Fast Oscillator Phase Noise Estimation Based on AC Analysis

A phase noise simulation method is discussed that is capable of estimating oscillator phase noise using standard AC analysis. Although its accuracy and scope are lower compared to available commercial tools, the simulation method has a number of useful and distinctive properties, including speed and good convergency. Moreover, its is ideally suited to numerically verify insightful behavioral models of oscillators. The principle of AC-based phase noise analysis is reduction of the open-loop gain in an oscillator to a value (1 - e), with arbitrary but small e. When the oscillator is simulated using AC noise analysis for a small e, the simulated output noise spectrum has a Lorentzian shape. By taking a noise value reading at an offset frequency fm on the -6 dB per octave part of this curve, dividing it by the carrier and subtracting 3 dB to remove the AM noise contribution, a value for L(fm) is obtained. Two simulation examples, one numerically verifying a behavioral model, and one predicting phase noise of a measured 3.6 GHz LC oscillator, illustrate the practical use of the simulation method. In addition, the AC-based phase noise simulation of a two-integrator oscillator is briefly discussed.