Phase noise in multi-gigahertz CMOS ring oscillators

An analysis of the phase noise in differential and single-ended ring oscillators using a time-variant model is presented. An expression for the RMS value of the impulse sensitivity function (ISF) is derived. A closed-form equation for phase noise of ring oscillators is calculated and a lower limit on the phase noise of ring oscillators is shown. Phase noise measurements of oscillators running up to 5.5 GHz are in good agreement with the theory.