juni- 

Laplarian (Hitential solver - l!l/Apr. 

Laser eomrol algorithm fi&'.\iig. 
Laser module . . . WFeb 

Lasers, 

estemal-i-avity ... 7, II, 20, 28, l)2..WFeK 

Lasers, semiconductor 7/Feli, 

Lasers, YAG - 7/Feb, 

Lale- binding 21/Jui)e 

Lathe/robot system 9/June 

Layereil miuiufacturing 30/Apr. 

LE[)s li/Aug, 

lifetime of a variable 4()/Aug. 

Liglnt-eniitliiig dioiUiS tVAug., li^/Dec. 

Linear arluai or .» S2/Dec. 

Linearity. ADC lOiyOct, 

Linearity '■orrection. ADC i'2/Ori. 

Literal object 24/Jiine 

LO feedthroiigli cancellation . 4n/Dec. 

LOloop 1 5/ Apr. 

LO, vector analyzer 41, 51/Dec. 

Logic syntliesis system , SS/Aug. 

Logical Description Formal 43/Aug, 

Long code !i4/Dec. 

Um-hiuid imtpiit section 2H/Apr 

U )tt -I I )hc rent e re fleet o meter 39^eh. 

I/)«-[|;insii'nl liesiMti ... , , , . . il7i'()cT. 

Luminous perfimnnnce, LED .... I^/Aup. 

M 

Manufacluring. signal generator . . HO/.^r. 

Matkcfs II l/Apr, 

Maximum jiower cum- IK/Feb. 

MDA 3l/,liinp 

Measuremenl sequencers 21/Dpc. 

Mi'cbanical clcsign. (tsciUoscope .. tUi/llct. 
Mec-|iimjcnl design, 

|)iiLsi' generator 71j'Apr. 

Meilic-al inrormatioii 

anil iiioniloriiig ^flistem , .47/-lune 

Meniiii'y chip . . , , l4/()ct, 

Meniuiy depth, oscilloscope . . . 14, 2Wf )ct. 

Memory teak.s 87/Auy. 

Mesa p-i-n riiiiiln-s 4i)/Feli, 

Message (llslrilmlion HWApr. 

Meiiuiierisii S3/Aug. 

Metrics, software inspections .... (li'.lune 
Methods ilevelDpiiieiit softwiuv H/.lutie 



MIE SiApc 

Mlchelson interferameter 53/Ftb. 

Microcircuits. 

hybrid .. 19.2I.46,K.C3,e/Apr_, 14/Oct, 

Microwave chain , 17/Apr. 

Micrciwai-e plaie ossein bljr &0/De<^ 

Miser. GaAs line .....4i»Apr. 

Modulation module 21/.\l>r. 

Modulation transfer fimction 

IMTF) , , - r-KJfiUg. 

Modulator amplifier 

microciri'uil 50/Apr. 

Modulator-based 

o])tii'al test systPtti SO/Feb. 

Modulators, optical T/Fel>. 

Module generators S-t/Aug. 

Moiiochnmie Display Adapter .... 31/June 

Mnoz mode 42/Dec. 

Motion, rolJot 14/Jime 

MPE directoiy stnictuce 41/June 

MPE/iX directory structure 42i'June 

Multimeter, liglitwave 81/Feb. 

Multimoding 37/Feb. 

Multijiitth 92/Dec. 

Multiple inherilaiici: 21/.lune 

Mtilliplexer !)B/OcL 

Mitlli pro ceasing 

operating s.vstem 72'Feb. 

Mulliraie digital bandpass filters .. 7:Wpr. 
Mullitlireaded kernel arVAug. 

N 

Native langizuge lielp lest 7'.Weh. 

Negative delay BS/Apr, 

Netltsts 3fl/Aug, 

Network tuialyzer Yd/Oct. 

Network monitoring 82/Apr. 

Node classes, C++ 80/Aug. 

Noise, ADC lOlVOcL 

Nonlinearities, time base 25/OcL 

0 

Object action manager (ObAM) . . , SI/,Iune 

Object, HP OpenODB 24/,!une 

Oliji'ct models 23/,!iinP 

Oliji«ct-i I ri em ed niodeling 20/June 

Object-oriented 

prograiiitiiing laiLgnage 24/Jtme 

Or'lavc-tiimd ;malysis 7U/Apr. 

Offset liiop U/Apr. 

OMVPE H/Ang- 

OUPL 24/,riine 

OpcnODB model 2Mune 

Optical deck ■115/Feb. 



© Copr. 1949-1998 Hewlett-Packard Co. 



RpcPiiiber IPHii HeWlett-PatltarU .Iiiiimal 1 11 



Optical frequency -ddn lain 

reflet'l imietry 52/Feb, 

Optical hfiterodyne test system .., S8/Feb. 

Optical impulse test system ST/Feb. 

Opiical specLtTini afialysis ()2/Deo. 

(Jptiral s[)e<'lnini analyzers (ifi/IJoc, 

Optical sources, tunable 1 1/Ft'h, 

(>f>lk-nl tiiiic-ilriniaiii 

t-eflcctcimet(y (OTIJR) 52, (il/Feli. 

()p(iniizatlfin, filter 77, 7S'Apr, 

OpiLitiizaiiopi logic 41/Aiig, 

0|>ttil)lock 13,24/Feb, 

ORCAmhdl e/.lune 

Ortliopedic siurgery kisvr ciifin'fil . . lifVAuR. 
Oscillfil'iriransienl 

mea.siirenieiits 12/Dec. 

Osclllcjscf>pes, 8-GSa/s 6/Ocl. 

07DR receiver fWFeb. 

Output module . , liJ/Apr. 

Overloaded functions 21, 2S/.lii]ic 

P 

Packi^e structuring iM/AuH- 

Packet capture HY/Apr. 

PA-RISC niachines 31 , 32/AiJj;. 

PL\icleo 31/Juile 

Peephole opCinuzatlona 41/Atlg. 

Perfomiance verification 57/Dec. 

Period generation 60/Apr. 

Phase accuracy 77/Ap]'. 

Pha.sp noise 14/Apr. 

Pliase noise nieasurements 12/Dec. 

Pilot Oder pclors H, 4H, S3, 85, 87/Feb. 

Photodiode. HP 71450A/51A 71/Dec. 

Phot 111 uminescence El/Feb. 

Photonic technology (i/Feb. 

Physical layer, DECT 99/Dec. 

PID semi loop 14/Feb. 

Pilot signal _ !)4/Dec. 

P-i-ii phoiiidetectors, HP S504A . . . 4SI/F('b, 

nxel viewer - . . -WMin- 

Planar p i n liiodes 5fl/Feb, 

Plug-ins, oscilloscope 11, (,i8/0cl. 

PMMA characteristics 68/Aug. 

Polarimeters S/Feb. 

Polai-izHlion S.yFeii., fiSt/Drc. 

Polarization diver^ty 

receiver 42, 55/Fi-h. 

Polarization sensitivity 71/Dec- 

Polytope opLiniizalioii 77/A|jr. 

Portable Operating System 

Interlace , , 41/Juite 

POSIX 41/June 

Power control, cellular 93/Dec, 

Power control, laser 29/Feli. 



PoWet leveling 26/Apr. 

Power spi'Clrai density ft4/Dec, 

Preamplifier 35/De('. 

Preliminaiy hazajrd analysis 51/Jnne 

Prpslressing 3G/0ci. 

Pri'tfwls ai/Apr. 

Pri-lriggi?!', oscilloMtiipp 7/Ocl. 

Priority inheritance 27, atl/AuR. 

Priority inversion 28/Ang. 

Piinleges, HP OpenODB 2i)/Jiine 

Probe, active 31J('M. 

Probe Fixliue. wafer lest 73'0c(. 

Prorpss, HP-RT 27/Aug. 

Proci'ss schctiuling, flP-RT :3I]/Aug. 

E^ograni acltvity 

mea-siiremeiila ll)!.l/Apr. 

fh-otocol, DECT !)8/Dpc. 

Pulse/data channel 56/A|ir. 

Pillsi' fonnatter , (iS/A]ir. 

PhIm' lleiglit accuracy 49/Ocl. 

Pulse generator 27, 52/Apr 

Puisi' inoiliilation 25/Apr. 

Pulse |iar:iiiii'ler accuracy 47/Oct. 

PulsL' width accuracy 4a'0cl, 

Pulse width general.ion (i4/Apr. 

Q 

Quantizing error lOT/Ocl. 

Quanttim efficiency il/Aug. 

Quanlum wells 7/Feh. 

R 

RAKE receiver , SZTJec. 

RAMDACs 36/June 

Real-time operating systems . . 23, 31/Alig, 

Real-time systems 23, 31, 9U/Aug, 

Receiver design, HP439GA .... 81,8ii/Oct. 

Receivers, lighrwai e 83/Feb. 

RecoristrucfJon, waveform 45/Qcl, 

Reeiilrancy 25i'Aug 

Reference loop IQ/Apr. 

Referential integrity 2D/June 

Keflectometjy 8, ;t9, (iO/t'eb. 

Register allocation, 

logic s.vTi thesis + . 4ti/Atlg. 

Reliiibility, 

iiidu.si rial workstation ivJ/.Aiig, 

Remote access 15/.Aug, 

Renitiie debugging yD/Dec. 

Resjiiiipling lO/Dec. 

Residual interpolator 28'OgL 

Resolutitm bandwidth 79/Oct. 

Retroreflector 41/Feb. 



Rpiuni loss meastirptnent. 

HPHllliA-.. Tfi/Feb. 

Reliirti loss tneasiirement, 

optical T!i/Feb. 

Reverse luik encoiiing tWi/Dec, 

RK shielfi design 82A)i-t. 

R?" source 54/Dec. 

RF vector anid.s7^r 47/Dec, 

RFl suppression :J3/Dec. 

Rise time accuracy ■17Aict, 

RMONMIB «2/Apr. 

Robot system li/Juiic 

RTOS measuremetU tool 97/Apr. 

Rules HWAug, 

Rim-l i nie-configurable 

hardware drivers 30/Dpc, 

s 

SAFI) inicrocirciiil 47/Apr. 

Sample rale, oscilloscope 14,21/Otl, 

Sample-and-t'itter lecluiiquc Ili/tlct. 

Sampler cliip 14, 17/l"lct, 

Samjiler, microwave IS/Apr. 

Sampling, electrooptic U/Feb. 

Scale fidelity 7S/(lct.. 

Scaling, fixed-point 79/Api'. 

Scallop error 91/0<;t. 

Scanner, color flesktop 52/Aiig, 

Scroll bars, sticky 102/Apr. 

Self-i;aliliratjiiti 44/Apr. 

Self-test , 43/Apr. 

Semaphores - 2!i'Aug. 

Sensitivity tests, DECT H)4/Dec. 

Separator, color 55/Aug. 

Seniceabilily, 

iniiiistrial workstation I)3/Aug. 

Ssiimp and longimp 37/Alig. 

Shadow I'egisd-rs 34/Aug. 

Shafi encoder 78/Dec. 

Shared dataslnicCures 33/Aug 

Shorl codes 114/Dec, 

Side-mode filter 13, 21/Feb. 

.Side mode suppression ratio G7/Dec, 

Signal averaging -U, f>5/Feb, 

■Signal gi'iieraloni li/Apr, 

Signal ]jrocessing, HP8.504A 4:i/Feb. 

SigjiiU-to-Hoise ratio 54. 63/Feb. 

Situ Illation, 

logic synlhesis system 4-1/Aug, 

Simultaneous RF/baseband 
measurements i4/Dec, 

SNllP 82/Apn 

Soft handoff B2/Dec. 



112 December 1083 Hewlett-Packard Joiinial 

© Copr. 1949-1998 Hewlett-Packard Co. 



SnftBench SS'Apr. 

Software defeol management '^'Aiijl. 

Software, DSP TiVApr. 

Software hazanls il/June 

Software inspeclions OQ/Jiine 

Softwacf^ localimtion 64'Juiie 

Sound mlensil} T-i/Apr. 

Soimtl pressure 'ili'Apt 

Soiirre, 

veotor signal aiiaiyzer . 43. 54/Dep. 

SoiircES, tunable laser IiyPeb. 

Spatial dh-ersily ... 92/Dec. 

Spatial resolution 54/Feii, 

Spectral power distribution 52/A\xs. 

Spectral reflectance _ 52i'Ai^ 

Spectrum analyzer TG/Oct, 

Spectrum analyzer 

DECT measurements OS/Dec, 

Spectnmi resolutioii - 90/Oct. 

Speech encoding, cellular 93/Dec. 

Spontaneous etnission, LEDs 63/Dec. 

Standard data formal SB^ec. 

Standard observ ers, 

CIEandNTSC 53/Aug, 

State machines 23/Dec. 

Statistics Ill/Apr. 

Stimulated emission, 

Fabij'-Perot lasers 155/Dec. 

Stitching, IIP 814(iA Ij4/Feb. 

Si ors^e, oscilloscope 7/Oct. 

Stored fiinction :!(i/Ju]ie 

Stored ijnicetlures SO/Aug, 

Strain gauge amplifier 83/Dec, 

Strain gauges 83/Dl'c, 

Slnictiireil analysis 

iuiii slnu'lnred design BO/Aug. 

Subscription, message . , EKVApr. 

SuperVT;A 31/June 

Surgii-al laser control tiii/Aug. 

SurrogHle olijecl 24/Jime 

SVGA 31'June 

Switches, p-i-n 23/Apr. 

Sweepers, microwave 3!1'^t. 

Synthesized signal generators 6/Apr. 



System administration 

manager (S.\M) S(K)ime 

System Stan WApt. 

T 

Task Broker 15/Aug- 

Task strucKiring , . ^Aug. 

Taumel etalon 23/Feb. 

Tfcarh pendant. n>bot i2/Jane 

Temperature calibration 7(V^r 

Temperature stabilization, laser . . . 28/Feb, 

Templates SS/Aug. 

Test library management system 

(TLMS) 53/June 

Ttst set, optical return loss S2/Feb. 

Test suite hierarchy 55/June 

Tlireads 27/Aug. 

Time base, oscilloscope 24/Oct, 

Time diversity 92/Dee. 

Hme-domain corrections 10/Dec. 

Time gated measurement B2/0cL 

Time selective 

frequency analysis 15/DeC- 

Tmieshare 31/Aug, 

Timing board, pulse generator .... 60/Apr. 

Topology graphs 4D, 44/Aug. 

TQC M/June 

TVamp errors 7L/June 

TYartsaction error handling 72/June 

Transceiver, DECT m/Dec. 

■n-jtisinipedance aniplirier, 

HP S5i)iA -mVeb. 

Tl^isinipeilance aiuplifier, 
HPTMoOA/filA 71/Dec, 

TVaveling-wiivo anipliri(?r 22/Apr 

Trigger syslem 24/nci, 

Trigger, vector analyzer 45/Dec. 

Triggers 80/Aug. 

Tsulsiili system 38/Aug. 

Timalile laser sources li/Feb, 

Two-axis niicmpasitioner SO/Dec, 

Two-step dec Mil at ion !)0/Oe1, 

TVpe, I IP OpenODB 25/June 



u 

Ulttasotind iransdiicer analysis , , , llTJec. 

Uncerlainty. return loss SO/Feb. 

Cser mlerface 47, 7ZTeb. 

tiser interface. HP 71450A/5LA . . . 61/Dec. 
User interface management system 
aiMS) so/June 

V 

Vector signal analyzers G/Dec. 

VGA 31/June 

Video Gr^hics .Array 31/June 

Video image procedures, 

assembly and tesl 35/Apr. 

Yidea RAM 34/Jtine 

Virtual instruments 46/Aug. 

VMEbus 23. 33, 64/Aug. 

Vocoder 93/Dec. 

VR.\M 34/June 

w 

Wafer test, amplifier eS^Apr. 

Wafer test fixture . , 73/Oct, 

Walsh codes 93/Dec, 

Water vi^or absorption B8/Feb. 

Wavelength caUbration, laser 25/Feb. 

Wavelength sweep . , la/Feb. 

WDM Il/Feb, 

Wliilc-Iight interferometiy 3SI/Feb. 

Widlli board, pulse generator 64/Apr 

Windows display driver IK/June 

Windows, synchronous , , - - lOl/Apr. 

Work groups, I IP Task Broker 20/Aug, 

Worm drive 77/Dec, 

XYZ 

YIG oscillator 51/Dec. 

YIG oscillatore 12, 4(VApr, 

Zero-span measurements 12/Dec. 

Zeroing and c hupping, 

HP 71450 A/SIA 73/Dec- 

Zoom nitcring 102/Oct, 



© Copr. 1949-1998 Hewlett-Packard Co. 



Derember 11)03 Hewlett-PackanlJijiinial 113 



Part 3: Product Index 



HP SSRCA Purtabie Real-TTrae Frequenty Aiialyzpr A])i', 

HP 43i)(iA I. M-tiHzVeftfir Network antlSpi-cimni Analywr .. <hi. 

HPHl;i^A:(-(iH;iPLilsf.f;pnerator Apr, 

IIPMHtiAOplical Tliiit-IJiJiriaiJi Rfflei'tometyr Fcti. 

HPai53AUgliLwiiveMiiillniel(;r Ft-h. 

HPSHiTATiiiiahlpUsfrSoviri-e Keb. 

HP sum Timal)If[-u^i-r Source Fvb. 

HP 8370 Series Signal Generators and Sweepers Apr. 

HPSo04A Predsion Rellectomeler , Feii, 

HF' 85911 E-Series Speclnini Aiiiilyzp.rs Dec. 

HP547l)lAAcave Pniiic Oct, 

lIP&4710A/l)OscUhiscope Oct 

HPr>4TllA Adfniiainr Piiig-in On, 

UPrj-lTiaAAliiljliTier Plug-in 

HP &4T13A Aniplifler Pius-in Ocl 

HP.54714AAiiiplirierPliig-m (JH. 

HPCvlTSIlA/DOscillusc-drH' Oct. 

HPr>4721AAmpliriiT Plug-ill Oct. 

HP 54722AAnenualor Plug-in Ocl- 

HP l)4?(10 Enlheitik'il Deliug Emlronment Apr, 

HP yuii lOA SyiilhesiKeil Signal Generator , Apr. 

HP 70341A Synlheshiecl Signal I'lenerator Apr. 

HP 714oOA ;unl "HiilA Optical Spectrum .\na!yzers Dw- 

HF' 815.34A Relum Loss Module Feb. 

HP S344II Series Liglilwavp Dpiectnrs , Feb, 

HP 837IIA Synthesized Signal Geni'rator Apr. 



IIP «i712A Synlhfsized Signal GeiienilDr Ajjr 

IIP 8:5731.^ Synlbesiwd Signal i;i'nf rator Apr. 

1 IP 837:HA .Synt.hesL/.i»l Kigiijil (ictienitor Apr. 

HP 837511 .Seiiifs Microwave Swei'iiers , , A})r 

HP St751A Microwave Sweeper _ , , Apr. 

HP8;t75Ui MicniwaveSweeper .Ar)r. 

HP 83T52A Micniwave Sweeper , Apr, 

HP83752RMicrt)wavi'Swepper Apr 

HP 857i-!A LlEtT Mea.'^iu-eruenr Personality Dec. 

HP 8!(4I0A Vector Signal ,\naly/i;r Der. 

IIPR9411A I.S-tlilzDown-Coiiverter Dec. 

HP WUHA RF St-ction Dec. 

HP M(I44UA Vo<-|or Signal .\nalyzer _. r>ec. 

llPttlJtlOMtHli'l 7l:!rl Cimiputer Ang. 

ni'E14a()AAD(.'nii).lill.> (.hi. 

HP nLM.\-Pi.iiii, 11,110. DiJio, KLon, CHiffl, DHOO, KHOO, urm 
ijgh:-Eijiiiiiii|^ Diuiics , Aiir, 

FFPI^Prolie 11 RMONMIB Network Momtor Apr, 

HIMliicnODB -, .Jiiiic 

ilP ORCA Robot System Jutie 

HP-iiTOperaring System Aug. 

HP ScanJet lie Scanner Aug. 

IlPTa.skHnikcr - Aug. 

HP Tisui.siul Uigic Synthesis System Aur. 

HPl'llniVliA - June 

MPE/iX Operaliiig Syston June 



Part 4: Author Index 



Adairu!, Niuu'y June 

Aliad, Rafiul June 

Aiizinger, (ieorge A. Aug. 

Armstrong. Allaji K .\pi , 

Assini. Kenato G _ Aiij;. 

Azary, Ziiltan Dec. 

Bailey, Da\id A Dec 

Barlz, .Manfred Dec. 

Baiinigiirtiier, William D Apr. 

Bayem. Keith A. Dec. 

Beck, Patricia .\ Feb. 

Seller. J ■isef Feb. 

Berger. .\mold S. Apr. 

Blasciak. .Andrew J Apr. 

Bloom, .Man R Apr, 

Blue, Keiiiielh J Dea 



Boosler, D. liowanl Feb, 

Eiosiak, I'hristophfr ,1 Apr. 

Braun, David M, . , Feb, 

Brenne man. John S Apr, 

Biirdii k, Matthew J A|)r 

Byrne, Patrick J Ocl. 

Campbell. John W. Ocl. 

Canesiri, Franco Aug 

Carey. Kent W, Feb. 

t.'hang, Kok Wai Feb. 

C'hau, Sarnnrl H. Ume 

Chens, "ni-nriK Jti'W 

CiiiMlor;i..Ia'i"iiA. Apr. 

Chou, Harry , Feb, 

Clertient.s. Brad .\ug, 

(-'oiuiolly. Brian June 



Crislid, Edward O Apr. 

Cuibertsun, W. Brui'e Aug. 

Cutler, lioberl T. Dec. 

D 'Alejandro. John A[)r. 

Deng, Yong June 

Diederichs, .loseph R Dec. 

Dipiion, Thfinias Apr. 

Dol.selli. Mike Apr. 

Duff, ClirislopherP Ocl.. 

Elo. Mark A Dec. 

EtLgel, Glenn R Dec. 

Es^'ovii^. WlUiam II Oct. 

EskeliisoiL. DiH id D Oct, 

Ferguson. Tliotnas C Aiir 

Rscher, Tliomjis Apr, 

Fisher. Peiei- H Apr. 



114 [itTiinbc-r Hewieft-PackMit ,! mii 

©Copr. 1949-199B Hewlett-Packard Co. 



Fletrfier, Roben M Aug. 

CienneUen. K. Doo^as 

(iiniler. ttUliani J Dw. 

Goodnow. John W. June 

GordtBuGaryB June 

(iiBFber, Roger R Apr. 

Graf, Tenenee P. Aug. 

Gdsell. Thomas L Apr. 

Groniimd, Robpn D .Apr. 

I talL Michael L Dec. 

Handing. Daiuet T, Oct 

Hantniond. CintlieA. ,liine 

Hart, Mii'hael G Feh. 

Heinz, William W. Apr, 

Hplgolh. Wayne F. OfL 

HeyniHii, EricV.V. .Apr, 

HUler, Dun Dec. 

HillstriHii. ■finioUiy 1. Dec. 

Hilton. Htiward E Oct, 

Hinch. Stephen W. Feb. 

Hoffmann. Briiui E .Aug. 

Howell, Douglas K Aug. 

Iniperato. John L Apr, 

Isliak, Wiiguih S Frti, 

.Talm, Robert Feb, 

James, Rick R Apr. 

.ruhrstin, Dana L Oei. 

,IiihiL'ifiu. Koiineth W. OcL. 

Johnson, Kpviii L , Dec. 

Jungenniin, Roger L Feb. 

Kiilkuhl, t'luisloph Apr. 

Kawahalji, Shigeru Oct. 

Keefer, David A - - Aijg. 

Kelliun, tiefiinairi Oci. 

Kcllcj, D^ividF Dec- 
Kin g, Randall Feb. 

Kiiiglil, J, Dougliis Dec. 

Kolselh, Caniaia H Apr. 

Kopfies, William A Juni' 

Kricger. .John J . Jnne 

Kuo. I'hihiiiiig . Aug. 

UilwiUli. R^esh . - June 

Ltirsoii, Diiuglas A ....... Apr. 



Leckel, Edgar Feb. 

Lewis, John M .^ufi. 

Long. Daiid W. Ocl 

Lusxcz. Joseph M Aug. 

MacUKHj.JeanM June 

Maildcn, (.'hrlsloplxTJ Feb, 

.Magnuson. Chnalophpr.l Ocl. 

Maier. Daiuel G. .Aug, 

Maier, Flank Feb. 

Maisonbacher. Benld Feb- 

Mason. Hoy L Dee, 

McC}uaU'. Darid J- Feb, 

McFigue, Michael T. Oct, 

MifsTici, Steven J, Feb. 

.Motilijo, .Allen (Xt. 

Mf>rgan, Ke\1n D AujJ, 

MiiUer, Enmierich Feb, 

Neuder, David L Apr, 

Nevvtoc, Steven A. Feb, 

Noless, MiirkH June 

.Miikiyaina, .Akira ., Ocl, 

NygHard, Richard .A„ Jr. .Apr 

O'Brien, Deiuiis P. - Dec, 

O'Koiiski. Uniolhy C Aug, 

( Isanie, Tbshiki ., .Aug. 

O.senlowski, "nniothy D Aug, 

Olsuru. Ynshisuke Aug, 

Percgrinn, Ricardo de Mello Apr, 

Pfaff, Andrea-s Ajjr 

PieBch, Jann's . . . , . . . , Dec 

r-less, Willried Feb, 

Holt., Michael - Feb. 

PratI, Ronald E. Apr. 

RaEiiel, Hince A lunv 

Rasper. John T. Ajjr, 

Ramsuii, Rolliii F. Feb, 

Reicherl. Wnirgang Feb, 

Hoark, .losi-ph C June 

RohbiiK, Virginia M. - . Aug. 

Rom. George June 

Hovell-Rixx. D. Kris - . June 

Riick, flemens , Feli, 

Rush. Kenneth Oct, 



Salomaa, Kan K. Feb. 

Scharrer, John A. ., OcL 

Si-hinzel. Peter 

Si hiacer. Rodney T. OCL 

Schleifer, .Arthur Jone 

Schmidt. Siegmar 

Schroaih. Leonard T. .... ■AtDC 

Schweikardt. Horet Fteh. 

Seeger, Harald fW». 

ShackiefonL J. Bany Aug. 

Sharpc Edward J Aug. 

Slion, Brian R .^r. 

Shuben, Karl Feb. 

Sloan, Susan Feb. 

Smith, Kevin G. Apr. 

Siirin. Wayne V. Feb. 

Steini>r, Hoif Fd). 

Steinle. Michael J. Aug. 

Sterner, John R, Oct. 

Slimple, .laniea R. Dec. 

Tanaka. Mni.oo Au& 

Thrantino, Joe Dec. 

Taylor, Gregoiy A. Apr. 

'n-utna, Williani R., Jr. ... Feb. 

Ibmer, .lames J. — Aug. 

l\itlie, MyrunR , June 

lihling, Thomas F Oct. 

WagtAer, Douglas li. . , Det 

Wagner. HansJiirgen Apr. 

Wiiiie, .liiini's W. Apr. 

Ward. Michai-I (' Aug. 

Warner, Sandra J .Iniie 

West, Jose[)h N Dec. 

Whi]iple, David P. Dec. 

Wiili'TTLiin. Donald A. Oct. 

Wildnauer, Kenneth R Dec. 

Wilson, Kenneth M June 

Yanagiiiiiilo. Voshiyuki Ort. 

Yu, Jiann Gmi Aug, 

Zellers, James R Apr. 

Zorabedian. Paul Feb. 

Ziirlingo, Ik'niaiiiin R. Dec. 



© Copr. 1949-1998 Hewlett-Packard Co. 



Deep iiib?r MIS Hcwicti-Packord ,I(>iiriial 115 



Authors 

December 1 933 




Vector Signal Analysers 



Kenneih J. Blue 

Ar RSD firmware angmeer 
a I ilie Lake Steuens Instru- 
menl Division, Ken Blue 
toinedHPrnigm He was 
an HP 3000 tumpulei pru- 
grammer for Two years tiE- 
fore mouing on to firmware 
desLgn He has coriribjied 
tn file riEi/eiopmert of the 
HP 3566DA dynamic signal analyzer and ihe HP 
358BA/B3A spectFumAielworli analyzers, an [i was 
the lead firmware engmger fnr Hie HP 894xxA vector 
signal analyzer series He developed tfie scalar mea- 
Eurement mode and hardviiate drivers used to control 
tlie HP B9440A RF seelian Ho also mtegrated the 
IBASIC pregrammmg language Ha is currenllv deuel- 
oping firmwatE for follaw-on products for the HP 
8941 DA and HP 89440 A analyzers and is working on 
Iow-IbubI digital signal processing assembly lan- 
guage development. Ken was born m San Antonio, 
Texas and graduated wilh a BSEE degree Irem the 
University of Washington, Seatlle, in 1986 He's mar- 
ried and likes boardsailmg. moLiniam biking, bungee 
jumping, running in the rain, and eating sushi 

Robert T. Cutler 

Bom in Lubbock, Texas, Boh 
Coder served for lour years 
in the US Alt Force as a 
sergeant before attending 
the University of Washington 
He recBiverJ a BSEE degree 
in 1984 and later completed 
work fnr an MSEE degree 
(1990) With HP since 1985, 
he is an R8D engineer at the Lake Stevens Insitumeni 
Division His past projects include the HP3583A and 
HP 356G0A dynamic signal analyzers He was respon- 
sible for the calibrations, coireciions, and resample 
algorithms for the HP B94xxA vector signal analyzer 
series, and is nr}w working on digital demodulation 
algorithms for the HP B9440A analyzer He's a member 
of Cie IEEE. Bob is matTied and has two sons. An 





amateur radio operator (call sign KE7ZJ], he also 
enjoys restoring classic v^neden bnais and cruising 
on Pjgei Sound witit his family. 

Dennis P. O'Brien 

Author's biography appears elsewhete in this section 
Douglas B. Wagner 

A Washington native, Doug 
Wagner was bom in Bellevue 
and attended Washingion 
State University, from which 
he received a BSEE degree 
in 198B Heconunued his 
[. ■ - studies at the University of 

M|^'A^i jjifj jj Illinois at Urbana -Champaign 
HBJHMMUB andcompletedwotkfnrhis 
MSEE degree in 1988. He held student intern positions 
at HP beginning in 1985 and joined the company full 
lime In 13B8 He's now an RSD engineer at the Late 
Stevens Instrument Division, where he specializes in 
software devalopment He was responsible for AM, 
PM, and FM demodulation software lor the HP B94«xA 
signal analyzers and is now developing algwithms for 
the digital demodulatian opiion for that series. Earlier, 
he worked on software for the HP 3&E3A and HP 
356B5A dynamic signal analysers He is named as the 
inventor in a patent on mixed-domain, mtxed-ratin 
frequency -response sampling and is a coauthot of a 
paper on digital lilLaring Doug is active in his church 
and likes travel, camping, volleyball, cross-cauntry 
skhng and photography 

Benjamin R. Zariingo 

Pradua marketing anginear 
Ben Zariingo I Dined HP's 
Love land Insifument Division 
in 1980 and later transferred 
lo the Lake Stevens Instru- 
ment Division. He has 
woffced on product defini- 
lion, applications, and sup- 
port lor over a dozen difler- 
eni HP synthesizers, network and spectrum analyzers, 
and vector signal analyzers His contributions lor the 
HP 894x!jA signal analyzers included applications 




research, producL definition, and technical training, 
and lie continues to wort: on applications for (he HP 
e9410A/44OA analyzers and future products He is the 
author of several product lech no logy articles published 
in trade press publicairons Ben was born in Glenwood 
Springs, Colorado and graduated Itom Colorado State 
University with a BSEE degree in 1380. Belore coming 
to HP he was a radio station announr^er and public 
service directoi He's married and says saa kayaking 
is his primary outside inieresi, 

17 Firmware Architectjre 



Dennis P. O'Brien 

A design engin Her at the 
Lake Stevens Instrument 
Division, Dennis O'Brien was 
responsible fnr ihe measure- 
mem architecture and firm- 
ware design of the HP 
a94s*A vector signal analyz- 
ers Alter he joined HP in 
1980, he was a production 
engineer lor voltmeters and scanners ai the Loveland 
Instrument Division At Lake Stevens, he designed 
linnware for nonvolatile memory management far the 
HP 35GIA signal analyzer and portions nl the measure- 
ment architecture for the HP 3565S measurement 
system He's now developing measurernenl firmware 
lor HP 894xkA analyzer options Dennis is a nahve of 
Denver, Colorado and a graduate of DeVry Institute of 
Technology IBS degree in electronic engineering tech- 
nology, 1973) and Aniona Stale University [MSEE 
197B1. Before coming to HP he worked at Sperry 
Flight Systems, where he designed ponions of one of 
the first cumnierciatly available digital flight -guidance 
control systems. He's married, has three sons, and 
coaches little league baseball He hkes woodworking 
and IS currendy linishing work on a hnuse he built 
He also en|Dys camping and canoeing with Ins family 




116 December IBaSHevttelt-PBcliBrdJouraBl 

© Copr. 1949-1998 Hewlett-Packard Co. 




31 Baseband Vector Signal Analyzer 



Manfred Bam 

With HP since iG"-" 
ManlfeU BaiTz . . ■ 
suoixji' engineering marage" 
al !)iB Lake Sletfin5 instm 
mem Diuiwon Heliasccn- 
[ributed ID the (ktefaam&il 
□t Ihe HP 3336B syrttHKueiS, 
■hp HP lymjiesizei, 
■ HP3577AneimorkaFia- 
l«ei, andiheHF ■..--■'"'"'"eswi't' a'alwer 
He was one ot Itie lead R&D engineers foi Ihe HP 
fflilOA irecjor signal analyzer He's cuiteiulv tespon- 
sible far service engmeenrg and qualiiy manegemenl, 
as well 3S for Ihe environmental test labniatDry and 
the calibration departmeni at Lake Stevens He is 
named as an immrtor in two patents and two fsndiiig 
patents on local osciliaiDr feedthiough nulling, diiliet 
error correction, a large-scale ditf)Bied convener, afHl 
a spread -spectrum linearization technique He's also 
the author ot three lectinical articles nn distortion, 
dittierad conveners, and future trends in analyzers. 
Manfred was horn in Cleueland, Ohio and receivetl a 
BS£E ilegree from the University ol Colnrado at flooi- 
der m 1979, an MS£E degree fromttie University of 
CalifofTiaai Berkeley in 1980, and an MS degree in 
compuier science frem the Umversilv of Washington, 
Seattle, in 19BS He is married and has a yoong 
daughter Outside work, he enioys skting, sailing, 
scuba diving, lly- fishing, lending his home oichard, 
and playing with his daughter 

Keith A, Bayem 

Keilh Bayern was fmrn in 
Princeton, New Jersey and 
graduated Irnin Montana 
State Uniuersity wiih a BSEE 
degrfieinl9B1 Hedesigned 
and prDgiammed industrial 
controls lor several compa- 
nies beturB|Diiung the 
flao laboratory at HP's Lake 
Stevens lostfument Division in 19S4 He hascontiib- 
uled ID hardware and software development for sev- 
eral products, including the HP 35660A dynamic signal 
analyzer and the HP 35636H progiammer's tool kit He 
designed the hardware lor several boards loi the HP 
89410A vector signal analyaer and is currently woriong 
on Eoftyyare foi a future VXI-besed product He has 
also published one leclmical atiicle Keith is married 
and has two children He's finishing a four-year pto|ect. 
a complete remodeling of his home, with the "help" 
of his two- year old son 

Joseph R. Diederichs 

flSD engineer Joe Diedetichs 
loined the Lake Stevens 
Instrument Division in ISB? 
and was a production engi- 
neer during his lust eight 
years at HP He was tespon- 
sitile for maintainmg the 
i design mlegnty of a variety 

' of RF networli and spectrum 

analyzo's, Rl^ sources, and dynamic signal analyzers 
He was also involved in a number of board redesigns 






and in test system itesign In the R&D fabofatDiy. he 
desigried the analog input circuits for the HP 89^1 OA 
vector signal analyjer He is (Kesemly WKliing on RF 
design lor 3 local oscillator for a VXI ttassil system 
Joe '^as Dom in Seattle. Washington and cwnpleied 
iMjrk lot his SSR degree l(om ttie Univcfsity o' 
Wathrnglon in 1^ He lists watetstiing, naJking, 
and raOio-cnntiDlied model helicopfeis as outside 
inieresis 

David F. Kelley 

A Uraversity of l!)olorado 
graduate (BSEE !9ff7l, R&D 
engineet Dave Kslley came 
to HP's Lake Stevens Insuu- 
ment Division the same year 
He has contributed to ihe 
development ot the IBflSlC 
instrument progfammiog 
language and the HP 
ElOIA, an eight-channel sigoal conditioning VXl 
caid For the HP SMIDA signal analyzer, he designed 
and developed or v/as responsible for the display 
senion. the CRT, the digital lillet board, the digital 
sDutcB boanJ, and the buffer/switch assembly He's 
cunenilv working full time toward an fvlSEE degree 
from the University of Washington under HP's resi- 
dent fellowship progrdm Dave was horn in Hartford, 
Connecticut and is mamed He en|uys backpacking 
and snow skimg. but his favorite sport is waierskimg. 
Ha and his wife get up eaily evety day from March 
through October to watet-slii. 

33 RFVector Signal Analyzer 
Robert!. Culler 

Auibot's biography appears elsewhere in this section 
William J. Ginder 

Bill Ginder started at HP m 
1979 at the Love I and Instru- 
ment Division and IS now an 
B&O engineer all lie Lake 
Stevens Insttuitient Division 
A native ol Kansas City, 
Missouri, he graduated from 
the Univeisiiy of Missouri at 
Holla with a BSEE degree 
the same year he |Oined HP An RF hardware special- 
ist, he has worked on Ihe HP 3D47A phase noise test 
system, the HP 35677 A/H lest set, the HP two- 
channel synthesizes, and the HP 3588A spectrum/ 
network aiialyzBi He developed Bf hardware for the 
IIP H944QA signal analyze! Bill and his wife have two 
sons and he likes to read, garden, cook, and bicycle. 

Timothy L Hillstrom 

RSiD engineer Tim Hillstrom 
designed and developed the 
local oscillator for the HP 
!i9440A signal analyzer Pre- 
viously, he worked on the 
RF/anslog system and circuii 
design for Ihe HP SSBflA 
sped rum/net work analyzer 
He also sper:ializes in solving 
problems involving electromagnetic compatrbrlily and 






lac.r, Tr-::QLiency imeiterenee for many ofdiEts al the 
laVe Stevens Insltomerrt Division He was bom in 
Hollywood, California anrJ anended the Univetsity ol 
PoOiand IBSEE 19821 atd the University of Wasmngion 
fMS£E!9B8l He nss been with HP since 1382 He is 
the author o( two patents and one pending paieni on 
jpecitum anaty;ei measuremem features and cuturis 
and has oublished several articles m a number of 
countries on osiillalDn. phase noise, ant) s-oaramewt 
cfwracietifatjon lim is mamed, has ttiiee children, 
and says he en]oys tennis, 'headbanger' music and 
esimg pancakes with his childieji 

Kevin L Johnson 

A msnu factoring develop- 
ment engineer at the Lake 
Stevens Insirument Division. 
Kevin Johnson lOined HP in 
19B4 He was born in Min- 
neapolis, Minnesota and 
completed work lor his BS 
degree in physics from Soise 
State University in 19SZaofl 
for his MSEE degree from Colorado Stale University 
in 1984 He developed the test system and automated 
test software for the HP SSMOA signal analyzer He 
has been a production engineer for the HP 3325A/B 
synthesizBi /function generators, the HP 3335A syn- 
Ihfisizei/level generator, the HP 3585 A/B spectium 
analyjers, and the HP 3577 A/B network analyzers. He 
IS the author of sevetal HP product notes and maga- 
zine articles on phase- locked loop measuremeni, 
noise measurement, and power measuremeoi Kevin 
IS married, has a son and Iwo daughlers, and is active 
in his church Outside of work, he spends most of bis 
time mowing the 2 5 acres of grass at his home, 
lending to his blackbeiry bushes, taking caie of his 
animals, and spending time with his children He also 
enioys hiking and mountain bicycling 

Roy L Mason 

m^^PiH With HP since 1964. Roy 

^^^^^r^^ Mason currently works at 
( I *• Ihe Disk Memory Division, 

* f where lie is responsiblE for 

;iiB design ol hard disk drive 
^^[^^^ products While at Ihe Lake 
^H^^^V Stevens Division, he was a 
^^^^^|r member of Ihe hardware 

design team lor Iha HP 
3 508 A spectrum/network analyzer and worked on the 
source section and several high-lreguency filter de- 
signs for the HP 89440A veciot signal analyzer He is 
named as an inventor in a pending patent nn RF pack- 
aging and interconnaci and is a coauthor of an HP 
Journal article on the HP 35B8A analyzer. A Lincoln, 
Nebraska native, Roy attended ihe University of 
f^ebraska ai Lincoln and leceived a BSEE degree In 
1984 He was a sergeant in the U,S Alt Fnme for four 
years and a siaff sergeant in the Nebraska Air 
National Guaid for another four years He and his 
wife have two children. His hobbies include hiking 
and woodworking 

James K, Pi else h 

Jim Pietsch is an RF hardware development engineer 
at HP's Lake Sievens Instrument Division. 



© Copr. 1949-1998 Hewlett-Packard Co, 



Decetnber lOEia HpwIeii-P-dt'kiU'il .Inuniiil 117 



60 Optica) Spectrum Analyzers 



David A. Bailey 

a^^" 'H Dave iSailey was born in 
AB^^^PtMH Fitchburg, Massachusetts 
^Otg V|] 'Bi^Biued a BSEE degme 

HV ^ 4|11 trnm Wnicesier PclvtenhniG 
r\ li'Eiiiuiein 1971 Hecontin- 

L '"^ studies ai ibe Uni- 

P '^^^L^ 'jflrsiiv of Caiifomia ai Sania 
Barbara, recBMrg a/i MS 
' Jegree m comiJiJler engi- 
■noiir.g III ''(ri:; Huiofl: lammg KP ttie sams year, lie 
worked at Rayiheon's ElBElrrjmagrebc Sysiems Divi- 
sion. Dave IS a snfiwara design engineer at Ihe Light- 
wave OpHiaiior of the Microwave Teclinologv Divi- 
sion and has coniributed to the dsvelapmHni pf the 
HP B5685A RF presaleclor and the HP 71400 lightwave 
Signal analyzer He also created the first itisuumeni 
shell soflmari: iihrary far HP and developed software 
for the HP 7H50A/51A optical spectrum analyzers. He 
Is a member of the IEEE and is a founding metnher of 
an Bdut:aiionai tree plantrng organization He and his 
wife have one son His hohhies Include Windsurfing, 
white wa'.er rafting, end Roilerblading. 

James R. Slimple 

Jim StimplE has been vfith 
HP since 1976 and is an 
HSD project manager ai the 
Lightwave Opera lion of the 
Microwave Technology Divi- 
sion He began his HP career 
as a materials engineer, then 
moved into R&D He has 
Conirihuied io [he develop- 
ment of the HP 8B62 and 859x spec t mm analyzers 
and was the project manager for Uie HP 71450A/51 A 
optical spectrum analyzers. Before joining HR he 
worked at Motorola's Communications Division He is 
named as an inventor m three patents and one pend- 
ing [)alent related to scanning antenna systems and 
optical spectrum analviers and has published an ar- 
ticle on optical spectrum analysers Jim was bom in 
New Castle, Pennsylvania and completed work for a 
BSEE degree from Northwestern University in 1 974 
He and his wife have three children, and his whole 
family is actively involved in breeding and Iraining 
dogs for the disat)led His other outside interests m- 
elude skiing, waiersfciing, tennis, running, bicycling, 
and playing guitar and saraphone. 

68 Double-Pass Monochromator 



Kenneth R. Wildnauer 

Kenn Wildnauei is an RSD 
engineer at the Lightwave 
Operation of the Microwave 
Technology Division and has 
been with HP since 19G0 He 
, , . — _ was born in Chatham, New 
^ Jersey and attended Cornell 
Hp4> ■ I Univetsily horn which he 

received a BS degree in ap- 
plied and engineering physics in 1979 and an MSEE 
degree in lOT Before working at HR he was em- 
ployed at AVCO Everett Research Laboratories and at 
Bell Telephone taboraiones His past HP projects 






inclutle IF and video circuit design for ihfi HP BM 
series of spectrum anatyzars He was responsihlH lot 
Ihe optical design of the HP71450A,/51A{iniical 
specTrum analyzers and his work has lasulled irt two 
patents related to those products. Kenri enioys teach- 
ing physics courses at a local university in addition to 
working at HR He is also a volunteet board inemher 
for a local recycling company His other oulsiije inter- 
ests are bicycling, backpacking, hiking. Windsurfing, 
tennis, saftbafi, and skimg 

Zultan Azar^ 

Software development engi- 
neer Zoltan Azaiy worked on 
Ihe digital hardware and 
firmware design for the data 
acquisition subsystem and 
the fimware design for the 
host processor for the HP 
7145DA/51A optical spec- 
trum analyzers He joined 
HP's Signal Analysis Division in 1981 and has been a 
reliabilitv engineer for modular spectrum analyzers, 
tias worked on software quality assurance, and has 
done software testing tor the HP 7DD0D series of 
spectrum analyiers. He was a firmware engineer for 
thBHP7DB10A optical receiver and a hardware/ 
hrmwaie engineer for the HP 71450A optical spec- 
trum analyzer He's now at the Lightwave Operation 
of the Microwave Technology Division, He is named 
as an inventor in a patent related m the sensitivity 
user interface for the HP 714S0A/51fl specitutn ana- 
lyrers toltan was born in Ano Arhor. Michigan and 
received a BS degree in electrical and computer engi- 
neering from the University of California at Santa 
Barbara in 1981 His outside inleresis include 
trapshooting and hunhng 

7S Diffraction Grating Rotation 
System 



Joseph N. West 

ACalifomia native, Jne 
West was horn in Berkeley 
and attended San Jose 
State College, from which 
he received a BSEE degree 
m 1970 fie worked no an 
early all-digital telephone 
switching system at a small 
company before joming HP 
in 137S An H&D enginser al the old Signal Analysis 
Division, he contfifiuied to the deveioptnent of Ihe HP 
7D0Q0 modular measurement system and later 
worked on vanous performance enhancements to Ihe 
HP 71 ODD spectrum analyzer family Presently he is 
an H&D engineer at the Lightwave Operation ol the 
Microwave Technology Division During the develop- 
ment of the HP71450A/5IA optical spectrum analyz- 
ers, he was responsible for several portions of the 
hardware, including the servo control loop and posi- 
tion sensors for the drff recti on giating positioner He 
also managed work on electromagnetic tompatihiliiy 
and toot the data acquisition board and the various 
servo control boards into production. He is named as 
a coinventor m a patent related to the HP 714BI1A opti- 
cal spectrum analyier He is a coauthor of a technical 
article and author of an electrical design guide pub- 
lished by HP Joe and his wife have four children and 





own a farm with several domesiic animals and a lot 
of wildhfe His hohbiHs include old steam locomotives, 
fly-fishing, camping, and reading, 

J. Douglas Knight 

Aiitnir'r. iiirjgraphy appears elsewhere in this section. 
80 Optical Fiber Alignment 
J. Douglas Knight 

F15D engineer Doug Knight 
began his HP career in igsz 
at Ihe Signal Analysis Divi- 
sion as a fabrication engi- 
neer and later, as a micro- 
electronics production 
engineer where he rede- 
signed HP's programmable 
step attenuators for lower 
cost and inipmvod reliability He was responsible lor 
the mechanical design of the monochromator used in 
the HP 7I4S0A/51A optical spectrum analyzers He's 
now at the Lightvrave Operation of the Microwave 
Technology Division. He is coinventor of a patent on 
douhle-pass monochromaint design Duug was born 
in Bakersbeld, California and received a BSME degree 
from the University of California at Davis m 19B1 His 
MS degree in manufacturing systems engineering was 
awarded by the University of Wisconsin at (vladison 
in ISSB He's married, has two i:hildten, and enjoys 
swimming, sailing, and family activities, 

Joseph N. West 

Author's biography appears else where in this section. 
85 Standard Data Format 



Michael L Hall 

Design engineer Mike Hall 
was born in Maiden, Mis- 
souri and received a BSEE 
degree from the University 
of California at Santa Bar- 
bara in 1979. He joined HP's 
Santa Clara Division the 
same year and later trans- 
ferred to the Lake Stevens 
Instalment Division. He has contributed to the develop' 
ment of software and firmware for several HP instru- 
ments, including the HP 3569A Option 550 frequency 
analyzer and the HP 356e5A, HP a5e6DA, HP 3563A, 
and HP 356ZA dynamic signal analysers He devel- 
oped utilities lor the Standard Data Formal and firm- 
ware for data acquisition and time capture for the HP 
E94iiA VBClor signal analyzers He's currently work- 
ing on enhancements for the HP B94i;xA and on MS 
Windows applications. He's listed as the inventor in a 
patent on dynamic linking of subprograms to main 
programs and is the coauthor of an HP Journal article 
on [he HP 35BZA dynamic signal analyzer Mike is 
married, fias two daughters, and is a tutor In BASIC 
computer programming at a local school A comic 
book colleclor, he has more than lO.OOD books His 
mam outdoor interest is landscaping his three-year- 
old house. 




1 18 Det rniber 1983 Hewleit-Patkard Journal 

© Copr. 1949-1998 Hewlett-Packard Co. 



90 North American Cellular CDMA 



98 DECT Measurements 




□avid P Whipple 

Dave WhiDple isanH&D 
Cro;ec: (nanage/ *oi riSlBtn 
ati^iiSECTure snfl is *tirknlg 
or CDMA leil equiomem 
aiifl siandaiflj With HP 
. srnce 1373. he haswciked 

^ %kr 3' the Sianfa'd Park D^isian 

and the Spokane Owismn 
'* Iniirall* he was a produinion 
engineer and ilwr ofoduction engineer ihg managei 
ioi srgnal generaiois Later ne was an H&D piojec! 
manage' lor the HP 3BS6B/57A signal geneiatDts. ihe 
HP H9ZaA communications test sat. the HPSSTlx 
GSM test 9B15. and Ihe HP H953DT TDMA tesi sys- 
tem He IS hsted as an mventoi in itiree patents, all 
dealing wrlh FM in phase-locked loops Dave was 
tominWilmingtan, Delaware and attended Purflue 
Uniuersilv, 'ram wtiitii he received a BS££ degree m 
1372andanWSEE degree in 1973 He is married, 
tias iwo children, and enjoys sknng, waters kiing, 
camping, and mountain billing 



MarhA-Elo 

-.enf engineer al 
^■^^^ .;sTy Mioowaue 

^g^^^L Division. Mail Ba Dom 
^^L^^^^^ in Preston, Lancashire. 
JK^^^^^ England He giaduated from 

''^^^'^ Salfotd University in 1992 
1^^^^^^ tviih 3 hachelai's degree in 
^^y^^^^^l engineering {electronics) arx] 

hasbeenwithHPstnce 
1990. when he held a siudeni Basiiinn Previously, 
was empldyed ai LC. Aotomalian, where ne worked 
on inrtusinal mfrared security systems He developed 
the HP B5723A OECT measurement personality and 
has participated m the DECT Radio Equipmert Speci- 
Rcaiton type approval working group. He is currently 
working on dowrlaa(toble progiams for niche martets 
lor Ihe HP 85K) E-Seiies SDectrum analyzers. Mark's 
interests ouiside work intJude music and sociaiizing 



tlecembec IWH Hewlelt-Parkotd Jounr^U 1 1 9 



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To- LEUIS. KAREN ^^'^^ 
' ■ -r ' - HEADQUARTERS 

' a'o&R 

IDR S98363 



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December 1993 Volume 44 • \iimber S 

Technical Information liam the Laboralaries of 
Hewlatt-Packaid Cgmpany 

HBwIen-Packard Cornpain, PO. io»5IS27 
fe\n Allc, CslilDnig, iiXl-tSlU U.S A 

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© Copr. 1949-1998 Hewlett-Packard Co.