[sdiy] expo accuracy? or integrator accuracy?, or both?

[Ian Fritz]

Hi Grant --

>As my understanding goes, each component is specified with a tempco, but
>then there is a tolerance on the tempco. For example polycarbonate caps with
>a +150 ppm +/-50 ppm or between 100 and 200 ppm.

Yes, that's correst.

>Now someone pointed out that the silicon will have a tempco tolerance and is
>only centered on 3300 ppm.

True, but that variable tempco is indeed properly compensated with a proper 
tempco resistor. The correct compensation is a resistance whose value is 
linear-through-the-origin, ie,  R = aT.  This dependence exactly cancels 
the 1/T term in the exponential.  The tempco of such a resistor is

(1/T)(dR/dT) = 1/T,

which indeed varies with temperature.  In other words, the Si and the 
tempco resistor vary together so as to always cancel the temperature 
dependence.

>So each unique collection of components will have a unique composite ppm
>drift.
>
>So if you develop a stable but temperature variable multiplier for the 1
>volt to ~18mv conversion, wouldn't you still have to do a zero and span
>calibration using an actual temperature chamber to calibrate it? Mucho work!

Absolutely!  You got it!!  It takes a couple of days of work (at least) to 
compensate a VCO accurately.

>I have also noticed that the 1 watt copper wirewound tempcos (KRL) exhibit a
>better short term stability, than the same circuit with an 1/8 watt film
>tempco (because the 1 watt has a much larger thermal mass?). I don't think
>you WANT to change the scale factor very fast. I get the impression that
>adds instability from air movement.

Yes, you also have to have things coupled properly in the thermal sense.

>It seems these exquisite design enhancements may have reached the law of
>decreasing returns.

Could be.  But I spent years retuning every 15 minutes whenever I had a 
recording project going, and now I never have to.

Depends on how you want to work.

   Ian