A TCXO or temperature controlled crystal oscillator is a crystal oscillator that incorporates circuitry to reduce the natural temperature sensitivity of the crystal. Usually a network of compensating components, either thermistors and capacitors or thermistors and varactors, are used to control the oscillator frequency so that it maintains the required frequency stability over temperature. Temperature controlled crystal oscillators offer better temperature performance than a clock oscillator and are smaller, less costly, use less power, and have no warm-up time like an ovenized oscillator.

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. These are primarily used as fixed reference sources for digital circuits, where stability is not as critical. Frequency stability, over 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. A VCXO or voltage controlled crystal oscillator is a crystal oscillator that allows the oscillator frequency to be controlled, "pulled", by an external voltage. 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 a VC, or voltage controlled. Therefore a voltage controlled crystal oscillator is called a VCXO. When 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 temperature controlled crystal oscillators in through hole, surface mount (SMD), and connectorized packages. The full DIP, half DIP, and 5 x 7 SMD packages are the most common. Temperature controlled crystal oscillator logic includes HCMOS/TTL, TTL /CMOS, and clipped sine wave. Voltages are typically 3.3 Volts or 5 Volts for temperature controlled crystal oscillators.