On the role of the additive and converted noise in the generation of phase noise in nonlinear oscillators

In a conventional approach, the oscillator phase noise due to noise sources near carrier is defined as additive phase noise by assuming that the oscillator operates in a near linear fashion. Nevertheless, fundamentally, an oscillator circuit is inherently nonlinear. In this paper, we show that the phase noise generated by noise sources around the fundamental frequency of oscillation is due to two simultaneous and correlated phenomena of the same order of magnitude: additive phase noise and converted phase noise due to conversion from one sideband to another. An analytical calculation applied to a simple purely theoretical circuit allows evaluation of the respective influence of the two above-mentioned phenomena. Numerical simulations performed on a realistic transistor oscillator circuit then confirms the importance of the conversion phenomenon already shown by the analytical evaluation. The converted noise results to be of the order of 6 dB higher than the additive noise. The term of additive phase noise must be intended to characterize the phase noise generated in linear components located out of the nonlinear oscillation loop and, for example, in buffer amplifiers following the oscillator itself.