Fast VCOs for wavetable

[Magnus Danielson]

>>>>> "DH" == Drew Hutchison <dhutch at kadets.d20.co.edu> writes:

 DH> Seems there has been a lot of discussion on this lately.  Anyway, here's
 DH> my input:  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 with enough
 DH> stability to reference a table.  Would it be possible to run a VCO
 DH> (Stopp's?) at a normal speed and send that through a multiplier of some
 DH> form?  I know there are many circuits out which do this at MUCH higher
 DH> frequencies than 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.

Then, you can rescale the VCO to make it go faster, chaning the
capacitor C2 and the resistor R17 is just one way. The main reason the
VCO will get stuck at high frequencies is the saturation of the
exponential transistor Q2 which can not deliver much current above 10 mA.

One way to double the frequency of a sawtooth is to waveshape it. You
can do this by consider this form of waveshaping.

VCO shape:

 /|        /|
/ |       / |
  |      /  |
  |     /   |
  |    /    |
  |   /     |
  |  /      |
  | /       | /
  |/        |/

Comparator output for detection of half level, detector output reduced
to half level amplitude:

---    |-----
  |    |    |
  |    |    |
  |    |    |
  |-----    |--

VCO shape subracted with the Comparator output signal:

 /|    /    |
/ |   /|   /|
  |  / |  / |
  | /  | /  | /
  |/   |/   |/

Amplify up with 2 and you got a full-swing double frequency ramp.

This method allows for recursive approach, thus you can cascade it
into become 4, 8, 16, 32 and 64 time the frequency. There is an issue
of speed of op-amps if you want to get higher speeds.

Note that when you casecade these, you can use the output of the
comparators as the output of a counter ;)

I migth drop a schematic of such a waveshaper if people requests it.

Cheers,
Magnus