[sdiy] PLL / VCO

[harrybissell]

Andrew Wyld wrote:
Hi there,

I am wondering if it is possible to use an arbitrarily weird VCO in a
PLL.
Basically I want to use a square wave from a computer (specifically a
horribly cannibalized Commodore 64) to drive an analogue VCO, making a
drift-free synth that can be sequenced and controlled completely from
the
computer console, but still sounds like an analogue synth.

I'm figuring I can do this if I do some kind of horrible gating
operation on
the VCO.  What would people suggest I read up on / look up on the net /
etc?

Of course its possible.  An arbitrary VCO needs to have an acceptable
range, and some
waveform that will be stable... specifially a square wave output.  You
need to feed this
to a phase discriminator, and filter the output and send it to the VCO.

If your 'arbitrary' VCO does not have a square wave out, you'll need to
make one.
The toughest challenge will porobably be the loop filter...which will
set the speed that
the loop can lock at. Too slow and you will have a permanent glide..too
fast and it will
bounce and overshoot.  Just right might not even be 'just right'...it
depends on how
fast you plan on running that sequencer.

Check the data sheet for the CD4046 which should get you started. Also
look up
Scott "Fatman" Gravenhorst of this list... who has used the CD4046 a lot
in his
PAiA Fatman modifications.

H^) harry