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:
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 temperature varies, such as heat buildup inside a transmitter.
Voltage Controlled Oscillators target radio applications.(M/ A-COM Technology Solutions Announces New Additions to Family of Voltage Controlled Oscillators)
Oct 13, 2011; Integrated into phase lock loop using divide-by-2 output, Models MAOC-009871, MAOC-009872, and MAOC-010334 feature frequency...
"Systems and Methods for Wideband CMOS Voltage-Controlled Oscillators Using Reconfigurable Inductor Arrays" in Patent Application Approval Process
Dec 06, 2012; By a News Reporter-Staff News Editor at Politics & Government Week -- A patent application by the inventors Park, Yunseo...