Fractional Spurs Reduction Technique Using Probability Density Shaping Sigma-Delta Modulator and Fractional Frequency Divider

A fractional spurs reduction technique using probability density shaping (PDS) sigma-delta modulator (SDM) and fractional frequency divider is presented. The PDS-SDM is utilized to reshape the quantization noise probability density. If a certain condition is obeyed, the loop nonlinearity would not induce fractional spurs. Moreover, in order to reduce the total quantization noise power, a fractional frequency divider, e.g., phase-interpolated (PI) frequency divider, is adopted to reduce the division ratio from 1 to 0.25, leading to 12 dB quantization noise suppression. By combining the PDS-SDM and fractional divider, the fractional spurs are greatly mitigated. A phase domain ADPLL Simulink model is proposed for verification and the simulation results demonstrate the proposed technique is effective for fractional spurs reduction.