Cryogenic sapphire oscillator with exceptionally high frequency stability

Extremely high short-term frequency stability has been realized in microwave oscillators based on liquid-helium-cooled sapphire dielectric 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. For integration times longer than 100 s, the frequency stability is approximately 3/spl times/10/sup -17//spl tau//sup 1/2/, and the relative drift per day is about 10/sup -13/. This stability is the best reported to date for any microwave source over integration times less than 100 s.