Narrow-bandwidth Television Association


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Chapter 13

NBTV Sawtooth Test Generator

To get the best from an NBTV signal it is important to use the full dynamic range of the signal with no crushing at black level or peak white. To check the linearity of a video path it is usual to use either a staircase or sawtooth test signal. The circuit described is a free standing NBTV sawtooth generator conforming to the standard of 1volt output level and complete with 31 line sync pulses.

Electronic circuit diagram

The 7555 timer and LM317 regulator form a linear sawtooth generator. Two timing capacitors are employed, one setting the active line time which may be trimmed by the pot, the second sets the line sync duration. The circuit is un-usual in that the LM317 performs two roles. It acts as constant current genera-tor to linearly charge the 0.22 mF capacitor, and also provides a very low output impedance.
The 4020 counter is used as a divide-by-32 fed from line pulses. The grounded base transistor differentiates the falling edge from Q5 of the 4020 and gates out one sync pulse in 32, the output from the gate being non-inverting. The remaining 31 sync pulses are mixed with an attenuated sawtooth to form a standard 70%-30% 1 V NBTV signal and delivered at a nominal 470 Ω impedance.
The circuit is sensitive to variations in the supply rail and the prototype was driven from another LM317 set to a nominal 8.5 V allowing use of a 12V supply. If a regulated supply is employed there is no need for this second regulator.

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