Quartz Crystal Oscillators

Quartz crystal oscillator is a timing device consisting of quartz crystal resonator and an oscillation circuit to generate an output waveform at specified frequency.  It can be classified into four main categories based on its typical applications.

XO - Crystal Oscillator

The device does not have temperature control or temperature compensation.  Frequency - temperature characteristics depend on the crystal units.  It is also called SPXO – Simple Packaged Crystal Oscillator or Clock.

TCXO - Temperature Compensated Crystal Oscillator

The device has temperature compensation circuit to suppress output frequency deviation caused by surrounding temperature.  That is, e.g. the output signal from a temperature sensor (e.g., a thermistor) is used to generate a correction voltage that is applied to a variable reactance (e.g., a varactor) in the crystal network.  The reactance variations compensate for the crystal's F vs. T characteristic. Analog TCXO can provide about a 20X improvement over the crystal's F vs. T variation.

OCXO - Oven Controlled Crystal Oscillator

The device usually contains an oven block where the temperature sensor, heating element, oven circuitry, and insulation function to maintain a stable temperature. By keeping the temperature of the crystal and other temperature sensitive components, great improvements in oscillator performance are realized such that the crystal's F vs. T has zero slope.  OCXOs can use either AT-, SC-, or IT- cut crystals depending on temperature range and aging performance.  Typical OCXO can provide a >1000X improvement over the crystal's f vs. T variation. The following table summarizes the best stability each type of oscillator can hold based on today’s typical manufacturing process.

Crystal oscillator that has tunable output frequency or modulated output frequency by external control voltage.  The connection of a variable capacitance diode (varactor) with crystal unit in series can lead the diode capacitance to be changed according to the voltage applied for frequency tuning, thus pulling frequency away from its normal value based on the load capacitance characteristics of the crystal unit.