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The origins of phase noise in oscillator systems are reviewed. A simple, self-excited 7.5 GHz oscillator incorporating either a conventional metal cavity resonator or a high temperature superconducting (HTS) cavity has been produced. Comparative measurements made at 77 K demonstrate significant improvements in the close-to-carrier noise performance of the oscillator incorporating the HTS cavity; 10 dB at a 10 kHz offset rising to more than 25 dB at a 10 Hz offset. The unloaded Q of the HTS cavity doubled to >10/sup 6/ on cooling from 77 K to 64 K suggesting that further improvements in noise performance could be achieved.