Speaker
Description
We show that low-phase noise and high-frequency stability can be simultaneously achieved in microwave sapphire oscillators. We describe the 9 GHz sapphire oscillator with interferometric signal processing, which was phase-locked to a stable RF reference by controlling microwave power dissipated in the sapphire resonator. The SSB phase noise of the oscillator was measured to be close to -170 dBc/Hz at Fourier frequency F = 10 kHz [1]. The fractional instability of the oscillator frequency was approximately 2x10^{-13} for integration times from 5 to 50 s.
The use of cryogenic sapphire resonators promises significant improvements in the phase noise performance of microwave oscillators [2]. Yet, serious attention must be paid to the noise mechanisms affecting the cryogenic resonators. The vibrations induced by cryocoolers and power-to-frequency conversion in the sapphire resonator are expected to be the leading causes of the oscillator's excess phase noise. In our recent experiments, we measured the power-to-frequency conversion of the cryogenic sapphire resonator as a function of Fourier frequency. We found that the resonator response to the fast variations of dissipated microwave power is similar to the transfer function of the 1st-order low-pass filter with corner frequency close to the resonator's loaded bandwidth [3]. The measurements were performed with three almost identical resonators cooled to 6 K and excited in the same whispering gallery mode with a resonant frequency near 11.2 GHz. Having measured the cryogenic sapphire resonator's power-to-frequency conversion, we predicted the phase noise spectrum of the cryogenic sapphire oscillator.
References
1. E. N. Ivanov and M. Tobar, “Low Phase Noise Sapphire Crystal Microwave Oscillators: Current Status”, IEEE Trans. on UFFC, v. 56, no.2, pp.263-269, 2009.
2. E. Ivanov and M. Tobar, “Noise Suppression with Cryogenic Resonators,” Microwave and Wireless Components Letters, vol. 31, Issue 4, pp. 405-408, April 2021, DOI: 10.1109/LMWC.2021.3059291, Print ISSN: 1531-1309, Online ISSN: 1558-1764
3. E. Ivanov and M. Tobar, “Power-to-Frequency Conversion in Cryogenic Sapphire Resonators,” Microwave and Wireless Components Letters, page(s): 1-4, Print ISSN: 2771-957X. Online ISSN: 2771-9588, Digital Object Identifier: 10.1109/LMWT.2023.3264975
