F-V converter and Ring Mod. stuff

Don Tillman don at till.com
Thu Feb 15 19:26:30 CET 1996


   Date: Wed, 14 Feb 96 09:07:38 PDT
   From: gstopp at fibermux.com

		    I then took Don Till's advice and tried the 
   exponential-discharge capacitor into a sample & hold idea. It looks 
   good on the scope, but I had trouble trying to get it to convert at 1 
   volt per octave using scope period measurement and a DVM. 

Hmmm, I wonder why.  It should work.

   Next I thought of an "exponentially discharging digital counter", in 
   which the 16-bit down-counter modifies its own clock in such a way 
   that the frequency division modulo is increased as the count 
   decreases. Now when the external input frequency transitions latch in 
   the period measurements, they will be at 1 volt per octave already (if 
   the counter modulo math is correct). Of course it's always possible to 
   have the period measurement value point to an address in a look-up 
   table, whose data value goes to the CV DAC. While this may seem 
   inelegant it will certainly work, but now we're getting into PROM 
   programming again. 

The lookup table is inelegant and has serious resolution problems.

Try this instead: Clock the counter off of a VCO driven by the DAC
from the counter.  The counter will start off clocking fast, and as it
counts down (up, whatever) the DAC voltage comes down and slows the
clock rate of the counter.  The result will be the digital equivalent
of a capacitor discharging.  Total incremental cost is single linear
VC0, so it's almost free.

   From: Christopher List <Christopher_List at sonymusic.com>
   Date: 14 Feb 96 13:48:26 

   Have you (or anybody) looked into the AD650 V-F / F-V converter? -
   They're pricey, like $18 a pop, but from what I could tell from the
   data sheet (not too much) it should have very good linearity on the
   output voltage for frequencies up to a few kHz. 

Not very good.  It's just a one-shot, so your input signal gets
converted to a fixed with pulse train that gets filtered.  So there's
the obvious tradeoff between response time and ripple in the output,
but also a single misshaped pulse will screw the output voltage signal
badly.

Actually measuring the cycle time of the input signal (like with
Gene's digtial method, my analog method, or something similiar, such
as a charge pump), requires much less filtering, and fewer tradeoffs.

On top of that, it's even better to have the ability to throw out
bogus measurements from mishapen waves.

  -- Don





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