A MASH DDSM is conventionally employed in a fractional- N frequency synthesizer as the divider controller. The quantization noise of the MASH DDSM adds to the output phase noise. It has been shown that the nonlinearity within the synthesizer causes the divider controller contribution of the noise to increase and spurious tones are generated as the result. In the past the pattern of the fractional spur has been reported and studied. In measurements as well as simulations, sub-harmonic spurs, i.e. spurs at a lower frequency than the expected fractional spur, have been observed. In this work, the pattern of MASH DDSM-induced spurs is studied through simulations and observations in a commercial fractional- N frequency synthesizer.