[sdiy] Temperature Compensated VCO attempt - help?

[Don Tillman]

   > Date: Mon, 03 Feb 2003 10:02:43 +0100 (CET)
   > From: Magnus Danielson <cfmd at swipnet.se>
   > 
   > My bid is:
   > 
   > Put a Gilbert-cell on top of an expo-converter. Put the linear
   > (sign-swapping) modulation on one of the Gilbert-cell input pair,
   > put the Schmitt-triggered output waveform on the other input pair
   > of the Gilbert-cell. The current output of the Gilbert-cell is
   > connected to an integrator and this is followed by a
   > Schmitt-trigger. The current in the Gilbert-cell is modulated
   > from the expo-pair. This should be both a compact and pretty
   > well-behaving oscillator and since there is non abrupt switching
   > of current but rather linear change in current balance I think
   > there is a good chance of a nice behaviour.

Well that's certainly elegant.

But I think something's missing (or perhaps I'm missing something).

Say we have it set to a positive frequency, the integrator has just
hit the positive reference voltage, the Schmitt trigger changes the
direction to slew down, and then at that point, if we set the control
voltage to negative, the integrator slews back up to the reference
voltage, but the Schmitt trigger is still in the positive state so it
won't notice to change direction again.

Graceful thru-zero a very difficult problem.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com