Crystal oscillators

Updated June 14, 2004

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We will start this page with some definitions:

Crystal oscillator (XO)
A crystal oscillator without any bells or whistles, can provide maybe 100 ppm frequency stability over a moderate temperature range. The problem is that crystal oscillators by nature are temperature sensitive.

Ovenized crystal oscillator (OXO)
Using specialized heating elements, controllers and temperature chambers, the stability of a crystal oscillator can be improved to 10^-10 (0.0001 PPM).

Voltage controlled crystal oscillator (VCXO)
This is a crystal oscillator that allows you to tune its frequency using an external voltage control. This is that heart of many a phase-lock loop, automatic frequency control and frequency-modulation scheme. The element that does the tuning is a varactor (voltage-variable capacitor).

Temperature-compensated crystal oscillator (TCXO)
Similar to a VXCO in that a varactor is used to vary the frequency of oscillation. An internal temperature compensation network using thermistors or other temperature sensors is used to provide a shaped voltage control signal to the device over temperature, so that stable frequency operation is provided.

Voltage controlled temperature-compensated crystal oscillator (VCTCXO)
Here an external voltage control is provided, but the signal is "reshaped" by the temperature compensation network so that stability over temperature is high but frequency is still controlled by an externally applied voltage.