Ultra-low-noise Cavity-stabilized Microwave Reference Oscillator Using an Air-dielectric Resonator

Ultra-low-noise microwave oscillators are often required to serve as reference signals in precision phase modulation (PM) noise measurement systems and in a host of other applications. We have significantly improved the spectral purity of NIST’s traditional cavity-stabilized microwave oscillator design, which uses a conventional air-dielectric cavity resonator as a frequency discriminator. We developed and tested an accurate model of the expected PM noise that indicates, among other things, that a conventional air-dielectric resonator of moderate Q will exhibit less discriminator noise than more esoteric and expensive dielectric resonators tuned to a high-order, high-Q mode and driven at the dielectric's optimum power. Additionally, we increase the discriminator's intrinsic signalto-noise ratio by use of a high-power carrier signal to interrogate an optimally coupled cavity, while the high level of the carrier is suppressed before the phase detector. We report exceptionally low PM noise levels from a microwave oscillator operating at 10 GHz.