Comparison of the effects of intermodulation and amplitude to phase conversion in a transistor stage upon the oscillator phase noise

Phase noise reduction is still an important goal in ultra stable oscillator design. As a contribution to understanding of phase noise generation, two identified mechanisms are analysed. The first one involves nonlinearities. It may be, for example, the nonlinear V-I relationship of the base-emitter transistor junction. In that case this nonlinearity causes the noise spectrum of transistor to be transferred around the RF signal carrier by means of intermodulation. There is no way to avoid such a nonlinearity in an oscillator. For this reason the amplitude of the RF signal is limited. The second one deals with conversion of amplitude noise to phase noise. This may be for example the effect of the parasitic capacitance of the collector-base junction modulated by the amplitude noise or by the signal itself. The effects of those two mechanisms are analysed with help of a simulation software and results are applied to the closed loop of a typical 10 MHz quartz crystal oscillator. Consequences on the power spectral density of its phase fluctuations and the resulting time-domain instabilities of the oscillator frequency are presented and discussed.