Advanced phase noise suppression technique for next generation of ultra low-noise microwave oscillators

In this paper we report an advanced phase noise reduction technique developed to improve the short term frequency stability of the microwave oscillators. The technique is based upon the ultra sensitive microwave frequency discriminator with the effective noise temperature close to its physical temperature. The frequency discriminator comprises a room temperature sapphire loaded cavity operating on a “whispering gallery” mode with an unloaded Q factor of 185000 at 9 GHz. The phase noise spectral density of the microwave loop oscillator, incorporating such a discriminator as a sensor for the frequency servo, has been measured to be equal to S_φ^osc(F)≈-60-10log₁₀(F³ ) dBc/Hz at frequencies below a few kHz. This corresponds to a level of phase noise as low as -120 dBc/Hz and -150 dBc/Hz at offset frequencies 100 Hz and 1 kHz respectively