Some synths have a knob called ‘Sub Oscillator’ or ‘Sub Octave’
This could be done with a VCO tuned an octave lower than the main VCO, but this would be an expensive feature. The way it can be done cheaply and effectively is by using a 4013 flip-flop
The 4013 is a dual D type flip flop, so this will divide up the input frequency, using one flip flop we get a square wave output which is an octave below the input, chain this into the second flip flop we get an output that’s 2 octaves lower.
Mixing these together gives an interesting waveform output.
On the rising edge of the clock pulse, the output is latched to what is on the data pin (high or low) because we feed back the inverted output this happens every other clock pulse
Dual flip flop circuit – Note this will only work with a square wave input.
The SH-101 used the ramp wave through a transistor circuit to get a square output, this way whatever PWM you have on your square wave won’t be on the sub waveform.
But it will work with the PWM, you’ll just get a -1 octave with the set duty cycle.
Above: Yellow – Input wave. Green: Output wave (one octave lower)
Above: Interesting mixed output of both waveforms (Green)