The effect of electrical noise on the spectral purity of oscillators

This thesis is a study of the effect of electrical noise in oscillators. An ideal noiseless oscillator would produce a line spectrum with all the oscillator power being generated at discrete frequencies. A real oscillator generates power over a wide band of frequencies, although most of it is close to the fundamental and harmonic frequencies. The power density spectrum of a noisy oscillator is shown in Figure 1.1, with the effects of the noise exaggerated so that it can easily be seen. The power generated at the fundamental frequency of oscillation lies in a very narrow band, with a 3 dB bandwidth usually much less than 1 Hz for a 100 MHz oscillator. The power density spectrum well away from the fundamental and harmonic frequencies is usually only a little above that due to thermal noise in the output impedance of the oscillator. Thus the effect of noise on the spectrum of most oscillators is very slight. For many purposes it is satisfactory to consider that an oscillator produces power only at discrete frequencies. There are however, some situations where the spectral purity of an oscillator is important. One of these situations occurs with the testing of VHF mobile radio receivers where a signal generator of high spectral purity is required. It was for this reason that Marconi Instruments suggested it would be worthwhile studying the effect of noise in oscillators. In this thesis variable frequency VHF oscillators of high spectral purity will mainly be considered. Much of the theoretical work however, is applicable to the effect of noise in other types of oscillators.