Combined stability of hydrogen maser and superconducting cavity maser oscillator

The authors combined a superconducting cavity maser oscillator (SCMO) and a hydrogen maser to form a standard which shows the medium-term performance of the hydrogen maser together with improved short-term performance available only from cryogenic oscillators. A double-loop receiver combines the two sources with a 100-MHz quartz crystal oscillator. Fractional frequency stability of 2×10^-14 was obtained for a measuring time of τ=1 s, and 1×10 ^-15 at τ=1000 s. The 1-s value is almost as low as the floor (1×10^-14) for the measurement system, while the 1000-s value is identical to hydrogen maser performance. The design is based on models of hydrogen maser and SCMO phase noise for offset frequencies down to 1×10^-6 Hz