[sdiy] VCO Linear Voltage

[David G. Dixon]

> Ian Fritz schrieb:
> 
> > You still have to design a time-reversible Saw core.  There's more to it
> > than just reversing the current.  My Teezer design gets it done, but it
> > wasn't trivial to get working correctly.
> 
> I haven't tried it, but hit Vmin -> reset to Vmax, hit Vmax -> reset to
> Vmin. Doesn't sound too complicated to me. But the devil is in the
> details.

The problem is in the fact that 2164 must feed to virtual ground.  Without
this limitation, using it for Saw-Core TZ FM would be almost trivial -- one
could simply sense the change in current polarity to switch the threshold
voltage on the integrator from ground to the top ramp voltage (say, 5V)
while switching to a different comparator, or using a window comparator
between 0 and 5V with a bit of hysteresis or some other provision to avoid
multiple resetting.  However, because 2164 can't feed anything other than
virtual ground, using it in this application would be difficult if not
impossible.  I think there must be a way to process the current from 2164
such that it can be fed into any voltage, but I haven't figured it out yet,
and I have a feeling that it would involve something fairly exotic.

I had the same problem trying to use it in a triangle-core VCO, and
basically gave up out of frustration.

One thing I don't know is whether the 2164 can be "tricked" into thinking
that it's feeding to virtual ground by tying its ground pin to the same
voltage.  However, I presume that this would also affect its gain response
to the control voltage.