Spur reduction technique for fractional-N frequency synthesizer with MASH 1-1-1-1 Sigma Delta modulator

Abstract

A fourth order MASH Sigma Delta modulator based fractional-N frequency synthesizer was designed and validated as a Matlab/Simulink macro- model. Use of MASH achieves a better noise shaping for the frequencies generated by the synthesizer. With a reference frequency of 20MHz, the fractional-N frequency synthesizer achieves a synthesized frequency range of 200MHz to 220MHz with 2 MHz resolution. The macro-model has a gain margin of 6.02dB and a phase margin of 39.7°, ensuring the stability of the closed loop system with settling time of 3.32μ sec. Spurs that were introduced by the fractional feedback and reference input are removed through notch type Comb filter.