Improved cryogenic sapphire oscillator with exceptionallyhigh frequency stability

Abstract
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 ~5.4 × 10-16τ-1/2 for integration times (τ) of 1 – 4 s and a minimum Allan frequency deviation of 2.4 × 10-16 at 32 s. For integration times greater than 100 s the oscillator frequency stability degrades approximately as 3 × 10-17τ1/2 This is the best stability reported to date for any frequency standard over integration times of 1 – 100 s.

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