Measurement of Frequency Stability in Time and Frequency Domains via Filtering of Phase Noise

A recently developed theoretical analysis has shown that it is possible to measure the Allan variance (a time-domain measure of frequency instability) without any statistical treatment of data from an electronic counter. The measurement is made via high-pass filtering of phase noise with a test set similar to the one used for frequency-domain measurements. The unique test set described in this paper relies on this principle and is capable of measuring the short-term frequency instability of the best quartzcrystal oscillators in both time and frequency domains.