Medium-term frequency stability of hydrogen masers as measured by a cesium fountain

Hydrogen masers are used extensively in time scale ensembles, but the frequency drift must be accurately known. NIST-F1, the cesium fountain primary frequency standard at the National Institute of Standards and Technology, now operates nearly every month and has provided an unprecedented opportunity to characterize the medium-term frequency stability (τ = 30 to 90 days) of six hydrogen masers. An unexpected observation was a moderate level of correlation in the frequency fluctuations of all the masers.