For crystal oscillators that have a voltage-control function, called a VCXO, the designer can tune the output frequency by varying the voltage control (VC) pin, typically pin 1. Internally, a voltage variable capacitor, or varactor, is put in series with the crystal. The varactor changes its capacitance based on the tuning voltage applied and in turn the crystal will change its associated frequency based on the capacitive load it sees at its input. For a VCXO, another important term to know is Absolute Pull Range (APR). An APR is the minimum pull range minus the frequency tolerance and aging.