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VHF/UHF TV modulator
Elektor january 1985
Computers, video games, video cameras, games computers; all of these
produce video signals that must be displayed via a television set. If the TV
receiver in question does not have a video input and its owner is reluctant to
vandalise it in order to fit one then this sort of modulator is the obvious
solution. It is a simple circuit that processes video signals to enable them to
be fed straight into the TV set's aerial input.
A 'TV modulator' is really no more than a transmitter. It is a very small
transmitter, admittedly, but none the less that is what it is. What does a
modulator actually do? In general -and this design is no exception to the rule -
it is a simple oscillator that generates a frequency somewhere in the VHF or UHF
region. The oscillator is modulated with the video signal and the modulated
carrier wave thus generated is fed into the TV set's aerial input via a cable.
Then all that remains to do is tune the TV to the correct frequency.
||A 'TV modulator' is really no more than a transmitter. It is a very
small transmitter, admittedly, but none the less that is what it is. What
does a modulator actually do? In general -and this design is no exception
to the rule - it is a simple oscillator that generates a frequency
somewhere in the VHF or UHF region. The oscillator is modulated with the
video signal and the modulated carrier wave thus generated is fed into the
TV set's aerial input via a cable. Then all that remains to do is tune the
TV to the correct frequency|
The layoutThe whole business is not quite as simple as we have just
suggested, of course, as the mini transmitter must meet certain requirements.
The frequency stability must be very good as, indeed, must the quality of the
display. The required frequency stability is achieved by the use of a crystal
oscillator. A well thought out choice of component values takes care of the
display quality: the modulator allows a resolution of 80 characters per line, as
this is a value that is often needed. A very important feature of the circuit
that must be decided is the transmission frequency. If this is only a single
channel, as suggested above, h gives rise to some practical problems. Different
users will want different channels, the carrier wave can become somewhat
difficult to locate, and unless the frequency is exactly spot on no signal will
be received. A much better idea is to ensure that the HF signal contains a large
number of different frequencies. This makes it much easier to tune the TV set to
one of the frequencies as there will surely be one to suit every user.
||The block diagram of figure I shows how this is achieved. The TV
modulator is made up of two parts, namely a modulatable crystal oscillator
and a harmonics generator. The oscillator operates at a frequency of 27
MHz, which is quite low so inexpensive crystals are readily available. The
harmonics generator converts the oscillator signal into a sort of
frequency spectrum containing all the multiples of 27 MHz up to about 1800
MHz. The TV modulator's output signal is made up of a large number of
little peaks, each of which is a complete transmitter signal. At least one
of these will always be in band I (VHF channels 2. . . 4), one in band III
(VHF channels S. . .12) and many of them will be in bands IV and V (UHF
channels 21.. .69).|
The circuit diagramLike the block diagram, the circuit (shown in figure
2) is very straightforward. The crystal oscillator is based on a very fast HF
transistor, Tl (BFR91), which performs the amplitude modulation. Apart from this
there is little to be said about the oscillator except, perhaps, that it is
essential to use the correct values for the components surrounding Tl. This is,
of course, simply common sense in this sort of HF circuit. The harmonics
generator is formed by two Schottky diodes, Dl and D2. These diodes must switch
very quickly in time with the 27 MHz signal so they provide strong harmonics up
into the gigahertz range. The modulation depth can be set with Pl, while the
oscillator's d.c. value can be varied by means of P2. The combination of these
two presets enables either positive or negative amplitude modulation to be
selected. This is essential as the harmonics produced vary in this respect. We
will discuss the calibration of Pl and P2 later in this article. The power for
the circuit can be provided by either an unstabilized 8...30 V or a stabilized 5
V. The latter could be taken from a computer's power supply and in this case ICI
is not needed.
ConstructionThe tiny printed circuit board designed for this circuit is
shown in figure 3. It is not double-sided as this was found to be unnecessary.
Construction is thereby simplified and readers who do not buy the board through
our EPS service (tut-tut) will find it easier to make themselves. Building the
circuit is simply a matter of fitting the components onto the printed circuit
board. The coils, often a source of much teeth-gnashing and hair-pulling, will
not be a problem in this case. Two of them, Ll and L2, are made by winding 3.5
turns of enamelled copper wire (about 0.2 mm thick) on a 3.5 mm ferrite bead.
Another, L4, is just one turn of copper wire (0.8. . . 1 mm thick) air-wound
with a diameter of 8 mm. The fourth inductor, L3, can simply be bought. Any
third overtone crystal with a frequency of between 25 and 30 MHz will work in
this circuit. A number of suitable values are advertised in this issue. The only
parts that might prove difficult to find are diodes Dl and D2. The ones stated
in the parts list are available at the moment but do not give up hope if your
corner shop does not have them. The only important thing is that they must be
UHF Schottky diodes; the actual type number is of little consequence.
CalibrationCalibrating the modulator calls for a certain degree of care
as it involves more than just 'set the presets to mid-position'. The setting
depends, in fact, on the harmonic to which the circuit is tuned. Calibration
should be carried out as follows:
Set the TV receiver to maximum brightness
Feed a video signal into the modulator (a video recording of a
test card, or a link to a computer's 'TV' socket, could be used) and connect the
circuit's output to the TV's aerial input.
Set P2 to mid-position and P1 to
minimum resistance (fully anti clockwise).
Tune the TV receiver to a
harmonic, preferably one of the VHF bands (channels 2. . .12). The tuning is
correct when the 'snow' on the screen disappears and/or the screen becomes dark.
Turn P1 very slightly until 'something' becomes visible.
Calibrate P2 to
give the best possible quality image. If the result is not very
wiper of Pl can be moved a bit more and P2 again trimmed to give a better image.
If this still fails to provide an acceptable result tune the TV to the next
This must give a decent image.
|The circuit diagram- figure 2
R1, R2 = 4k7
R3, R4 = 56ohm
P1 = 100 ohm
P2 = 500 ohm preset
C1 = 4mf7/16 V
C3 = 220p
C5 = 47n, ceramic
C6 = 100n*
C7 = 330n*
L1, L2 = 3.5 turns of 0.2 mm (SWG 35 or 36) CuL
on a ferrite bead of about 3.5 x 3.5 mm
L3 = 1 microH
L4 = 1 turn
of 0.8. . .1 mm (SWG 19...21) CuL, air wound with a diameter of 8 mm
D1, D2 = 1N6263 (Ambit/Cirkit)
T1 = BFR91 (Ambit/Cirkit)
IC1 = 7805*
X1 = crystal, 27 MHzd(3rd overtone) or other
3rd overtone crystal between 25 and 30 MHz
*= not needed if the circuit is powered from a stabilised 5 V
||Figure 3. Fortunately the printed circuit board for the
modulator is only single-sided. The largecopper surface acts as a ground
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