[sdiy] Henry Walmsley's page

[David G. Dixon]

> I had a look at the scheme, i'm still a noob but I was wondering how
> this VCO would track...
> I don't see any tempco so I think I understand why Davind mentions a new
> design with an 2164 ;-)
> 
> I'm intrested if I can help David... (testing / parts... )

Walmsley's VCO current source is at the following link:

http://homepage.ntlworld.com/henry01/waveform_synth/waveform02-02.pdf

It is a standard matched-pair-and-tempco arrangement based on LM394.  From
this one current source, he is tapping three separate currents for various
uses: "INT-CC" drives the interpolator, "HF-CC" drives the high-frequency
clock oscillator (this is basically the "core" of this oscillator, as it is
what defines the frequency of the VCO output), and "TRACK-CC" drives the
cutoff frequency of the tracking filter.

My inclination would be to replace this current source with the 2164 Expo,
but probably leave the rest of the circuit more or less as is.  Of course, I
can never leave well enough alone, so I'd probably simulate the whole mess
and go looking for tweaks, but based on Mr Walmsley's write-up, he seems to
have thought the circuit through exceedingly well, and the proof is in the
scope pictures.