VCO

A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is governed by a control voltage applied at its input. The control voltage, typically supplied by a keyboard, sequencer, or a phase-locked loop, modulates the frequency so that changes in voltage produce corresponding changes in pitch or carrier frequency. In audio applications, VCOs generally cover the musical pitch range from roughly 20 Hz to several kilohertz, while RF VCOs operate at much higher frequencies.

Most VCOs use a core circuit such as a capacitor-resistor network or a transistor current source whose effective frequency is altered by the control voltage. To produce musical pitches that are evenly spaced in semitones, the frequency often varies approximately exponentially with the control voltage, necessitating an exponential converter or carefully matched transistor networks. Temperature and supply voltage variations can cause drift, so designers employ temperature compensation, stabilization schemes, or feedback from a phase-locked loop to improve frequency stability.

VCOs typically provide multiple waveform outputs, including sine, triangle, sawtooth, and square waves. Some designs offer adjustable symmetry or pulse width for the square wave and include features such as synchronization, frequency drift compensation, and range switching. In analog synthesizers the VCO serves as the primary pitch source, while in RF equipment it functions as a voltage-to-frequency converter within a broader frequency synthesis system.

Related concepts include numerically controlled oscillators (NCOs) and direct digital synthesis (DDS), which generate waveforms digitally rather than from an analog core.