ODP: thru zero VCO questions

[Rene Schmitz]

At 22:17 10.09.99 -0700, Harry Bissell wrote:
>Boy this is a "brain booster"
>
>Yes...but
>
>There are two cases... if the modulating signal is of the correct polarity
>then we are integrating back into range, so we do nothing...
>
>Or if the modulating signal is wrong polarity, then we could look that the
>previous state (the flip flop is the memory) and "reverse" it. A couple of
>gates would work... (no extra comparator..)
>
>Or would it ?????

Mmmh, one could derive the input of the D-flipflop with a little logic,
which pre-calculates the next direction from the polarity of the input
signal, and the previous state. No "extra" toggling of the FF. Maybe a
JK-FF could save extra gates. 
But here the offset problem comes back into play, the voltage of the OTA
for zero Hz, and the zero detect voltage should be equal! It could occur
here that the "real" direction has changed before or after the polarity
detector notices it!

Or yet another solution, perhaps, trigger a short monostable with the
comparator output, after the mono has expired, look again if the comparator
is zero (did reverse correctly) and all is well. If it isn't apply second
pulse to the FF. (And also back to the input of this circuit, if the
polarity has changed again...)
Mono time would be slightly longer than the reset pulse that the comparator
would normally produce.
Here we would not need a polarity signal derrived from the FM input,
avoiding the different offsets.

Or would I buy the solution with another problem here?

>P.S. Your idea is good, because there would always be the possibility that
>the modulating signal (if strange enough...) could slip past BOTH SETS of
>comparators and still latch...

The worst case signal I can think about would be white noise! 
(Blue noise even worse perhaps...)

Bye
 René


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