Analysis and Prediction of Spurs in a Fractional-N Frequency Synthesizer with Discontinuous Nonlinearity

The output phase noise of fractional-N frequency synthesizers shows spurious tones due to nonlinearities present in the system. Recently, Donnelly and Kennedy have provided a numerical method to predict the locations and amplitudes of the spurs. This has been tested and confirmed for charge pump (CP) PLLs with different types of continuous nonlinearities. This paper applies the method to cases where the nonlinearity is discontinuous. This type of nonlinearity can be considered as being representative of digital PLLs with quantization in the loop.