[sdiy] Temperature Compensated VCO attempt - help?

[sbernardi at attbi.com]

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
> ------------------------------------------------
> 
>