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 an Allan deviation of about 5.4/spl times/10/sup -16//spl tau//sup -1/2/ for integration times (/spl tau/) of 1 to 4 s and a minimum of 2.4/spl times/10/sup -16/ at 32 s.