Allan variance
The
Allan variance, also known as
two-sample variance, is a measurement of accuracy in
clocks.
It is defined as one half of the
time average over the sum of the squares of the differences between successive readings of the
frequency deviation sampled over the sampling period.
For most real-world systems, the Allen variance depends on the time period used between samples: therefore it is a function of the sample period, as well as the distribution being measured.
A low Allan variance is a characteristic of a
clock with good stability over the measured period.
The Allan variance is conventionally expressed by σy2(τ). The samples are taken with no dead-time between them.
Source: from Federal Standard 1037C
See also: