# Time deviation

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Time deviation (TDEV), [1] also known as ${\displaystyle \sigma _{x}(\tau )}$, is the time stability of phase x versus observation interval τ of the measured clock source. The time deviation thus forms a standard deviation type of measurement to indicate the time instability of the signal source. This is a scaled variant of frequency stability of Allan deviation. It is commonly defined from the modified Allan deviation, but other estimators may be used.

Time variance (TVAR) also known as ${\displaystyle \sigma _{x}^{2}(\tau )}$ is the time stability of phase versus observation interval tau. It is a scaled variant of modified Allan variance.

TDEV is a metric often used to determine an aspect of the quality of timing signals in telecommunication applications and is a statistical analysis of the phase stability of a signal over a given period. Measurements of a reference timing signal will refer to its TDEV and maximum time interval error (MTIE) values, comparing them to specified masks or goals.

## Definition

The most common estimator uses the modified Allan variance

${\displaystyle \sigma _{x}^{2}(\tau )={\frac {\tau ^{2}}{3}}\operatorname {mod} \sigma _{y}^{2}(n\tau _{0})}$

where ${\displaystyle \tau =n\tau _{o}}$. The 3 in the denominator normalizes TVAR to be equal to the classical variance if the deviations in x are random and uncorrelated (white-noise).

or TDEV, which is the square-root of TVAR, may be derived from MDEV modified Allan deviation

${\displaystyle \sigma _{x}(\tau )={\frac {\tau }{\sqrt {3}}}\operatorname {mod} \sigma _{y}(n\tau _{0})}$

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