[sdiy] Analysis of frequency variation in analogue synths

[JH.]

>The topic of weither the jitter & wander aspects is part of the "analog" or
>"fattness" has been debated here and elsewhere to death before. I still
>consider that issue unsettled.

It just came to me - I may have read it somewhere, though I don't remember:
The slow ("wander") part of the pitch variation could easily be detected 
with a PLL setup, couldn't it?
Use a very stable VCO (can be linear, to avoid the possible drift from a 
non-ideal expo converter) for this PLL, feed the VCO- under-test into the 
PLL, and trace the CV of the PLL's own VCO. I'm not sure if you'll get the 
"jitter" part (probably not, because of the PLL's loop filter), but you 
should be able to trace the slower modulations easily. No?
Has anybody tried this?

> Personally I advocate the point that
> it *might* be phase modulations or for that matter amlitude modulations

I think when there is a variation in frequency, this can always be expressed 
as a variation in phase, so there certainly *is* phase modulation if there 
is a change in frequency at all. And amplitude modulation may be an addition 
effect to some (small) extent. For instance, when the comparator thresholds 
in the VCO core are noisy: This will cause both frequency/phase modulation 
and amplitude modulation. Same for parasitic soft sync effects that work on 
the comparator thresholds. Eveything that works on the CV processing 
circuitry (expo converter etc.), will only cause phase/frequency modulation.

JH.