thru zero VCO questions

[Don Tillman]

Hi.

I've been following this discussion closely, but I haven't said
anything yet because I haven't had anything to offer... the only
interesting comments I've come up with have been expressed better by
others.

I am completely fascinated by the problem though.  And I had a chance
to think about some of the issues over the weekend.

I think the current approach, the integrator/flop/invert-the-signal
setup, is doomed to have a region of instablity or a region where the
oscillator just dies.  Small offset voltages will ruin the performance
of the circuit in several ways and the best workaround appears to be
spewing pulses to the flop, as if to say "Will this polarity work?
No?  Try this one.  No?  Okay let's go back...".  

I think an *elegant* thru-zero VCO design is going to have to work in
two dimensions.  By "elegant" I mean a VCO that works just as smoothly
around the peak voltages as it does over the rest of its range.  And
by "two dimensions" I mean you'll need two capacitors working in a
quadrature arrangement.  I don't have a schematic to offer yet, but
I'm thinking of having two integrators running through four states:

   state 1: integrator 1 rises, integrator 2 holds at -Vref
   state 2: integrator 1 holds at +Vref, integrator 2 rises
   state 3: integrator 1 falls, integrator 2 holds at +Vref
   state 4: integrator 1 holds at -Vref, integrator 2 falls

This way you can run the oscillator forward and backward, however
gradually, without hitting any funny states.

  -- Don