XO

An ideal clock oscillator is an active device that produces a single frequency output for a DC input. A crystal clock oscillator is also known as a XO. Of all the crystal oscillator types, the XO is the simplest and least costly. However the XO typically offers the least stability over temperature.

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. Many engineering applications require a frequency source that is electronically tunable. Tuning is typically accomplished by the use 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 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. In the case of no temperature compensation or voltage control the device is called a clock oscillator. These are primarily used as fixed reference sources for digital circuits, where stability is not as critical. 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 is limited to below 600 MHz.

Programmable XO’s are now available. Programmable clock oscillators are programmed at the factory to the exact frequency requirement. The main advantage of a programmable clock oscillator is the short lead time and low cost.

Filtronetics Inc can supply XO’s in through hole, surface mount (SMD), and connectorized packages. The full DIP, half DIP, and 5 x 7 SMD packages are the most common. Tri-state crystal clock oscillators are available. XO logic includes TTL, HCMOS, CMOS, TTL/HCMOS, TTL/CMOS, ECL, and PECL. Voltages are typically 3.3 Volts or 5 Volts for XO’s.