- Voltage-controlled oscillator
A voltage-controlled oscillator or VCO is an
electronic oscillatordesigned to be controlled in oscillation frequencyby a voltageinput. 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).
Control of frequency in VCOs
For high-frequency VCOs the voltage-controlled element is commonly a
varicapdiode 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.
Voltage-controlled crystal oscillators
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 factorof 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 stabilize the oscillator frequency in applications where
temperaturevaries, such as heatbuildup inside a transmitter.
VCO time-domain equations
::: :::::::* is called the oscillator gain. Its units are hertz per volt.::::* is the symbol for the time-domain waveform that is the VCO's tunable frequency component.::::* is the symbol for the time-domain waveform that is the VCO's output phase.::::* is the time-domain symbol of the control (input) voltage of the VCO; it is sometimes also represented as
VCO freq-domain equations
VCOs are used in:
*electronic jamming equipment
*the production of
electronic music, to generate variable tones,
frequency synthesizers used in communication equipment.
* [http://my.integritynet.com.au/purdic/voltage-controlled-oscillators.htm schematics]
* [http://www.electronics-tutorials.com/oscillators/voltage-controlled-oscillators.htm Practical Example of VCO design ]
Wikimedia Foundation. 2010.