Comparative spur performance of a fractional-N frequency synthesizer with a nested MASH-SQ3 divider controller in the presence of memoryless piecewise-linear and polynomial nonlinearities

Fractional-N frequency synthesizers are notorious for exhibiting nonlinearity-induced spurious tones (spurs) in their phase noise spectra. One cause of these spurs is interaction between the shaped quantization noise produced by the digital delta-sigma modulator (DDSM) in the divider controller and nonlinearities in the phase-frequency detector and charge pump (PFD-CP). A novel type of divider controller called the Successive Requantization (SR) is claimed to have better spur performance than the DDSM in the presence of loop nonlinearities. In this paper, we compare the integer boundary spur (IBS) performance of a fractional-N synthesizer with three different divider controller architectures: a conventional MASH 1-1-1 DDSM, a Successive Requantizer (SQ), and a recently-proposed MASH-SQ hybrid. While the MASH is best in the linear case, the SQ has lower spurs in the presence of typical PFD-CP nonlinearities. The performance of the MASH-SQ hybrid is comparable to that of the SQ, but the former requires significantly less hardware and power.