Jun 8, 2011
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Oscillator Short Term Stability: (Root) Allan Variance

Normally, when trying to determine how tightly grouped a set of data is, the standard deviation (s, or square root of variance, s2) of that data is calculated. However, when making this calculation on a group of frequency measurements taken from an oscillator, long term stability variables (such as aging and ambient effects) can begin to obscure the short term stability of that oscillator. In this instance a calculation called Allan variance (named after David Allan of NIST) provides better results. Simply put, instead of calculating the variance for an entire set of data at once, the variances from progressing pairs of data points are averaged, minimizing non-short term factors.