A voltage-controlled crystal oscillator (VCXO) 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.
A temperature-compensated VCXO (TCVCXO) is normally used so that temperature does not alter the piezoelectric effect, thereby causing frequency drift due to heat buildup inside of a transmitter.
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