Optimisation and synthesis of low phase-noise microwave-feedback oscillators

A procedure is presented for the improved design of low phase-noise microwave-feedback oscillators. The measured circuit parameters of the individual circuit elements are used in a nonlinear model which, in conjunction with a parametric computer-aided-design technique, allows one to predict the operating point of the circuit, namely the oscillation frequency and output power level. From this, together with the amplifier noise figure and Q-factor of the feedback element, the phase-noise spectral density of the oscillator output signal is derived. Theoretical and experimental results are compared and found to be in good agreement. The computer-aided design technique is also used to study the effects of various S-parameter variations upon the output power and phase spectral density. From perturbation studies it is shown how optimum sets of S-parameters can be derived to obtain either maximum output power or minimum phase noise, as desired. A correcting network for a first amplification stage is synthesised and the reduction of phase noise is experimentally verified.