Cryogenic sapphire oscillator with exceptionally high long-term frequency stability

A cryogenic sapphire microwave resonator oscillator (CSO) has been constructed, which now consistently exhibits the best measured long-term and fractional frequency performance for such an oscillator. The single oscillator long-term square root Allan variance was measured at about 2 x 10^-17radictau with a negative drift of about 2.2 x 10^-15/day for integration times tau >1000 s, limited by diurnal cycles. The short-term fractional-frequency is highly reproducable and also state-of-the-art: 5.6 x 10^-16 for an integration time tau = 20 s. The CSO´s short-term stability represents a performance improvement by a factor of 40 compared to the 1990 version of the CSO, but by a factor of 5 on the 1995 version, yet by a factor of 10 on the long term performance of the latter. Also a comparison is made with the 2000 version.