A voltage-controlled oscillator or VCO is an electronic circuit that uses amplification, feedback and a resonant circuit to generate a repeating voltage waveform. The frequency, or rate or repetition per unit of time, is variable with an applied voltage, while alternating current audio or other signals may be fed into the VCO to generate FM (frequency modulation).

A version using a quartz crystal is sometimes found in radio transmitters for frequency-modulating an input signal. The piezoelectric effect of quartz causes it to vibrate at a very high frequency, producing radio waves. VCOs are also used in the production of electronic music, to generate variable tones.

See also: oscillation, electronic oscillator, modular synthesizer