voltage controlled crystal oscillator (VCXO)
The voltage controlled crystal oscillator, Voltage Controlled Crystal Oscillator (VCXO), like the Voltage Controlled Oscillator( VCO), uses a capacitance diode controlled by a variable DC voltage for the frequency change. In contrast to VCOs, only minimal frequency changes can be achieved with VCXOs, since the crystal is the frequency-determining component.
The frequency variations, the Absolute Pull Range( APR), therefore range from only a fraction of a part per million( ppm) to several hundred parts per million. Key characteristics of a VCXO are linearity and tuning range, frequency stability and drift, phase noise, and output level as a function of frequency variation.
As for linearity, it is defined as the percentage deviation from the total tuning range. A linearity of 1% means that within a tuning range of, say, 100 kHz, the frequency deviation can be 1 kHz. This 1 kHz frequency deviation can occur anywhere in the tuning range. The tuning range itself is set by the tuning voltage with a capacitance diode. As for drift, it is extremely small in VCXO oscillators. However, it can occur after frequency tuning, before the crystal and the capacitance diode reach thermal equilibrium.