[sdiy] Temperature Compensated VCO attempt - help?

[patchell]

    Looking at my diagram...the VCA consists of R44, U9a and U9b.  R44 and U9a create
an attenuator (U9a is connected as a simple voltage controlled resistor).  U9b is just

your normal VCA stage.  As the temperature changes, the resistance of U9a will change,

compensating for the gain change that occures in U9b...thus the overal tempco of the
VCA will be 0 (well, close to it).  The two OTA's must be in the same package.

    I am already starting to mod a breadboard I did of this originally using your band

gap circuit.  I will let you know how it works out.  It will probably be Saturday
(when I can make measurements from morning to early after noon, temp change over that
time is about 15 degrees F at this time of year), so I should find out if indeed this
is the mysterious 400 ppM that I have been trying to locate off and on for a couple of

years now.

sbernardi at attbi.com wrote:

> I started looking at combining your OTA configuration with my bandgap reference
> to generate the temp dependent current for the OTA. I did some analysis, and
> noticed that the temperature dependent current derived from the transistor
> array drives the first OTA and cancels the kT/q term in the transconductance,
> and then the kT/q in the transconductance of the second OTA is what is used to
> compensate for the q/kT term in the exponential transistor's Vbe. So ideally,
> the OTA and transistor array should be thermally connected. I guess you could
> glue the CA3046 on top of the LM13700, or via versa. Like a dead bug with its
> legs in the air.
> >     Good point.  That might explain where the 400 ppm is coming from.  I will
> > have
> > to look into that some more.  Sort of glad you brought that up.  I was planning
> > on
> > doing another prototype run of VCO boards...I might have to come up with a few
> > more
> > options before I send out again...
> >
> > Scott Bernardi wrote:
> >
> > > 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
> > > > ------------------------------------------------
> > > > * Visit:http://www.silcom.com/~patchell/
> > > > *-----------------------------------------------
> > > > *I'm sure glad Merry Christmas comes just once a year
> > > > * -Yogi Yorgensen
> > > > ------------------------------------------------
> >
> > --
> >  -Jim
> > ------------------------------------------------
> > * Visit:http://www.silcom.com/~patchell/
> > *-----------------------------------------------
> > *I'm sure glad Merry Christmas comes just once a year
> > * -Yogi Yorgensen
> > ------------------------------------------------
> >
> >

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