Apa itu Allan deviation?

The Allan deviation $\sigma_y(\tau)$ of a clock is the root mean square of the change in its average fractional frequency between successive intervals of length $\tau$, divided by $\sqrt2$: $\sigma_y^2(\tau) = \tfrac12 \E\bigl[(\bar y_{k+1} - \bar y_k)^2\bigr]$. From phase samples $x_i$ spaced $\tau$ apart it is $\sigma_y^2(\tau) = \E\bigl[(x_{i+2} - 2x_{i+1} + x_i)^2\bigr] / (2\tau^2)$.