Tunable microwave oscillator for low phase noise applications

A low-phase noise 9 GHz oscillator with a tuning range of +/-12 kHz has been developed. The SSB phase noise of the oscillator in the middle of the tuning range is of the order -145 dBc/Hz at I kHz Fourier frequency. The oscillator phase noise is suppressed by a frequency control system based on an ultra-sensitive frequency discriminator. The latter consists of a microwave interferometer with a high-Q resonator in one of the arms and a phase sensitive microwave readout system. Suppressing the carrier at the output of interferometer enables the small signal operation of the microwave readout system and effective cancellation of the oscillator phase noise. The frequency of the oscillator is tuned by varying the phase mismatch of microwave interferometer without affecting the high-Q resonator. To minimise the degradation of the oscillator phase noise during frequency tuning, an automatic carrier suppression system is introduced to keep the microwave interferometer balanced.