Cryogenic-sapphire-oscillator-based reference signal at 1 GHz with 10(-15) level instability

We describe a 1 GHz reference signal with 10-15 level fractional frequency stability for averaging times longer than 1 s, synthesized from a 10.8 GHz oscillation frequency of a cryogenic-sapphire-resonator-based ultra-stable oscillator and phase locked to a hydrogen maser with a time constant of about 1000 s. The degradation of the short term stability of the reference signal by phase locking to the hydrogen maser was evaluated. We also report on the implementation of the 1 GHz signal down converted to 5 MHz as a reference oscillator for a cesium atomic fountain frequency standard.Ken-ichi Watabe, Shinya Yanagimachi, Akifumi Takamizawa, Takeshi Ikegami, Shin-ichi Ohshima, Giorgio Santarelli, Clayton R. Locke, and John G. Hartnet