Skip to main content

Full text of "Folkscanomy Electronics Articles: ATV Handbook"

See other formats


Amateur 
Television 
Handbook 



By John L. Wood, G3YQC and Trevor Brown, G8CJS 



Acknowledgements 



The British Amateur Television Club expresses its gratitude to the following companies, societies 
and individuals who have provided material and assistance for this publication. 



Blean Video Systems, Canterbury, Kent 
Plessey Semiconductors Ltd., Swindon, Wiltshire 
Radio Ref. Paris 
Radio Society of Great Britain, Doughty Street, London 
Wasco Electronics, Queens Street, Leicester 




C. Browabridge 
M.Chamley 
M. Crampton 
C. G. Dixon 
A.Emmerson 
J. Goode 
A.F.Wood 



F3YX 
G8DLX 
G8CGK 
G8PTH 

G3RDC 



D.EJones GW8PBX 

J.Lawrence GW3JGA 

T. Mitchell G3LMX 

R. S. Roberts G6NR 

R.T.Russell G4BAU 

N. Walker G8AYC 

British Amateur Television Club, 1981 



This special A4 sized edition edited by Ian Pawson, August 1998 

Editors note: This book was originally printed A5 size. This version has the same content, but has 
been rearranged to A4 size. The quality of some of the diagrams and pictures is not up to our usual 
standard as they have been scanned in from an original paper copy. 



Amateur Television Handbook 



Page 1 



Contents 



Contents 



Acknowledgements 1 

Contents 2 

Preface 3 

Principles 3 

Scanning 3 

Television Standards 4 

The Modulating Waveform 4 

Bandwidth and Channel Space 5 

The Station 6 

Aerials 7 

Feeder Systems 7 

Receivers 9 

The ELC1043 Series Tuners 9 

A High Performance Wideband Tuner 10 

An Amateur Television Receiver 12 

Transmission 16 

Video Modulators 16 

A Modular Linear Amplifier 16 

Vision Sources 20 

An Electronic Character Generator 20 



Page 2 



Amateur Television Handbook 



Preface 



Preface 



Amateur television is a large and complicated subject 
and to do justice to its many facets a book of several 
volumes would be required. 

Amateur television handbooks in the past have tried to 
cover as many subjects as possible and consequently, 
due to size limitations, each subject has been treated in 
rather less detail that one could have wished. The 
original conception of this handbook was to deal in 
greater depth with the more complex and in some cases 
less-well publicised techniques used in the modern 
amateur television station. You will therefore find less 
information on basic principles, aerials, operating 
techniques, licensing requirements and even 
transmitters, all of which are adequately covered in 
books and frequently found in magazines. Instead, 
emphasis has been placed on subjects such as modern 
receiving systems, electronic video sources, vision- 
processing techniques and of course colour television. 
These subjects are particularly susceptible to changes in 
modern techniques and innovations and therefore the 
designs need to be periodically up-dated. 

The newcomer to ATV has not been forgotten however 
and a chapter explaining such things as the composition 
of the modern TV signal and the organisation of an 



Principles 



amateur station has been included. There is also 
guidance on aerials, feeders, simple receiving 
equipment and colour television principles and it is 
hoped that this information will adequately augment the 
large amount of already published data in other books 
and periodicals. 

Almost all of the projects in this volume have never 
before been published and indeed some were designed 
especially for this book. Printed circuit boards will be 
made available to the constructor in order that the more 
complex circuitry may be successfully built by less 
experienced constructors. The video projects are all 
compatible with each other and the PC boards have 
been made to a standard size and use standard edge 
connectors that enable them to be installed in a 
card4rame cabinet system if required. This ensures 
complete flexibility and permits the use of only those 
units that are required. 

The British Amateur Television club is pleased to 
present this book in the hope that it will encourage and 
stimulate television amateurs throughout the world to 
strive for technical improvement and will help 
newcomers to enjoy this fascinating hobby. 



There are several methods of picture transmission; high 
and low definition television, slow scan television and 
facsimile (FAX) for still pictures, pictures built up using 
radio teletype, and so on. This book is mainly concerned 
with high definition television and sets out to describe 
the equipment necessary to build a modern amateur 
television station. 

The broad principles involved in television transmission 
are well known, and this brief review is intended to 
highlight many of the important features of a modern 
system which are dealt with in full detail in the 
following chapters. 

All forms of picture transmission 
and reception differ from normal 
'seeing' with a human eye in one 
important respect, the human eye 
uses about 150 million 
simultaneous channels of visual 
communication, but an electronic 
system uses only one channel at 
any instant in time, consequently a 
process termed 'scanning' has to 
be used whereby the visual 
information to be transmitted and 
received is explored bit by bit and 
translated into electrical terms for 
modulation of a transmitter. The 
received signal is demodulated and 
used to build up a reconstituted 
picture on the screen of a cathode- 



ray tube. 

Scanning 

To simplify the explanation we will consider a picture 
made up of only eight lines and displaying a black 
square in the centre of the screen. 

Scanning requires, firstly, that the picture to be 
transmitted is framed in a field of view having an 
'aspect ratio'. The standard aspect ratio for television is 
4x3 units, as shown in Fig 1(a). It is seen that the 
actual picture size is of no importance so long as the 
aspect ratio is correct. 

aerial 



television 


— * — 












modulator 


— »— 


transmitter 




camera 


i 
i 




— »-] 




i 








( 




t 








c 


other video 


i 
i 
i 

. j 


television 




70cm 




sources 




receiver 


■ < 


tuner 


— <- 



Fig 4. 



A simple amateur television station. 



Amateur Television Handbook 



Page 3 



Principles 



Fig 1(a) shows a scanning spot that traverses the field 
line by line, (similar to the manner in which we read a 
book page), translating the variations of light and shade 
(and possibly colour) into voltage variations which are 
used to amplitude-modulate the transmitter. The camera, 
with its optics and electronics carries out this operation. 
At the receiver, a CRT beam is swept across the face of 
the tube in synchronism with the camera scan, and the 
demodulated signal is used to modulate the beam 
current, thus writing a reproduction of the picture 
scanned at the transmitter. 




Fig. 2(d) 

Fig 1(b) shows the voltage obtained by scanning (say) 
line four of the picture. Because electronic circuitry 
cannot respond instantly the changes from white to 
black and from black to white at the edges are not 
sharply defined. To improve 
resolution the spot is made 
smaller and the number of 
lines increased. 

Television, in dealing with 
moving pictures, requires a 
complete scan of the field to 
be so fast that, compared with 
any movement taking place in 
the scene, each complete scan 
is of a virtually still picture. 
Standard broadcast television 
in the UK scans 25 pictures 
per second. 



the field. There are many reasons why amateur 
television should follow existing broadcast standards, 
not least of which is the availability of receivers. There 
are two UK broadcast standards, the original 405 line 
'black and white' system A, and the later 625 line 
system I which includes colour. Both use an aspect ratio 
of 4 x 3, and both transmit 25 pictures per second. The 
highest modulation frequency generated during 
scanning in system A is about 3MHz, whilst 5 to 
5.5MHz can be generated in system I. 

Any TV system can include a sub-carrier with colour 
information, the normal 625 line system uses a colour 
sub-carrier frequency of 4.43MHz. A black and white 
system does not of course require a colour sub-carrier. 

The Modulating Waveform 

The scanning system output will be used to amplitude- 
modulate the transmitter, and it is necessary for the 
receiver tube beam to be in the same two-dimensional 
position as the scanning beam in the transmitter camera. 
As stated earlier, a single communication channel can 
only handle one bit of information at any instant of time 
but, in addition to the video information, it is necessary 
for the transmitter to send synchronising information to 
the receiver indicating the precise position of the 
scanning spot in both horizontal and vertical planes. 

W > 




Fig. 1 (a) 



volts 



black 



white 



All broadcast television 
systems use a technique 
called 'interlaced scanning', 
this means that the screen is 
scanned and every other line 
is displayed onto the screen, 
during the next scan the in- 
between lines are displayed 
thus completing the picture. 
Referring to the eight line 
picture in Fig 1(a), interlaced 
scanning would require that 
the complete field would be 
scanned by lines 1, 3, 5 and 7 
and then the gaps would be 
filled by re-scanning the field 
with lines 2,4, 6 and 8. 



Television Standards 

Picture quality is determined by the scanning spot size 
and, therefore, the number of lines required to fully scan 




Fig. 1 (b) 



Fig. 1 (c) 



Fig 2(a) shows the modulating waveform during a one- 
line scan. The video signal varies the transmitter output 
according to its amplitude. Time is taken from the 
complete video line scan by 'blanking' the video signal 



Page 4 



Amateur Television Handbook 



Principles 



for a fraction of the total line period. During the 
blanking period a line-synchronising pulse is inserted 
which takes the transmitter output from 30% to near 
zero. This pulse is processed in the receiver to 'tell' the 
sweep circuits when to start the line scan across the 
CRT face. When the line scan reaches the bottom of the 
field, a field blanking pulse blanks several lines and a 
train of broad pulses are inserted during the blanking 
interval (Fig 2(b)). The receiver processes this train of 
pulses to return the CRT beam to the top of the display 
tube to retrace its vertical sweep. 

Fig 2(a) shows what is termed 'positive' modulation as 
used in system A, in which peak white corresponds to 
maximum transmitter output. The 625-line system I uses 
the same principle but an inverted waveform ('negative' 
modulation) is used in which sync tips drive the 
transmitter to maximum output, and peak white is near 
zero. 



cycles) is inserted on the back porch, together with the 
timing associated with the line blanking pulse. 

Bandwidth and Channel Space 

Television is characterised by the need to handle very 
high video frequencies throughout the system from the 
camera to the receiver, and this includes the aerial 
system Amplitude modulation of the transmitter would 
produce the normal double sidebands which, for system 
A, would require a channel space of about 6MHz, and 
up to 1 1MHz for system I. Including a sound channel to 
either system would increase the channel width by about 
another 1MHz. 

It was realised very early in the history of broadcast 
television that the heavy demands for channel space 
would limit the number of available channels, and a new 
system for saving channel space was evolved and called 
'vestigial sideband' (VSB), or 'asymmetric sideband' 



An 



CARR16& 
AMPLITUDE 



PEAK | 0<> 7 

WHITE I . 



VISION 
SIGNAL 



W.ACK- '• lo'fc 
PULSES 



LINE 
6LANK1I*; 



FRONT 
PORCH ' 



• FIELD BLANKING 25H + l2-05yuS *■ 



If T 



nmrnjunnrfft^Pinn it 

i 1 : hr-»l 



• 2-5H • 



2-5W- 



LINE PERIOD H.— * 



lr yiniTTpLMdrTTifTyYin if 



if 



FIE 
BLANKINQ 



•FIELD BLANKING 25H + I2.05y-S- 



-SACK PORCH 



TIMING REF* 



1 LINE 



For a black and white system, a complete picture 
requires two cycles of video and synchronising 
information, as shown in Fig 2(b). Colour requires, in 
addition, further information in the form of a 'burst' of 
about ten cycles of sub-carrier on the back porch. This 
burst experiences a phase change on every line and, 
although interlacing is completed in two scans, the 
complete cycle of blanking, pulses and colour-burst 
phase requires four fields, as shown in Fig 2(c). Fig 2(d) 
shows how the burst of colour sub-carrier (about ten 



(ASB). Fig 3 shows the channel spectrum for one 
system I channel. VSB involves filtering off a large part 
of the lower sideband, leaving only about 1 MHz or so, 
and an overall channel width (with guard bands) of 8 
MHz. With suitable tuning of the receiver IF circuits 
distortion of the vision signal by the loss of part of a 
sideband can be reduced to negligible proportions. 

The bandwidth required for a single system I channel is 
8 MHz. The 2-meter band is thus quite unsuitable for 
television transmissions. The 430 MHz band can only 



Amateur Television Handbook 



Page 5 



Principles 



support one channel by the use of nearly the whole of 
the band. Higher carrier frequencies such as 10.5GHz 
are much easier to modulate than are low carrier 
frequencies and, of course, can provide the much- 
needed channel space. Another reason for using a band 
such as 10.5GHz is that risks of interfering with other 
users are minimised because the aerials used at both the 
transmitter and the receiver are highly directional and 
have a narrow beamwidth. 

The Station 

The modern amateur television station is neat, 
compact, usually solid-state and by no means 
complicated or difficult to construct and use. 
Gone are the days of 6ft racks of equipment, 
huge valved linear amplifiers and their 
associated lethal power supplies and large 
modulators. Gone also are the large valved 
excommercial cameras with their boxes of 
control equipment which took two strong men 
to lift. 

A modern 10-Watt vision transmitter can be 
housed in a small neat cabinet often smaller 
than a H.F. bands transceiver. The tuner can 
also be incorporated in the same cabinet or tucked away 
inside the station TV set. If a camera is used it will 
typically be an ex-commercial surveillance type such as 
those seen in supermarkets and department stores. 

The block diagram of a station is shown in Fig 4 and 
many amateurs use no more equipment than this. 

It is often possible to modify an existing FM or SSB 



Fig. 3 



transmitter for video service without jeopardising its 
intended mode of operation. Solid-state linear 
transverters are usually quite suitable for vision use with 
fairly simple modifications. Custom-built transmitters 
are not difficult to construct using modern techniques. 




freq 
MHz 



AAA 



-FIELD BLANKING INTERVAL = 25 LINE PERIODS + LINE BLANKINQ 



= 26 H + 12'05/uS 




i_ji_ju^ji_jirxiririj — u — it 

314- I 315 I 316 | 3n I 318 I 319 



t 4 



^ji_JLJiJLJiri]nrir-u--LJ — lt 

I 621 I 622 | 623 I 624 I 625 | 1 I Z I 3 I 4 I 5 I 6 I 



t 




i_n_Ji_Ji_n_nrir 

3I3 I 314 I 315 I 316 
I 



317 



LT 

318 | 3i 



621 



BURST 
8LANKINQ 



LJULJULJinnj^^ 



622 ] 623 | 624 | 625 | 1 | 2 | 3 | 4 | 5 | 



FIELD SYNC. 
DATUM. 



FIRST FIELD 



SECOND FIELD 



THIRD FIELD 



If 



- 4tk 
FieLO 



320 



32I 



U 
I 8 



1st 



u — 

9 I 320 



2nd 



i : t 
~u — 

I 7 | 8 
i 

I I 



3«L 



FOURTH FIELD 



F1G.2C 



Burst phase 



t+135° 
| -135° 



Page 6 



Amateur Television Handbook 



Aerials 



Aerials 



Aerials suitable for amateur television are similar to 
those used for other modes of amateur radio, although 
the actual selection is limited by some specialised 
requirements. The two most important considerations in 
ATV work are gain and bandwidth. For anything other 
than local operation an aerial gain of lOdB should be 
considered to be the minimum requirement. Since 
television requires a stronger received signal for 
adequate results care should be taken to obtain the 
maximum possible ERP from the aerial system over the 
whole of the 70cm band. Poor aerial design will degrade 
or lose colour sub-carrier and may cause you to miss 
foreign DX at the top end of the band. 

Polarisation, though technically non-critical, should be 
horizontal since this is the standard used throughout 
Western Europe. Height and location of the aerial are 
prime factors and should be chosen carefully to ensure 
the shortest possible feeder length. The aerial should 
clear any local obstructions-including trees which 
absorb RF-and should have as clear a take-off as 
possible. Height gain increases usefully up to 50ft or so, 
above this the increase in gain due to height is offset by 
increased feeder losses, so ultra high aerial systems are 
often not ideal unless special arrangements are made to 
reduce losses. 

In practice three types of aerials are in common use in 
the UK. The first is the eight-over-eight skeleton slot 
that has good forward gain (around 12dB) and adequate 
bandwidth for television work. It is small and light 
making it easy to handle and erect. 

The second is the eighteen element Parabeam which has 
been around for many years. This aerial, although not so 
popular today has a good performance in amateur 
television service. It exhibits high forward gala and a 
fairly good polar pattern together with sufficient 
bandwidth for modern colour transmissions. 

Lastly there are the 'X' element types of aerial 
commonly known as 'Multibeams' . 

The 48 element Multibeam achieves a gain of 15. 7dB 
and has a beamwidth of 26 degrees. It is 1.83 metres 
long. There is also an 88-element version that has a gala 
of 18.5dB. This is a very big aerial however, almost 4 
metres long, and the beamwidth is so narrow (19 
degrees) that it is easy to miss signals through incorrect 
beam alignment. The 48 element Multibeam is therefore 
considered by many as an ideal compromise and is one 
of the most popular ATV aerials presently in use. It has 
been noted by a number of amateurs however that the 
performance of Multibeams often falls off during wet 
weather, presumably due to the water on the element 
insulators shorting out the elements since these points 
are at high impedance. 

Feeder Systems 

Feeder cable should be 50ohms coaxial and should be 
low loss hard-line. Uniradio 67 is probably the 



minimum acceptable quality, anything flimsier or 
thinner will only be a disappointment. In particular 
beware of so called 'bargains' in 50ohm cable. 

When choosing and installing coaxial cable try to ensure 
that the braiding is densely woven to provide adequate 
screening and preferably should be made from tinned 
copper strands since bare copper will oxidise in time 
and prevent good electrical contact between the 
individual strands. The inner dielectric should 
preferably be solid rather than semi-airspaced to try to 
minimise the ingress of moisture over the years. 

When installing the cable avoid sharp bends and undue 
strain or pressure on any part of the feeder run and make 
sure that both ends of the feeder are properly sealed to 
stop moisture getting under the outer sheath and 
corroding the braiding. Finally, ensure that all 
connections to aerials, plugs and sockets are properly 
made, scrupulously clean and adequately protected 
against moisture. 

The subject of RF connectors in VHF and UHF amateur 
stations is one that is often neglected but many would be 
surprised at the losses incurred by using poor quality or 
incorrectly chosen connectors. 

Ideally an RF connector will be transparent to the 
signal, i.e. it should appear like a continuous piece of 
coaxial cable. 

There are many different types of connector available 
today and it is well worth establishing a standard 
throughout the station. Since large diameter coaxial 
cable is almost essential for effective 70cm work 'UHF' 
or 'N' types are to be preferred. 'UHF' (PL259, S0239) 
are adequate if correctly fitted but the 'N' type is 
undoubtedly superior and is used throughout the modern 
electronics industry. It is important that good quality 
plugs are obtained and fitted according to the 
manufacturers instructions, this is most important if the 
connector is to perform at its best. 

'N' type and 'UHF' connectors however are rather 
bulky and it is good practice to use smaller connectors 
such as 50 ohm BNC and thinner cable for general 
purpose video and IF connections inside the shack. Do 
not use long runs of this coax at 70cm otherwise 
unacceptable losses will result. 

Note that with 'N' type plugs and sockets 50 and 75- 
ohm types are NOT interchangeable because of 
different sized centre pins. If one decides to standardise 
with say 50 ohm 'N' type then it is strongly 
recommended that 75 ohm versions be completely 
excluded from the shack, junk box etc. to avoid costly 
mistakes. 

The well known 'Belling Lee' type of plug and socket 
used commonly on broadcast television sets should 
never be used in an amateur station, the losses and un- 
reliability caused by these connectors make them totally 
unsuitable. In case you ask why, broadcast television 



Amateur Television Handbook 



Page 7 



Aerials 

signals are almost always very strong and therefore insignificant, 
losses in poor quality cable and connectors are often 



Page 8 



Amateur Television Handbook 



Receivers 



Receivers 



The most popular amateur television system in current 
use consists of a standard black and white 625 line TV, 
(the modern portable set sold in the high street is a 
popular choice) together with an external commercial 
tuner modified for 70cm. 

aerial 



o 

20- 

30- 

O 

O 
«>- 



50- 
60- 



IF may be set to channel 1 or 2 in band 1 and fed to the 
VHF tuner (if the set is dual standard), in this case 
though the timebase will need to be changed to 625 
lines and the modulation sense should be correct for 
receiving negative modulation. The advantage of this 
system is that the gain of the VHF tuner is 
used as IF amplification giving perhaps a 
small advantage. 



+12v 



AGC 



+12v RF 
tune volts 



lk preset 
— jw- 



10k 



tune 
lklln 
— \AA^ — 



✓77 

+12v osc/mixer 
test point 
IF output 

Fig.l 



case 
Ov 



Fig. 1(a) 



RF amplifiers 



o 



o 



A favourite tuner used extensively by amateurs is the 
Mullard ELC1043 and ELC 1043/05. These are readily 
available and inexpensive, the later versions may not 
quite tune down as low as the 70cm band but by a 
simple modification can be made to do so. The tuners as 
purchased are quite sensitive and suitable for receiving 
local and semi- 
local amateur 
transmissions but 
the addition of a 
good pre-ainplifier aerial - 
improves the gain 
and sensitivity 
making the system 
suitable for longer 
distance working. 

The IF output may 
be left tuned to the 
standard IF 
frequency and fed 
straight to the IF 
input of a domestic 
625 line TV set, 
great care should 
be taken when 
using a mains 
operated set with a 
live chassis. If 
required a switch 
may be installed in 
the TV to change 
over from the aerial, 
internal to external 
tuner thus preser- 
ving the set for 
domestic use. 
Alternatively the 



A slightly different converter makes use 
of the fact that only one 625 line 
television channel can occupy the band 
between 434 and 440 MHz at a time. The 
converter is adjusted such that it can only 
receive a television channel in this 
segment, thus there is no need for a 
3 tunable converter since this is the only 

place where television on 70cm is 
permitted. 

This type of converter usually out- 
performs modified commercial tuners 
since it uses relatively high Q bandpass filters which 
considerably attenuates out of band signals and give a 
better overall noise performance. Low noise transistors 
are used in the RF and mixer stages and the mixer is 
usually more capable of handling strong interfering 
signals without causing intermodulation distortion. 



I 



o 



Fig. 2 



TOP VIEW 



twisted wires 
chip caps O O 



Fig. 2 (a) 



BOTTOM VIEW 



Some domestic 
TV receivers- 
particularly it 
seems the 
Japanese sets- will 
tune directly to 
70cm without 
modification, in 
this case all that is 
needed is the 
addition of a good 
low noise pre- 
amplifier to make 
a suitable amateur 
TV receiver. 

Tlie ELC1043 
Series Tuners 

Fig 1 shows an 
ELC 1043 type 
tuner and gives the 
layout and pin 
connections. A 
circuit for wiring 
up the tuner is 
given in Fig 1(a) 
the IK linear 
potentiometer is 
the main tuning 
control that should 
preferably be 



Amateur Television Handbook 



Page 9 



Receivers 



mounted onto a 10-1 ratio reduction drive. 

The later model ELC1043 and ELC 1043/05 may not 
tune low enough to cover the 70cm band and 
consequently will need slight modification to enable 
them to do so. Two modifications are described here 
and either or both may be used as required. 

The first and most effective is simply to lengthen the 
tuned lines in the mixer and oscillator compartments, 
this is done by unsoldering the main lines (those 
connected to the varactor tuning diodes) and pulling the 
line out of the PC board as far as it will go whilst still 
leaving enough line protruding through the print side to 
enable it to be re-soldered properly, this will effectively 
lengthen the line by up to an eigth of an inch overall, 
this should be sufficient to allow the whole of the 70cm 
band to be tuned. The RF amplifier lines may be 
lengthened in a similar manner and the tuner re-aligned. 

The second modification increases the capacitor values 
in the oscillator and mixer tuned circuits. 

Refer to Figs 2 and 2(a) and locate the ceramic chip 
capacitors at the ends of the oscillator and mixer tuned 
lines, these protrude through the print side of the board. 
Take two pieces of thin hookup wire about 1 inch long 
and solder one to each side of the oscillator and mixer 
chip capacitors. Set the tuning voltage on pin 3 to about 
0.3 volts and with the aid of a strong local 70cm signal 
twist together the oscillator wires a little at a time until 
the signal is tuned in. In a similar manner adjust the 
wires on the mixer for maximum signal. It is important 
to use as little extra capacitance as possible since too 
much may stop the oscillator. The remaining tuned 
circuits should then be re-aligned for maximum signal. 

A High Performance Wideband Tuner 

This converter is a high performance unit which 
receives amateur television transmissions on the 70cm 
band. It is fixed tuned and covers the range 434 to 440 
MHz and therefore needs no tuning control. The 
converter has a sensitivity and immunity to cross 
modulation which considerably exceeds that found in 
most commercial tuners, the performance owes much to 
the correct adjustment of the tuned circuits to reduce out 
of band signals. 



438. 5 MHz. Total power consumption is around 40niA 
at 14 volts. 

Circuit Description 

Two BFR91 low noise transistors provide RF 
amplification of the signal before it is applied to the 
40673 MOSFET mixer. The local oscillator is a self- 
oscillating push-pull circuit. 

The input signal is applied to a quarter wave line LI 
which matches the impedance of the aerial system to the 
first RF amplifier and provides input selectivity. The 
first transistor is used in a common-emitter 
configuration with the emitter lead soldered directly to 
the earth plane, because of this a compensation network 
is provided in the collector circuit to provide bias for the 
transistor in the absence of an emitter resistor. 

The signal is then applied to the second RF amplifier via 
a bandpass filter consisting of two over-coupled tuned 
circuits L2 and L3 and then to the mixer via L4. The IF 
output is fed to the transformer that has adjustable 
capacitive coupling. The secondary has an adjustable 
capacitive potential divider for correct matching to the 
IF unit and to control the damping on the output tuned 
circuit due to the output load. 

The push-pull oscillator circuit uses printed inductors 
and has been found sufficiently stable for TV reception. 
Oscillator injection is applied to gate 2 of the mixer via 
a 9pF trimmer capacitor. 

Construction 

The tuner is built on a double sided printed circuit 
board, most of the components are mounted on the 
circuit side, the underside is used as an earth plane and 
for the point-to-point wiring of the 4.7 ohm decoupling 
resistors in the transistor output circuits. 

The 47pF coupling capacitors must be low impedance 
disc ceramic or chip types, all decoupling capacitors 
should be good quality disc ceramics. The trimmer 
capacitors are not critical with the exception of that 
which tunes Li, this should be an air spaced type. The 
trimmers are placed flat on the PCB and the earth 
connection, where needed, is passed through the board 
and soldered onto the earth plane. The 4.7-ohm 
decoupling resistors should be mounted on small stand- 



This project is intended for 
those with access to proper test 
equipment but may also be 
successfully built by those less 
fortunate but who are prepared 
to spend time on the alignment. 

The overall gain is of the order 
of 30dB with a supply of 14 
volts. The measured noise 
factor of several specimen 
converters was measured at 
between 1.8 and 3dB. The 
bandwidth is nominally 6 MHz 
but can be adjusted as required, 
the passband ripple is better 





_ 

i 


TP 

i »i- 

x _ ' 


AAII6 
c J 

-Vi" ^ ~ >TEST 




AERIAL 
' • * 

r ~ " 


1 

. 1 ■ 
1 1 1 


. i 


+I2-Ilfrv 


i 






! 










L—.iL 


- 10 












! if 


V*) t *" 7 








J 

- -1 

1 








i i 




1 
1 











than 0. 5dB between 434 and Fig. 5 EARTH PLANE SIDE. 



Page 10 Amateur Television Handbook 



Receivers 



off insulators on the earth plane side (see Fig. 5). 

RF chokes CR1 and CH3 are made from 12 turns of 
28swg enamelled copper wire close wound using a one 
eighth drill and made self supporting on SHORT leads. 
01 12 and CH4 have 10 turns on a IK ~W resistor. 

The transistors should be mounted last, the BFR91's are 



mounted on their edges with the emitter leads passing 
through the board and soldered to the earth plane in 
such a way that they are as short as practicable. Bend 
the leads of the oscillator and mixer transistors about 
half a millimetre from the case and solder them with a 
small iron keeping the leads as short as possible. Special 
care should be taken to ensure that the iron used to 




Amateur Television Handbook 



Page 11 



Receivers 



solder the FET is properly isolated, in practice it is wise 
to unplug the iron from its supply just before making the 
joints. 

Screens are used to isolate the RF stages from each 
other, positioning is shown on the circuit diagram 
(Fig. 3). The screens may be made from sheet brass, 
copper, tin or double-sided copper laminated board and 
fixed to pins pushed through the board and soldered 
through to the earth plane. Care should be taken to make 
cut-outs in the bottom of the screens to prevent fouling 
of components or shorting of the printed tracks. A long 
cut-out should be made in the screen separating the 
bandpass filter lines to enable the shorting link to be 
adjusted during alignment. 

Adjustments 

The converter is intended to have an IF frequency on a 
convenient channel in band 1 but can be modified for 
the standard TV IF frequency or any other suitable 
frequency up to about 250MHz. 



socket. Adjust the four trimmers around the output 
transformer to achieve a curve similar to that shown in 
Fig. 6. Finally re-adjust the capacitor on L4 to peak in 




Fig. 6 



430 432 438.5 440 

RESPONSE CURVE. 



MHz 





COMPONENT LAYOUT. 

Adjustments are best made in three stages with the aid 
of a sweep generator, detector and oscilloscope display 
(a Polyskop or similar RF analyser is ideal). 

Connect the swept source to the aerial input and the 
detector probe to the test point on IA. Adjust the 
capacitors on LI, L2, L3 and L4 and the position of the 
shorting strap between L2 and L3 to obtain a passband 
curve having a bandwidth between 434 and 440MHz at 
the -ldB points. 

Using a grid-dip oscillator, frequency counter or a 
receiver adjust the local oscillator frequency to that 
required for the chosen IF output. 

The oscillator is positioned on the low side of the input 
and is calculated from: 

F oscillator = F input - F if. 

eg. for an IF frequency of 39. 5 MHz and taking the 
centre of the band as 437 MHz the oscillator frequency 
is: 

F oscillator 437MHz - 39.5MHz = 397.5MHz. 

For final adjustments connect the swept source to the 
aerial input and the detector probe to the IF output 



the middle of the band. 

In the absence of proper test equipment the converter 
may be adjusted by trial and error using a strong local 
amateur TV signal. If a signal generator is available a 
probe can be made using a germanium diode and a 
decoupling capacitor the output of 
which is connected to a sensitive 
voltmeter, the probe should be 
connected to the points described 
earlier. By manually tuning the 
signal generator and plotting the 
response on a piece of graph paper 
it should be possible to obtain the 
correct curve. 

If a high IF frequency is used it 
may be necessary to short circuit 
one or more turns of each output 
transformer winding and to 
increase the values of the 
oscillator injection and tuning 
capacitors. 

If the standard IF frequency of 
39.5 MHz is chosen it will be necessary to increase the 
four trimmer capacitors around the output transformer 
to roughly twice their original values, this may best be 
done by selecting fixed capacitors and placing them in 
parallel with the existing trimmers. It may be necessary 
to change the 3pF coupling capacitor to around lOpF to 
achieve the correct response. 

The completed converter should be housed in a screened 
box which can conveniently be made from sheet metal 
soldered together or from copper laminated printed 
circuit board. 

A drilled and tinned printed circuit board is available for 
this converter, details of which may be found at the 
back of this book. 

An Amateur Television Receiver 

This receiver differs from the majority of systems used 
by television amateurs in that it includes IF selectivity, 
amplification, demodulation, AGO and AFC circuits 
and its output is 1 volt peak-to-peak video. Thus there is 
no need to use the RF circuits of a standard broadcast 
receiver, display being via a video monitor. 



Page 12 



Amateur Television Handbook 



Receivers 




u 
« 

o 
w 
« 



The tuner can be a modified ELC1043 commercial type, 
the broadband converter shown elsewhere in this 
chapter or any other tuner having an IF frequency of 
39.5 MHz. The tuner IF output is matched to the IF pre- 



amplifier (IC1) with the tuned circuit Ll/Cl, Rl ensures 
correct broadband termination. 

IC1 is a high gain IF amplifier and is needed to offset 
the large loss caused by the selectivity filter. 



Amateur Television Handbook 



Page 13 



Receivers 




Page 14 



Amateur Television Handbook 



Receivers 



Selectivity is achieved using a surface acoustic wave 
filter (SAW). This filter is specifically designed for the 
UK 625 line broadcast television standard and is a 
significant advance on the multitude of tuned circuits 
previously required. 

The differential output from the SAW is applied to 102 
which is an IF amplifier, video demodulator, AGC and 
AFC generator all on one integrated circuit. The tuned 
circuit L2/C2 is a critical part of the synchronous 
demodulator and must be of good quality and high Q. 
L3/C3 is part of the AFC generator, its adjustment is 
described later 

The video signal from pin 12 of 102 is applied to an 
emitter follower which matches the output to 75 ohms 
suitable for feeding a video monitor. 

Construction 

Component layout is not particularly critical providing 
care is taken to preserve symmetry around 102, input 
and output circuits should be kept well separated and all 
leads should be as short as possible, particularly those 
on the bypass capacitors. Connections to the SAW filter 
should be made in such a way that the input and output 
leads are kept as short as possible and are kept away 
from each other otherwise the filter passband 
characteristic may be distorted. 

The printed circuit board should be mounted on top of 
the tuner using 3/lOinch spacers. The two top corners 
and one of the bottom corners should be earthed to the 
tuner case. 

If the whole of the broadcast band is not required the 28 
volt tuning supply may be omitted and the 12 volt 



supply used instead. Band-spreading for 70cm may be 
obtained by fitting a series resistor between the tune 
control and the tuning volts rail, this resistor should be 
chosen to give a tuning volts range of between and 1 .5 
volts. 

A drilled and tinned printed circuit board is available for 
this receiver, details of which may be found at the back 
of this book. 

Alignment 

Alignment is best carried out using a strong television 
signal such as one of the local broadcast transmitters. 

Adjust the core of LI to about half way and the IF gain 
control to mid-position, switch off the AFC. 

With no aerial connected measure the dc voltage 
between video output and ground, adjust L2 and note 
the voltage reading at each end of the adjustment range, 
adjust the core for a voltage reading exactly mid way 
between the two. 

Connect the aerial and tune in a strong signal. Switch on 
the AFC and adjust L3 until the signal is brought back 
on tune (this adjustment is fairly critical). 

If an oscilloscope is available monitor the video output 
waveform and adjust the 'scope to display one or two 
lines of video, check the adjustment of L2 and if 
necessary re-adjust slightly for minimum distortion of 
the video waveform. 

Finally adjust LI and the tuner IF output coil for 
maximum signal. 




Amateur Television Handbook 




Transmission 



Transmission 



Video Modulators 

Modern amateur television transmitters are usually low 
power and invariably have transistorised final 
amplifiers. It is often possible to vision modulate an 
existing FM or SSB "black box". These will typically 
produce HF levels up to about 10 watts. 

Vision modulation is quite straightforward and details 
are given here of two modulators suitable for 
transmitters up to about 2 watts or 10 watts respectively. 
The circuit of the low power modulator is given in Fig 4 
and is very simple whilst that required for higher power 
is shown in Fig 5. 



If possible both the final amplifier and its driver stage 
should be modulated to achieve a reasonable depth of 
modulation. 

The bias control sets the black level and the video gain 
control sets the level of the picture signal. With the 
transmitter switched on and with no video input, adjust 
the bias control until about three-quarters of the 
transmitters output power is observed on a power meter, 
then connect a video signal when it will be seen that the 
power output will drop, this indicates modulation and it 
will be expected that the power will drop to about half 
which indicates a good depth of modulation. Final 
adjustments should be carried out by using a video HF 




0+i2v 



probe and monitor 
receiving the signal. 



or a local station 



Video modulated 
output to TX 



OOv 



Fig 4 



HEAT SINK 



VIDEO MODULATOR FOR 2W TRANSMITTERS 



From modulator (short lead) 



X 



3-i 



choke 



choke 



Drive 



Driver 



X 



x 

X 



ch 



PA 



Showing a typical driver /PA with video modulator connected. 

"High level" modulation is used in both cases and on 
the low power version the bandwidth has been reduced 
to 3MHz so that the resulting signal comprising carrier 
and both sidebands can be contained within the limits of 
the 70cm band. 

Both modulators are wired in series with the HT supply 
to the final amplifier of the transmitter. It is important 
that a very short lead is used and that all bypass 
capacitors which exist in the original transmitter HT 
circuit to the stages to be modulated are removed, 
otherwise the video information will be lost. A choke 
will prevent HF from entering the modulator. Bypassing 
may be accomplished with LOW value capacitors 
(lOOpForless). 



The modulators should be built in well- 
screened boxes to exclude HF and 
should have adequate heat sinks for the 
output transistors. 

A Modular Linear Amplifier 

The Motorola MHW710 hybrid module 
an integrated circuit and transistor 
device which is encapsulated in blue 
epoxy and mounted on a heat sink 
flange. It was originally designed as a 
15 Watt class C amplifier for radio 
telephone use and has been used as the 
final amplifier in many 70cm amateur 
FM transmitters-principally in the US. 
When correctly biased and driven 
however it can be made to work as a 
linear amplifier though with reduced 
output power. 

To achieve linear amplification suitable 
for television service an HF level of 
80mW is sufficient to drive the output 
to lOWatts peak, care should be taken 
not to overdrive the module otherwise 
compression of the video and sync 
pulses will result. The power supply 
must be 13 volts plus or minus 1 volt 
and very well regulated, power leads 
should be as short as possible and certainly not longer 
that 18 inches. Supply levels greater than 15volts may 
destroy the module. The amplifier is otherwise virtually 
indestructible and will tolerate a high VSWR or even no 
load for short periods. 

The following notes regarding amplifier adjustment and 
operation apply equally to other amplifiers used for 
television transmission. 

Although this unit will deliver lOWatts peak to a 50 
ohm load a power meter will show considerably less, 
typically half peak power. This is because with a 
negative modulation sense such as that used by most 
amateurs peak power will only be acheived on sync tips, 



mi 



Aerial 



-OOv 



Page 16 



Amateur Television Handbook 



Transmission 



black level will produce about 7Watts and the video 
information will drop the power towards zero thus the 
power meter reading is proportional to the average 
power developed by the complex modulating signal. 



A technique of 'pre-distortion' is often used to raise the 
average power generated by the video information 
whilst ensuring that the sync pulses are not crushed. A 
suitable sync-stretch pre-distortion circuit is shown in 




05 



1 
< 

H 
o 

§ 

§ 

< 

8 

o 

w 

a 



in 

tto 
.i-i 



Hli— E 



Amateur Television Handbook 



Page 17 



Transmission 



Fig. 2. Using this circuit the power amplifier can be The video signal is amplified, inverted, DC restored and 

driven into sync compression until the syncs are reduced applied to the gate of a FET. DC restoration is necessary 

to their proper size. so that the amount of distortion is independent of the 




> 



Page 18 



Amateur Television Handbook 



Transmission 



average signal level. The gain of the FET stage is unity 
but during the sync period diode Dl switches on and 
effectively reduces the value of the source resistor thus 
increasing the gain. The amplitude of the syncs can be 
adjusted with the sync amp control. DC restoration is 
again applied to re-establish black level before the 
signal is passed to the emitter follower output stages. 



The output signal of an amateur television station 
should always be monitored with a HF probe and an 
oscilloscope to check that an undistorted signal is being 
radiated. Fig. 3 shows a suitable probe. 

A pickup wire is connected to a tuned circuit and the 
signal demodulated to produce a video signal which is 
fed to a two-stage emitter follower, the resultuig output 




2 



u. 



E 



^v/^-Lxb^ — — i 




ci 
•i-i 

ts 

s 
s 

CO 

n 

Ctt rv 
CD Pv 

N « 

§ S3 

ITS O 
* 

u 

PCI 

K 



ni 

• r~4 

ti 

0) 

i 

s 

CO 

a 

Q. 

O • 

O 

_ S3 

ll 







S3 

0) g 

bO O 

r B* 

«|* CO 



0> 



Amateur Television Handbook 



Page 19 



is suitable for display on a video monitor. 

A 1mA meter is provided to give an indication 
proportional to the HF output power and is useful in 
tuning the transmitter since, because of the tuned circuit, 
it only responds to power at 70cm. 

Construction. 

Construction should preferably be on a printed circuit or 
plain copper laminate board which should be firmly 
secured to the aerial feeder. To insert the pickup wire 
cut out a small square of outer covering from the coax 
cable and push open the braiding, thread a thin piece of 
connecting wire under the braiding for a distance of 
about half an inch and connect the free end to the 4.7pF 
coupling capacitor using as short a lead as possible. The 
actual length of wire will vary according to the HF 



Vision Sources 



output power of the transmitter but will usually be 
between a quarter and half an inch. 

Adjustment. 

Connect the video output to an oscilloscope terminated 
with a 75 ohm resistor. Transmit a properly modulated 
television signal and adjust the input tuned circuit for 
maximum, adjust the length of the probe pickup wire 
until about 1 volts peak to peak is displayed on the 
oscilloscope. Set the meter adjustment control for a 
convenient reading, usually about two-thirds deflection. 

The MHW710 module is available in the UK mounted 
on a finned heatsink and wired as shown in Fig. 1 from 
Blean Video Systems, 4 Mount Pleasant, Blean 
Common, Canterbury, Kent, CT2 9EU 



An Electronic Character Generator 



Page 20 



Amateur Television Handbook