Low phase noise cylindrical cavity oscillator

A low phase noise oscillator operating at 9.2 GHz is presented. A cylindrical metal cavity with air as dielectric is used as the resonator. To minimize the phase noise of the oscillator, the resonator must be designed to have a high quality factor (Q-factor). A high Q-factor is obtained by designing the resonator to operate in the TE₀₁₁ mode. A tuning screw is used to tune the resonant frequency without significantly affecting the Q-factor, while also separating the resonant frequencies of the degenerate TE₀₁₁ and TM₁₁₁ modes. The signal is coupled to the resonator by means of rectangular apertures. The coupling is designed to minimize the phase noise of the oscillator. A dual mode waveguide filter was developed and inserted into the oscillator loop in order to prevent oscillation at unwanted frequencies. The output signal of the oscillator was measured at different locations in the loop and clearly showed that the resonator can be used as a filter to minimize the phase noise. Phase noise levels of -115 dBc/Hz and -146 dBc/Hz were obtained at offset frequencies of 10 and 100 kHz.