The VCO module consists of two parts:
The VCO is a classical transistor exponentiator, current controlled Miller integrator and a monoflop controlled reset circuit.
At the wave shaper's core is a 1-transistor sawtooth triangle converter. Sine waves are generated by over-loading the LM 13700 current controlled amp. which is also used as the VCA part
Rectangle waves are generated using a standard comparator design.
Analog multiplexers select the chosen waveform.
The VCO has a final output stage as a voltage controlled amplifier (VCA), in fact two VCAs which are feed with the same input
The VCO module has two current outputs. This came in naturally as the LM 13700 contains two current controlled amps.
Current outputs allow for easy mixing into a simple resistor load.