[sdiy] Temperature Compensated Exponential Converter Using SSM2164

[David G. Dixon]

Excellent point, Ian.  I was just trying to demonstrate the difference
between relative and absolute error, but perhaps that is not terribly
appropriate for this particular problem.  I'm going to have to do more
analysis to wrap my head around all of this, and preferably not at 1:00 in
the morning, after nine hours on the road.

In any case, the maximum relative errors I'm getting with the 2164 analysis
are in the neighborhood of 0.2%, and significantly less if one uses the
"proper" values of tempco trim and feedback resistance, and doesn't stray
too far from the reference temperature, particularly if one errs on the
warmer side (cooler temps give more error).  Hence, I think, with a little
care, this expo can be adjusted to give very slow beat frequencies
everywhere in the audio spectrum.

> >Missing the next octave by 1 Hz is no big deal
> >if your target is 1760 Hz, but it is a very big deal if your next octave
> is
> >55 Hz.  Capiche?
> 
> Ummm ... no.   :-)
> 
> What is most objectionable about mistuning is the beating it creates.  In
> your example there would be a 1 Hz beat rate (against a perfect
> reference)  for either case.  Not bad at all.
> 
> This why I set my VCOs up so that deltaVbe at the converter pair
> corresponds to something around 2 kHz.  (deltaVbe = 0
> --->  exp(a*deltaVbe/kT) = 1, so there is no T dependence at this
> point)  Drift above this point isn't too important, because you rarely use
> fundamental frequencies much above 3 kHz.  Drift below this point becomes
> less and less critical as you go down, because a 1 Hz beat represents
> smaller and smaller fractional error.
> 
> Just my personal philosophy, I guess.  :-)
> 
> Ian