Voltage-controlled oscillator

A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillationfrequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency. Consequently, a VCO can be used for frequency modulation (FM) or phase modulation (PM) by applying a modulating signal to the control input. A VCO is also an integral part of a phase-locked loop. VCOs are used in synthesizers to generate a waveform whose pitch can be adjusted by a voltage determined by a musical keyboard or other input.

A voltage-to-frequency converter (VFC) is a VCO which has a very linear transfer function (i.e., the ratio of output frequency to input voltage is nearly constant) over a wide range of input control voltages.