A cavity oscillator comprising: an amplifier operable to provide energy to a local oscillating signal; a superconducting resonant cavity arranged to receive the local oscillating signal and being operable to reflect a portion of the local oscillating signal for use in a feedback circuit, to support an electromagnetic wave related to the local oscillating signal in a desired TM mode, and to output a filtered signal of a desired frequency or frequencies based on the geometry of the cavity; a feedback circuit operable to generate an error signal based on the local oscillating signal and the reflected signal; and a phase shifter arranged operable to adjust the filtered signal based on the error signal to generate an adjusted filtered signal. The amplifier can amplify the adjusted filtered signal.