Spur free DDSM design with random dither

Multi-stAge noise SHaping (MASH) based Digital Delta-Sigma Modulator (DDSM) architecture is widely used in Fractional-N Frequency Synthesizers. However, the DDSM is a Finite State Machine (FSM) with a constant input, and its output sequence presents an intrinsic periodicity, this structure is apt to generate spur tones around the carrier. This paper proposes a novel method to implement an improved random dither to the MASH1-1-1 structure. The improved random dither is transformed from the traditional dither signal generated by the Linear Feedback Shift Register (LSFR). Theoretical results prove that the quantization noise is asymptotically white and uncorrelated with the input; it is verified by simulations and implementations. Compared with the existing methods, the novel method is more effective in suppressing the spur tones of the spectrum.