Improved cryogenic sapphire oscillator with exceptionally high frequency stability

Extremely high short term frequency stability has been realised in oscillators based on liquid helium cooled sapphire resonators with a modified mounting structure. These oscillators have exhibited a fractional frequency stability (Allan deviation) of similar to 5.4 x 10(-16)tau(1/2) for integration times (a of 1-4s and a minimum Allan frequency deviation of 2.4 x 10(16) at 32s. For integration times greater than 100s the oscillator frequency stability degrades approximately as 3 x 10(-17)tau(1/2) This is the best stability reported to date for any frequency standard over integration times of 1-100s.