Fast VCOs for wavetable

[Don Tillman]

   From: Magnus Danielson <magnus at analogue.org>
   Date: Sun, 22 Nov 1998 05:37:02 +0100

   > "DH" == Drew Hutchison <dhutch at kadets.d20.co.edu> writes:
    DH> It has been fairly well agreed that no VCO we know of can
    DH> handle running at 32-64 times the speed we normally use them 
    DH> with enough stability to reference a table.  Would it be possible 
    DH> to run a VCO (Stopp's?) at a normal speed and send that
    DH> through a multiplier of some form?  I know there are many
    DH> circuits out which do this at MUCH higher frequencies than
    DH> audio, and they seem pretty stable.  It seems like a 
    DH> phase locked loop might do it.  Any thoughts?

   To start of with, the VCO that Gene put in the ASM-1 is good for just
   above 100 kHz (116 kHz, my notes and memory missmatches). It will
   go nonexponential in tracking between 102 and 116 kHz.

A sawtooth VCO is just not going to do well at high frequencies
because the speed and accuracy requirements of the reset transition
get mighty extreme.  Frequency multiplication by manipulating sawtooth
the waveform gets ugly because it multiplies any waveform
imperfections.  And frequency multiplication with a PLL is a pretty
much a trainwreck if you want to use this VCO over musical frequency
ranges (though possibly musically entertaining!).

The most straighforward approach is a triangle VCO.  No reset
transition issues, very simple and well behaved at high frequencies,
just make the caps small and use a really fast comparator.

But Magnus' suggestion got me thinking... you could also but together
something with a regular audio-range sawtooth VCO, a counter connected
to a DAC, and a comparator circuit.  The sawtooth reset pulse zeros
the counter, and as the sawtooth ramps up you compare the sawtooth
voltage with the DAC output, when the sawtooth voltage is delta-V
above the DAC you clock the counter.  Pretty clean, and you get a
simultaneous analog sawtooth.

  -- Don