# Modified Allan variance

The modified Allan variance (MVAR),[1] also known as mod σy2(τ), is a variable bandwidth modified variant of Allan variance, a measurement of frequency stability in clocks, oscillators and amplifiers. Its main advantage relative to Allan variance is its ability to separate white phase noise from flicker phase noise.

The modified Allan deviation (MDEV),[1] also known as mod σy(τ), is the deviation variant of the modified Allan variance.

## Background

The Allan variance has a drawback in that it is unable to separate the white phase modulation (WPM) from the flicker phase modulation (FPM). Looking that their response to Power-law noise it is clearly seen that WPM and FPM has almost the same response to tau, but WPM is linearly sensitive to the system bandwidth fH whereas FPM is only weakly dependent on it. Thus, by varying the system bandwidth the WPM and FPM noise forms may be separated. However, it is impractical to alter the hardware of the measurement system. By post-processing the sample-series and implementing a software bandwidth a modified Allan variance measure can be given capable of resolving the noise forms.

## Definition

The modified Allan variance is defined for [2] using time error samples as

$\operatorname{mod}\sigma_y^2(n\tau_0) = \frac{1}{2\tau^2}\left\langle \left[ \frac{1}{n}\sum_{i=0}^{n-1}x_{i+2n}-2x_{i+n}+x_i\right]^2 \right\rangle$

or with average fractional frequency time series

$\operatorname{mod}\sigma_y^2(n\tau_0) = \frac{1}{2}\left\langle \left[ \frac{1}{n}\sum_{i=0}^{n-1}\bar{y}_{i+n}-\bar{y}_i\right]^2 \right\rangle$

where n is the integer number of samples averaged over.

## Estimators

The modified Allan variance estimator for time error time series is

$\operatorname{mod}\sigma_y^2(n\tau_0) = \frac{1}{2n^4\tau_0^2(N-3n+1)}\sum_{j=0}^{N-3n} \left\{ \sum_{i=j}^{j+n-1} x_{i+2n}-2x_{i+n}+x_i\right\}^2$

or with average fractional frequency time series

$\operatorname{mod}\sigma_y^2(n\tau_0) = \frac{1}{2n^4(M-3n+2)}\sum_{j=0}^{M-3n+1} \left\{ \sum_{i=j}^{j+n-1} \left( \sum_{k=i}^{i+n-1}\bar{y}_{k+n}-\bar{y}_k\right) \right\}^2$

## References

1. ^ a b NIST SP 1065: [1] Handbook of Frequency Stability Analysis
2. ^ David W. Allan and James A. Barnes: A modified "Allan variance" with increased oscillator characterization ability, 35th Annual Frequency Control Symposium, 1981