How Low Can They Go?: Oscillator Phase Noise Model, Theoretical, Experimental Validation, and Phase Noise Measurements

Noise is associated with all the components of the oscillator circuit; however, the major contribution of the noise in an oscillator is from the active device, which introduces amplitude modulation (AM) and phase modulation (PM) noise. The conventional wisdom is to ignore AM component of the noise because the gain limiting effects of the active device operating under saturation, allowing only little variation in the output amplitude due to the noise in comparison to PM noise component, which directly affects the frequency stability of the oscillator and creates noise sidebands. But in reality, many oscillator topologies create significant AM noise, therefore effective noise contribution is the combination of 1/f spectrum with the 1/f 2 effect in all PM, makes the low-frequency noise much greater, and that´s where the information in most modulated signals resides.