Incorporating the Single-Loop Delta-Sigma Modulator in Fractional-N Frequency Synthesizer for Phase-Noise Improvement

Abstract

This research experimentally evaluates the improvement of phase noise in a fractional-N frequency synthesizer that uses the single-loop delta-sigma modulator (DSM) instead of the conventional multi-stage noise-shaping (MASH) DSM. The single-loop DSM can achieve a smaller range of quantization levels so as to be much less susceptible to the phase-locked loop (PLL) nonlinearity than the MASH DSM. The advantage is to effectively suppress the growth of quantization noise spectrum due to the PLL nonlinearity, resulting in low phase noise for the fractional-N frequency synthesizer. The experimental results show that a 4th order single-loop DSM design can improve the phase noise of fractional-N frequency synthesizer by 28 dB within the PLL bandwidth when used to replace the same order MASH DSM design.