IEVref: 561-03-02 ID: Language: en Status: Standard Term: Allan variance of fractional frequency fluctuation Synonym1: Synonym2: Synonym3: Symbol: Definition: unbiased estimate of the preferred definition in the time domain of the short-term stability characteristic of the oscillator output frequency given by ${\sigma }_{y}{}^{2}\left(\tau \right)\cong \frac{1}{M–1}\sum _{k=1}^{M–1}\frac{{\left({Y}_{k+1}–{Y}_{k}\right)}^{2}}{2}$ where Yk are the average fractional frequency fluctuations obtained sequentially, with no systematic dead time between measurements; τ is the sample time over which the measurements are averaged; M is the number of measurementsNote 1 to entry: The confidence of the estimate improves as M increases. Note 2 to entry: This entry was numbered 561-04-18 in IEC 60050-561:1991. Publication date: 2016-07 Source: Replaces: 561-03-02:2014-11 Internal notes: CO remarks: TC/SC remarks: VT remarks: Domain1: Domain2: Domain3: Domain4: Domain5: