An investigation of noise in microwave oscillators

Abstract

During recent years the demands on the sensitivity of microwave systems, particularly radar systems, have increased considerably, and a better understanding of the noise properties of microwave oscillators has become an important necessity.

The thesis describes the development of suitable techniques for measuring all oscillator noise quantities of interest. The methods used can be conveniently grouped into direct-detection methods, which determine either a.m. or f.m. noise together with background noise, and the superheterodyne method, which measures total noise. Both types of measuring system may necessitate the suppression of the oscillator carrier, and suitable carrier-suppression filters have been designed. Provision has also been made for a visual display of noise spectra. A method is given by which the modulation noise components can be separated from the background noise. This should help to provide a better insight into the noise generation mechanisms. Further methods for the measurement of the degree of correlation between individual noise quantities are described, and a study was made of the noise generated in microwave mixer crystals which form an essential part of any measuring system.

Detailed investigations, using the measuring techniques developed, have been carried out on two types of low-power oscillator, the backward-wave oscillator and the reflex klystron. The modulation noise spectra obtained for some of the backward-wave oscillators showed undesirable phenomena due to Ion effects which, however, could be eliminated by ion drainage. It is also shown that the backward-wave oscillator noise can rise markedly near the frequency at which spurious oscillations are possible. Noise in the non-oscillating state of the tubes was investigated, and measurements of carrier frequency deviation and the degree of correlation between various noise components, were determined.

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