Time deviation

Time deviation (TDEV), also known as ${\displaystyle \sigma _{x}(\tau )}$, measures the time stability of a clock source's phase over an observation interval, expressed as a standard deviation of the time variations. This indicates 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), symbolised 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.