The allan variance – challenges and opportunities

The Allan variance has historically been measured using heterodyne measurement systems, which have low noise and preserve the carrier phase information needed for long-term stability. The single-sideband phase noise has traditionally been measured using phase detectors that suppress the carrier in order to achieve even lower noise. The recent development of the direct-digital phase noise measurement technique makes it possible to estimate both statistics simultaneously from the same time series of the phase [1]. Our comparison of the three techniques has revealed several challenges to the accurate estimation of the Allan variance including undesired aliasing, biased estimators, and spurious signal generation. Successfully overcoming these challenges has resulted in faster, easier, more reliable, and more accurate measurement methods.