Abbreviation for Voltage Controlled crystal Oscillator. A VCXO is a type of oscillator used in many modern digital devices as a master clock source. Essentially it is a stable clock source (as you would expect from a crystal), but with an ability to have its rate “pulled” up or down by changes of input voltage. VCXO’s provide manufacturers with a way to have a stable clock in their devices while still allowing them to be synchronized to an external clock source like word clock. This is often accomplished by using a PLL or Phase Locked Loop to compare the external clock to the output of the VCXO, which will generate a control voltage based on the difference. This voltage is used to vary the frequency of the crystal, thereby keeping it in sync with the external clock.