Efficient Dithering in MASH Sigma-Delta Modulators for Fractional Frequency Synthesizers

The digital multistage-noise-shaping (MASH) ΣΔ modulators used in fractional frequency synthesizers are prone to spur tone generation in their output spectrum. In this paper, the state of the art on spur-tone-magnitude reduction is used to demonstrate that an
M
-bit MASH architecture dithered by a simple
M
-bit linear feedback shift register (LFSR) can be as effective as more sophisticated topologies if the dither signal is properly added. A comparison between the existent digital ΣΔ modulators used in fractional synthesizers is presented to demonstrate that the MASH architecture has the best tradeoff between complexity and quantization noise shaping, but they present spur tones. The objective of this paper was to significantly decrease the area of the circuit used to reduce the spur tone magnitude for these MASH topologies. The analysis is validated with a theoretical study of the paths where the dither signal can be added. Experimental results of a digital
M
-bit MASH 1-1-1 ΣΔ modulator with the proposed way to add the LFSR dither are presented to make a hardware comparison.