[sdiy] VCO tuning philosophy re-visited

[Ian Fritz]

>As an aside, there's another kind of design called a "charge pump" that I 
>think avoids the droop issue; instead of discharging the integrator cap 
>through a "resistor" (i.e. a FET or some other transistor acting as a 
>switch), it actually shoves current in the opposite direction. The Rhodes 
>Chroma oscillator does this. I think the ultrasonic VCO Ian Fritz uses in 
>his Double Decka (huh, it came up again) does something like that too

Right. The DD uses an LM331 V-F converter in its core. This works on the 
charge balance principle, which does indeed involve a charge 
injector.  This chip has phenomenal 0.01% tracking, which is a consequence 
of the integrating current flowing over the whole period of a cycle (no 
error due to discharge time).

Ian