[sdiy] Temperature Compensated VCO attempt - help?

[patchell]

    From what I saw on your schematic, everything looks basically OK...the
circuit you are using is a bit different from the one I considered to be
Bogus...(the one I was using came from an EDN design Ideas collum...it
looked good, but it had a tendency to latch up under certain conditions).

    Just a couple of comments from what I have learned.  Using the OTA
linerizing diodes will only give you about 0.4% linearity.  I can't remember
what the linearity of the circuit I am using is exactly, but I seem to
recall that it is around 0.1%...or there abouts...can't remember where I got
the circuit, but somebody on this list posted a link to it.  I like it also
because it uses fewer parts.  But the one test I did I was getting +/- 2
cents over 8 octaves and the scale drift was about 400ppm/C (using an
"ASM-1" osc core)

Scott Bernardi wrote:

> 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?

--
 -Jim
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