[sdiy] Temperature Compensated VCO attempt - help?

[Magnus Danielson]

From: Scott Bernardi <sbernardi at attbi.com>
Subject: [sdiy] Temperature Compensated VCO attempt - help?
Date: Wed, 29 Jan 2003 13:39:54 -0800

> I've made an attempt at an electronically temperature compensated
> exponential generator and VCO. I built it into a thru-zero FM VCO based
> on an Electronotes design. The VCO works, but the exponential generator
> suffers from linearity problems. I can tune it to a nice 1v/octave
> response using my VCO tuner
> (http://home.attbi.com/~sbernardi/elec/og2/vco_tuner.html) at 0v CV, but
> as soon as I deviate from that either higher or lower in frequency, the
> scaling is off.
> I've got a pretty detailed theory of operation and schematic at
> http://home.attbi.com/~sbernardi/elec/og2/tempcomp_vco_theory.htm.
> Jim (Patchell), I was especially interested in your temp compensated
> VCO, and notice you changed the circuit because the previous one was
> "bogus".  I believe on your previous circuit you were using a delta Vbe
> bandgap reference like mine - was that giving you problems? I also
> notice you are using a different configuration for your multiplier OTA
> that "linearizes" it.  I suspect it might be my OTA multiplier that's
> causing the problems.
> Anybody care to comment?

I've done some analysis of the problem and come to the conclusion that the
culprit is that you do not provide the signal to the OTA in diffrential mode
only, the common mode part of the OTA input should be a steady current. When
you use single ended form as you do, you will feed both differential and common
mode signal, and the common mode will rescale things in an unwanted fashion.
Other than that it seems like the overall strategy is pretty OK.

Cheers,
Magnus