vhfuhf tv modulator


Posted on Feb 6, 2014

The electronic circuit is a TV modulator that 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. T


vhfuhf tv modulator
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he 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 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.




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