Cryogenic Oscillators
Cryogenic sapphire low-phase-noise microwave resonators
NPL has developed a design of microwave oscillator offering, at 9.2 GHz, a fractional frequency instability below one part in 1014 for any time between 1 and 50 seconds, and a (double-sided) phase noise of –87.5 dBc at a 1 Hz offset (referenced to 9.2 GHz).
The stability of this oscillator derives from a cryogenic resonator containing a ring of high-purity mono-crystalline sapphire that supports a ‘whispering-gallery’ electromagnetic mode with a Q-factor on the order of one billion. This mode provides a frequency reference to which the oscillator is locked by way of a Pound servo. The sapphire ring is maintained, within a cryostat, at a temperature close to the boiling point of liquid helium (4.2 K), where the frequency of the whispering gallery mode exhibits a turning point as a function of temperature. This turning point, combined with the high Q-factor of the whispering gallery mode, provides extreme frequency stability and low phase noise.
The applications of this type of microwave oscillator include use as the local oscillator for a caesium fountain primary frequency standard or a rubidium fountain frequency standard.