The ES20 starts with a classic sawtooth oscillator core and adds a voltage controlled wave shaper to the output.
Unlike most conventional VCOs' the ES20 does not have a 1V/Octave input. However the scalable input [CV A] of the ES20 can be used in conjunction with the [FREQUENCY] control to provide tonal tracking.
The [CV B] knob can be used as a FINE TUNE control for the oscillator.
The wave shaper allows the [SHAPED] output to swing from a sine wave to a modified sawtooth output and can be both manually and voltage controlled.
See the Patch Of The Week below from Doug for a comparison with the original Serge 1973 VCO.