A VCXO or voltage controlled crystal oscillator is a crystal oscillator that allows the oscillator frequency to be controlled, "pulled", by an external voltage. Many engineering applications require a frequency source that is electronically tunable. This is a necessary for any modulation scheme used in information transfer. Tuning is typically accomplished by the use of a varactor diode in the resonant circuit. The diode capacitance is a function of the applied voltage. Thus, the applied voltage controls the output frequency of the oscillator. The prefix for such a device is VC, or voltage controlled. Therefore a voltage controlled crystal oscillator is called a VCXO.

A crystal clock oscillator is also known as a XO. Of all the crystal oscillator types, the clock oscillator is the simplest and least costly. However the clock oscillator typically offers the least stability over temperature. Crystal oscillators are primarily used as fixed reference sources for digital circuits, where stability is not as critical. Frequency stability, over a broad range of temperatures, is usually improved by employing a biasing network of thermistors that compensate the oscillator circuit with a quadratic least-squares fit. This class of oscillator is termed temperature compensated crystal oscillator or TCXO. However, the best stability obtainable is usually on the order of 1 ppm. Higher stability, on the order of 1 ppb, is achievable using an ovenized crystal oscillator, or OCXO. These oscillators have a built-in oven that keeps the crystal at a constant temperature – typically equal to the turning point of the crystal. When the voltage tuning and temperature compensating features are combined it is called a VCTCXO or TCVCXO. If an inductor or microstrip is used as the resonator the nomenclature VCO is used. Similarly a dielectric oscillator is termed a DRO. Crystal oscillators as a group offer the greatest stability but suffer from the lowest tuning range. Phase noise is typically the best for this group of oscillators, particularly for the OCXO’s. The frequency range for crystal oscillators is typically limited to below 600 MHz.

Filtronetics Inc can supply VCXO’s in through hole, surface mount (SMD), and connectorized packages. Common VCXO packages are the full DIP, half DIP, and 5 x 7 SMD. VCXO logic includes TTL, HCMOS, CMOS, HCMOS/TTL, and CMOS/TTL. Voltages are typically 3.3 Volts or 5 Volts for voltage controlled crystal oscillators.