Prediction of phase noise in CMOS distributed oscillators

This paper presents a frequency-domain analytical method for predicting the phase noise in distributed oscillators. The method is based upon a first-order linear model, which is then augmented with the nonlinear effects. Closed form formulae are derived for predicting the distributed oscillator phase noise which contributes from the additive noise and mixing (intermodulation) noise. The principal issues related to phase noise are discussed. Harmonic balance (HB) simulation in advanced design system (ADS) is used to ascertain the accuracy of the phase noise predictions. It is found that the theoretical conclusions are in a good agreement with the phase noise observed in the simulation, which in turn is within acceptable limits. In five test cases, namely, three 4-stage distributed oscillators operating at the three different frequencies and two distributed oscillators in 4-stage and 5-stage respectively operating at the same frequency, the maximum calculated/simulated phase noise error is about 5.0 dBc/Hz at 1MHz offset from the carrier.