Voltage Controlled Crystal Oscillator (VCXO): In some applications it is desirable to be able to tune or adjust the frequency of the oscillator in order to phase lock it to a reference in a phase locked loop or possibly to modulate the waveform. A VCXO provides this capability via an Electronic Frequency Control (EFC) voltage input. The tuning range specification for a VCXO may vary from ±10ppm to ±100ppm or even higher for some specialized devices.
TCVCXO and VCOCXO: A TCXO or OCXO will often include an EFC input voltage. This allows adjustment in order to calibrate the output frequency precisely to a nominal value.