[sdiy] Temperature Compensated VCO attempt - help?

[Scott Bernardi]

Those are pretty good specs, I'll look into it.
One other question. You derive your temperature dependent current from the
exponential transistor Vbe directly, rather from a delta-Vbe.  Doesn't that then
include the temperature dependence of the transistor saturation current also
(doubles every 10 degrees)?

patchell wrote:

>     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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> * Visit:http://www.silcom.com/~patchell/
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> *I'm sure glad Merry Christmas comes just once a year
> * -Yogi Yorgensen
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