Doepfer A-110 Standard VCO

Module A-110 is a voltage-controlled oscillator. This VCO's frequency range is about eight octaves. It can produce four waveforms simultaneously - square, sawtooth, triangle, and sine wave. The Frequency or pitch of the VCO is determined by the position of the octave (Range) switch and tuning (Tune) knob, and by the voltage present at the CV inputs.

Frequency modulation (FM) of the VCO is therefore a possibility. Footage (the octave of the fundamental) is set by the Range control, and Fine tuning controlled by the Tune knob. You can control the pulse width of the square wave either by hand, or by voltage control - Pulse Width Modulation or PWM.