Low noise microwave oscillators based on high-Q temperature stabilized sapphire resonators

This paper reports on development of X-band microwave oscillators incorporating recently developed sapphire resonators. Two resonators have been constructed for oscillator development. One resonator is a room temperature thermoelectric (TE) stabilized sapphire resonator operating at 9.00000 GHz and configured as a two port device with an unloaded Q factor of 187000 and port couplings of 0.74 and 0.17. The second resonator is a liquid nitrogen cooled sapphire resonator with two stale temperature control operating at 8.95 GHz with an unloaded Q factor of about 40 million. The single sideband phase noise of a free running loop oscillator incorporating the room temperature resonator has been measured to be -104 dBc/Hz at 1 kHz with a f^-3 dependence and is limited by the resonator Q and the flicker noise of the active components. By incorporating specially designed feedback electronics we have measured a reduced single sideband phase noise of -125 dBc/Hz at 1 kHz with a f^-3 dependence. For this oscillator configuration the phase noise was limited by the flicker noise of the phase detector in the feed back electronics. Based on the room temperature results we predict that we will obtain better than -155 dBc/Hz at 1 kHz with the liquid nitrogen cooled resonator-oscillator