[sdiy] HF VCOs and tracking problems

[Russell McClellan]

You can get probably to a few 100 kHz with good linearity by using a
charge pump-type VCO.  These work by draining a fixed charge from a
secondary capacitor into the integrating cap to reset the ramp rather
than shorting the integrating cap.  This means that you don't have to
worry about reset time, as the integrating cap is always integrating.
A slightly convoluted example is the "10kHz V-to-F" in the Jim
Williams app note referenced previously (this is the famous "zoo
circuit" I mentioned in a previous thread).  I think the Rhodes Chroma
does something similar, but without the CMOS inverter trick.

-Russell