VCO's for homebuilt synths

[Joachim Verghese]

On Thu, 10 Aug 1995, SuperBAD MoFo wrote:

> As the Chroma SM put it, the Charge Pump design 
> is often found in instrumentation, but rarely in synths.  The only catch 
> is, I don't know how available the charge pump chip...

I've used this design for some of my VCOs, and it works very well.
The 4151 V-to-F converter has been manufactured by at least Raytheon
(RC4151) and Exar (XR4151). I've been able to get the XR4151 for around
$2 in small quantities. There's also the RC4152, which has better
linearity, but is also much more expensive. National has a second
source for the latter (LM-something...).

> Apparently the charge pump is there to assure that the 
> sawtooth retraces w/o slew at high frequencies.

That's right. The retrace time is around 20 microseconds, during which
the integrating cap is (dis)charged with a constant current. Part of
this current is "stolen" by the exp generator, so there's no HF-tracking 
problem.

To get a sharp sawtooth retrace edge, the saw output is forced to gnd
during retrace by means of an open collector output on the 4151. At
high frequencies this slightly alters the saw shape, making it more
squarish (attenuated even harmonics), but only few harmonics are
audible at high frequencies anyway.

The 4151 together with a temp controlled CA3046 should make for a very
compact and easy-to-set-up VCO.

-joachim