A VCXO is a Voltage Controlled Crystal Oscillator whose output/oscillating frequency changes when a control voltage is applied to it under a constant supply voltage. A VCXO consists of a crystal oscillator with a varactor diode and supporting circuitry. The maximum variation in VCXO's output frequency over changes in control voltage under a constant supply voltage and temperature - specified in ppm - is called Pullability or Total Pulling Range.

A VCXO is different from a VCO - Click here to learn about the difference between a VCO and a VCXO.