Improved cryogenic sapphire oscillator with exceptionally high frequency stability

Extremely high short term frequency stability has been realized 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 approximately 5.4x10 super(-16) tau super(-1/2) for integration times ( tau ) of 1-4 s and a minimum Allan frequency deviation of 2.4x10 super(-16) at 32 s. For integration times greater than 100 s the oscillator frequency stability degrades approximately as 3x10 super(-17) tau super(1/2). This is the best stability reported to date for any frequency standard over integration times of 1-100 s.