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A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation (FM) or phase modulation (PM); a VCO with digital pulse output may similarly have its repetition rate (FSK, PSK) or pulse width modulated (PWM).

For high-frequency VCOs the voltage-controlled element is commonly a varicap diode connected as part of an LC tank circuit. For low-frequency VCOs, other methods of varying the frequency (such as altering the charging rate of a capacitor by means of a voltage controlled current source) are used. See Function generator.

A voltage-controlled crystal oscillator (VCXO) is used when the frequency of operation needs to be adjusted only finely. The frequency of a voltage-controlled crystal oscillator can be varied only by typically a few tens of parts per million (ppm), because the high Q factor of the crystals allows "pulling" over only a small range of frequencies.

There are two reasons for using a VCXO:

• To adjust the output frequency to match (or perhaps be some exact multiple of) an accurate external reference.
• Where the oscillator drives equipment that may generate radio-frequency interference, adding a varying voltage to its control input can disperse the interference spectrum to make it less objectionable. See spread-spectrum clock generation.

A temperature-compensated VCXO (TCVCXO) incorporates components that partially correct the dependence on temperature of the resonant frequency of the crystal. A smaller range of voltage control then suffices to stabilise the oscillator frequency in applications where temperature varies, such as heat buildup inside a transmitter.

VCOs are used in:

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

• VFO
• VCF
• VCA
• modular synthesizer
• Numerically-controlled oscillator (Also "digitally-controlled oscillator" or DCO)
• Phase-locked loop

• schematics
• Practical Example of VCO design