This VCO uses analogue switches to achieve the thru 0 function, based on ideas presented in Electronotes EN#75 which were in turn based on suggestions by Bob Moog.

If nothing is patched into the CV INPUT, the CV pot functions as a tuning pot to set the module up for higher frequency operation. Once a CV signal is patched in, the pot becomes an attenuator and the signal is summed with the signal from the 1V/OCT jack. This only happens if you install the “optional” 10k resistor on the bottom PCB, I suggest you do install it.

SELF CONTROL allows the sine output signal to be fed back into the Frequency modulation sub-circuit, the DEPTH control will need to be turned up for this to have an effect, or (better) use a CV on the FM ENV input. SAW and PULSE are both controlled by the PW pot and PWM input. PULSE will have its width varied, SAW will become chopped and morph into a complex, harmonically rich waveform eventually its frequency will double.