AN ULTRALOW NOISE MICROWAVE-OSCILLATOR BASED ON A HIGH-Q LIQUID-NITROGEN COOLED SAPPHIRE RESONATOR

Two liquid nitrogen-cooled sapphire loaded cavities (SLC's) operating at about 80 K have been successfully constructed, Both cavities were d esigned to operate on the whispering gallery (WG) E(12,1,delta) mode a t a resonant frequency of 8.95 GHz. The first SLC was used as the freq uency-determining element in a loop oscillator, while the second was u sed as a frequency discriminator to measure oscillator phase noise, Th e single sideband phase noise of a free running loop oscillator incorp orating the first SLC was measured as -133 dBc/Hz at an offset frequen cy of 1 kHz, and was limited by the SLC Q-factor and the amplifier fli cker phase noise, By using specially designed feedback electronics the oscillator phase noise was reduced to -156 dBc/Hz and -162 dBc/Hz at 1 and 10 kHz offset, respectively. This measurement was shown to be li mited by the electronic flicker noise imposed by the phase detector in the feedback electronics. To our knowledge the phase noise and resona tor Q-factor of 6 x 10(7) represent the best results ever measured at liquid nitrogen temperatures or above.