TTI TG501, Руководство по обслуживанию, Страница: 10 / 33

Waveform Generation - Comparator and TTL output
IC13 operates as a window detector and determines the peak to peak amplitude of the triangle
wave on the range multiplier capacitor, which is approxi/-1.2V. C17-C22 compensate for
comparator and loop delays ensuring that the triangle wave amplitude remains constant with
increase in frequency. The two internal NAND gates in IC13 are wired as a flip-flop to ensure
positive switching of the comparator. C23 provides a small amount of positive feedback to ensure
jitter free operation. Some comparators oscillate with the dial at 0.005; this can be prevented by
fitting C66.
One of the comparator outputs is routed via IC17a to parallel gates IC17 c and d which drive the
TTL output. Q15 level shifts IC13 output to be about ground to drive the diode gates. Q16 is a VBE
multiplier and its total voltage is just under two VBEs. R45, R46 and C16 improve triangle linearity
at 5MHz.
IC15a and level shifter Q17 are used to drive the squarewave shaper circuit. When the squarewave
button is out, pin 4 of IC15a is held low and so the squarewave is gated off at the source.
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