[sdiy] Hybrid VCO

[Tim Parkhurst]

Hello All

It seems a LOT of effort goes into building a VCO that tracks well and has
little or no drift with temperature. I was wondering, has anyone tried using
a microcontroller as a 'feedback element' to adjust the frequency of a VCO
to take out temp drift? The steps would be

1) Micro uses A/D converter to sense CV
2) Micro calculates or looks up desired frequency
3) Micro measures frequency at output of VCO and adjusts input CV or
charging current accordingly
OR
3) Micro takes two quick samples of sawtooth wave, calculates frequency from
slope and adjusts input CV or charging current

The second version of step 3 would be faster and might allow the micro to
update the CV in lots of quick, small steps so that the compensation
wouldn't produce audible frequency jumps. The end result might be a $500
module, but I would think that a reasonably fast micro might be able to keep
tabs on 2, 3 or even 4 VCOs (so that $500 might buy a quad ultra-stable VCO
module). Also, this would take some of the tempco burden off of the analog
circuitry, making it cheaper and easier to build. This is essentially a
digitally tuned analog VCO, and I think you could dial in enough drift to
keep the sound "phat" (in fact, you should be able to dial in exactly how
much drift you'd like). I'm sure this has been done before in modern
polyphonic systems, but I've never seen it done (real time) in a modular
VCO. Any thoughts?

Tim Servo




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