Frequency stability of 1x10(-13) in a compensated sapphire oscillator operating above 77K

We report on tests of a frequency-stable temperature compensated sapphire oscillator (CSO) at temperatures above 77 K[1]. Previously, high stability in sapphire oscillators had only been obtained with liquid helium cooling. Recent improvements include a more careful analysis of the ac frequency-lock ''Pound'' circuitry that now enables the oscillator to reliably attain a stability 6 million times better than its fractional resonator linewidth. The frequency stability of the current resonator with a quality factor of Q approximate to 2x10(6) is approximately the same as for the very best available quartz oscillators. The apparent CSO flicker floor is 7.5x10(-14) for measuring times between 3 and 10 seconds, with stability better than 2x10(-13) for all measuring times between 1 and 100 seconds. We project a stability of 2x10(-14) for a resonator Q of 10(7) value about one third of the intrinsic sapphire Q at this temperature.