An ultra-low noise microwave oscillator based on a high-Q liquid nitrogen cooled sapphire resonator

Two liquid nitrogen cooled Sapphire Loaded Cavities (SLC) operating at about 80 K have been successfully constructed. Both resonators were designed to operate on the Whispering Gallery (WG)E_{12, 1, δ} mode at a resonant frequency of 8.95 GHz. The highest unloaded Q-factor measured was approximately 60 million for the first SLC and 40 million for the second. The first was used as the frequency determining element in a loop oscillator, while the second was used as a frequency discriminator to measure oscillator phase noise. The single sideband phase: noise of a free running loop oscillator incorporating the first SLC was measured as -133 dBc/Hz at an offset frequency of 1 kHz, and was limited by the SLC Q-factor and the amplifier flicker phase noise. By using specially designed feedback electronics the oscillator phase noise: was reduced to -156 dBc/Hz at 1 kHz. This measurement was shown to be limited by the electronic flicker noise imposed by the phase detector in the feedback electronics. Optimising the oscillator further, we measured by self discrimination a phase noise of -162 dBc/Hz at kHz. Further analysis reveals that this type of liquid nitrogen cooled oscillator has the potential to reach -177 dBc/Hz at 1 kHz