Second generation 50 K dual-mode sapphire oscillator

Low-temperature, high-precision sapphire resonators exhibit a turning point in mode frequency-temperature dependence at around 10 K. This, along with sapphire's extremely low dielectric losses at microwave frequencies, results in oscillator fractional frequency stabilities on the order of 10/sup -15/. At higher temperatures the lack of a turning point makes single-mode oscillators very sensitive to temperature fluctuations. By exciting two quasi-orthogonal whispering gallery (WG) modes in a single sapphire resonator, a turning point in the frequency-temperature dependence can be found in the beat frequency between the two modes. A temperature control technique based on mode frequency-temperature dependence has been used to maintain the sapphire at this turning point and the fractional frequency instability of the beat frequency has been measured to be at a level of 4.3 /spl times/ 10/sup -14/ over 1 s, dropping to 3.5 /spl times/ 10/sup -14/ over 4 s integration time.