Fig, 2, Bfock diagram of the television transmitter used at K3ADS,

336 lines. This is an excellent picture capable of very fine detail. The average TV receiver, considering ghosting and other problems, normally will give a 250 line picture with a 200 line picture being entirely satisfactory.

Therefore to obtain a 200 line picture, the video amplifiers and modulator must have a bandwidth of at least 2.5 MHz. As a practical matter, a 2.5 MHz bandwidth is really easy to obtain. This wide bandwidth, however, results in amplifiers with low gain so several stages of video amplification are necessary. One of the most popular surplus modulators is the CRV series, originally used in a 250 MHz TV transmitter, This can be used to grid modulate tubes such as 5894 or 6939.

Most modulators for TV use grid or a combination of grid and cathode modulation, In AM work, this would be a poor modulating scheme, but in TV the modulation is essentially negative ┬┐is per Fig, and therefore even though grid modulation is used, the tubes may be run at full CW inputs. Notice that sync modulates positive about 15% and that video modulates negative about S8&, More on this depth of modulation later, When using cathode

0 0

Post a comment