What is Short Term Stability in a Crystal Oscillator?

Short-term stability of a crystal oscillator is a measure of the oscillator’s frequency variation over a short period of time. It is typically measured for 1 to 10 seconds and is primarily caused by noise from electronic components in the oscillator circuits.

The short-term stability can be measured by four main parameters: Allan variance (the most common one specified in oscillator data sheets), phase noise, spectral density of phase deviations, and the spectral density of fractional frequency deviations. Most manufacturers specify the oscillator's short-term stability in terms of Allan variance.

Allan variance is a statistical tool (also known as a two-sample variance) and is the most commonly used time-domain measure of frequency stability (short-term stability) in oscillators, clocks, and amplifiers. This technique is a unitless measure of frequency stability, and it can be used to quantify the short-term stability of crystal oscillators. In crystal oscillator datasheets, the short-term stability is specified as a number (for instance 5 x 10^-11 – called Allan variance). A low value of Allan variance indicates good stability over the measured period (i.e., it has negligible short-time frequency variation) over a measured period (typically 1 sec).

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