A voltage controlled oscillator (VCO) is an oscillator where the principal variable or tuning element is a varactor diode. The VCO is tuned across its band by a "clean" dc voltage applied to the diode to vary the net capacitance applied to the tuned circuit.

VCOs are used in virtually all Spread Spectrum, RF and wireless systems. Every synthesizer and PLL has at least one VCO in it — thus designers should know something about them. Pick one of the links below to find out more information about this important subject!

A voltage-controlled crystal oscillator is used when the frequency of operation needs to be adjusted by a relatively small amount or when exact frequency or phase of the oscillator is critical, or, by applying a varying voltage to the control input of the oscillator, to disperse radio-frequency interference over a range of frequencies to make it less objectionable. Typically the frequency of a voltage-controlled crystal oscillator can only be varied by a few tens of parts per million (ppm), because the high Q factor of the crystals allows only a small "pulling" range of frequencies to be produced.

VCOs are used in:

function generators
the production of electronic music, to generate variable tones,
phase-locked loops
frequency synthesizers used in communication equipment.

Also Read: Multi Wave Oscillator, Crystal Oscillator, Voltage Controlled Oscillator, Colpitts Oscillator, Oscillator Circuit, Mcclellan Oscillator, Clock Oscillator, Hartley Oscillator