[RE: VCOs]

[Martin Czech]

> Some addl thoughts... the ESD diodes shouldn't be a player in the inaccuracy
> of the circuit because the operating point will be far away from the point
> where they would be biased even a little bit. The circuit will run at 1/2 Vcc
> plus or minus the hysteresis band. The very limited amplitude should keep
> these errors almost constant. That doesn't mean it works, it means this isn't
> the biggest problem. Can anyone think of a use for a sawtooth wave where we
> have no control over amplitude, bot can control frequency. It's no sweep
> generator... ;-) Harry Bissell

NO!

The junctions are not forward biased , but there is diode leakage current.
These diodes are parasitic elements of the CMOS process, the technology is
not designed to do very low leakage diodes, just NMOST & PMOST.
Leakage rises exponentially with temperature. @-40C these parts may have
~1nA input leakage, @120C 1uA (mil grade).

> 
> The problem is even worse than diode leakage. C-MOS schmitt-trigger
> threshholds vary with manufacturer and aren't guranteed. Sawtooth amplitude
> is proportional to the hysteresis. And the C-MOS gate has a very limited
> ability to recharge the cap. especially with the series resistor and diode.

As long as the threshold margins are constant, I don't care about
amplitude.  The schmitt's are quite fast, you have to pay for that with
temp and supply variation (via supply you could adjust the amplitude).
The temp error can be a problem for large temp variations (thermostat ?).

If the drive capabilty is nut sufficient one could add a switching tranny
instead of diode, like I proposed.

m.c.