[sdiy] Current driving a CD4046

[Grant Richter]

Just as a note of interest.

I needed a very wide range VCO for playing the 256 step pages of the  
Envelooper MARF.

I knew a CD4046 would run 256 kHz at 5 volts which I needed to play  
the 256 steps in one millisecond.

I hooked pin 9 to +2.5 volts through a voltage divider and ran a  
regular NPN exponential current source through a 10K to pin 11 (max  
frequency). It produces a square wave from 15 Hz to 280 kHz with a  
100 pF timing cap.

Since I don't need perfect 1 volt per octave tracking (just close  
enough to half / double rhythms), I didn't test for precise 1 volt  
per octave pitch performance.

With more work it might work well enough to drive a counter for a  
wavetable VCO, a Walsh generator or something else that needs an RF  
clock at 5 volts.