Spur Immunity in MASH-Based Fractional-N CP-PLLs with Polynomial Nonlinearities


Spurious tones appear in the output phase noise spectrum of a fractional-N frequency synthesizer when nonlinearities are present in the loop. These spurs are generated by the interaction between the nonlinearity and the quantization error that the digital Delta Sigma modulator (DDSM) introduces into the system. This paper analyzes the spurious effect of polynomial nonlinearities on fractional-N CP-PLLs equipped with multi-stage noise shaping (MASH) DDSMs of different orders. This leads to a proof of immunity from spurs for PLLs based on higher order MASH modulators in the case of certain polynomial nonlinearities.

Publication Title

IEEE Transactions on Circuits and Systems I: Regular Papers