ODP: thru zero VCO questions (4th attempt)

[Roman Sowa]

My advise is this:
don't look at other designs. If you have simpler idea - try it.
As a matter of fact I wanted to do something exactly like your idea,
but due to epidemic disease eating free time I didn't.
What bothers me is, when the D-flop was just toggled, integrator
voltage started to travel from max to -max, and FM input changed polarity,
then D-flop should change again really soon.
OK, little histeresis on window comparator would solve nose-switching
problems, but what if it all happens really close the maximum integrator
voltage, near the switching point? VCO will lock at opamp maximum voltage.
OTOH such condition might appear once in a milion years during
normal use.

Don't know if I express myself clearly.

Roman

-----Oryginalna wiadomooeæ-----
Od: Rene Schmitz [mailto:uzs159 at uni-bonn.de]
Wys³ano: 9 wrzeoenia 1999 04:26
Do: synth-diy at mailhost.bpa.nl
Temat: thru zero VCO questions (4th attempt)


I wan't to develop a thru zero capable VCO, and I have some questions
regarding it. I can see from "prior art" that one uses integrators that can
go up and down, depending on the polarity of the FM input signal. In the
ones I've seen the FM input is rectified, fed into the reference input of
the expo convertor, and the whole oscillator is switched between forward
and backward. Why is it done that way? I think all one would have to do is
to replace the schmitt-trigger with a window comparator which toggles a
D-flipflop between forward and backward when the thresholds of the
comparators are met. The output of the flipflop would be switching the
polaritiy of the FM input sum. Or am I missing something here? 
My brain already hurts, so any comments highly welcome.

Bye
 René