F-V converter and Ring Mod. stuff

Don Tillman don at till.com
Fri Feb 16 06:54:45 CET 1996

   Date: Thu, 15 Feb 96 18:18:38 PDT
   From: gstopp at fibermux.com

   I started to calculate what kind of circuitry it would take to make a 
   digital exponential counter with granularity comparable to a 
   continuous exponential converter, and it's a bitch. I'd need something 
   like 64 bits of exponent register just for six bits of final output - 

Hmmm, I calculate maybe 10 bits for an octave, 16 for a six octave
range if you're using an exponential lookup.  Maybe 13 bits if you use
the exponential counter.  Add a few more bits if you value accuracy.

   Then I got to thinking - no matter what method is used to drive an 
   exponential VCO with a pitch tracker, it's still open loop - meaning 
   that the VCO will never track the input to within anything closer than 
   several hertz, at best.

I'd say that depends more on the subtleties of the design than
anything else.  The biggest problem is going to be extra or missed
transitions from a funny-looking waveform, and that will screw up
either approach, unless you take precautions.

   So the next logical step was to try a phase-locked loop, and use the 
   VCO loop control voltage as the recovered frequency voltage output.

I've wanted to try this out for a while, but never got to it.  It's
cool that your giving it a go.

The major problem I see here is that, with an exponential VCO, the
loop transfer function (the loop's response to a change in input
frequency) changes drastically with the running frequency, to the
point of ringing at some.  I don't know how much of a problem this is
in real life.  And of course it can be compensated for too.

[Later... Wait a second, this effect might also be a feature.  I have
to think about this some more.]

  -- Don

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