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 transconductance amplifier, 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). There are in fact two VCAs, which are feed with the same input, but their outputs can be used independently. This came in naturally as the LM 13700 contains two transconductance amplifiers. Hence, the VCO module has two current outputs. Current outputs allow for easy mixing into a simple resistor load.