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RF/Microwave Low Noise Oscillators

Theories have been developed which show how minimum sideband noise can be obtained in oscillators. It is shown that there are a number of optimum coupling coefficients between the resonator and amplifier as illustrated in the figure below. Typically the optimum noise performance occurs when QL/Q0 is 1/2 or 2/3 dependent on the definition of power within the oscillator. In General QL/Q0 = 1/2 is optimum. Low noise oscillators operating from 1 MHz to 10 GHz have been built which use Inductor Capacitor (LC), crystal, Surface Acoustic Wave (SAW), Dielectric, Helical and Transmission Line Resonators. These oscillators demonstrate noise performance usually within 1dB of the predicted minimum. MMIC low noise oscillators have been designed and fabricated. Tunable low noise oscillators (both hybrid and MMIC) have been built and are under further investigation. Non linear modelling techniques, using Volterra Series, have been developed which accurately predict the output power and the operating frequency. Ultra-low noise reference standards for operation up to 20 GHz are being developed using sapphire dielectric resonators. Present performance at 7.5 GHz exceeds -143dBc/Hz @ 10 kHz offset. Improvements of around 35dB are currently under investigation. New techniques have just been developed (Sept. 2000 see publications, patent applications and book below) which suppress the flicker noise by 20dB down to the thermal limits for offsets above 10kHz. The phase noise results are now within 1/2 dB of the theoretical minimum noise performance set by thermal noise. This is achieved by using feedforward amplifiers as the oscillator sustaining stage and incorporating the limiting elsewhere.


  • Jeremy Everard
  • Konstantinos Theodoropoulos
  • Simon Bale
  • Min Xu


  • Start: October 1998