High-Q TE stabilized sapphire microwave resonators for low noise applications

Two low-noise high-Q sapphire loaded cavity (SLC) resonators, with unloaded Q values of 2 × 10⁵ and very low densities of spurious modes have been constructed. They were designed to operate at 0°C with a center frequency of 10.000000 GHz. The cavity was cooled with a thermoelectric (TE) Peltier element and in practice achieved the required center frequency near 1°C. The resonator has a measured frequency-temperature coefficient of -0.7 MHz/K and a Q-factor which is measured to be proportional to T^2.5. An upper limit to the SLC residual phase noise to L(100 Hz) = -147.5 dBc/Hz, L(1 kHz) = -155 dBc/Hz, and L(10 kHz) = -160 dBc/Hz has been measured. The authors have created a free running loop oscillator based on the SLC resonator and measured its frequency noise using the other as a discriminator