The performance of fractional-N frequency synthesizers in wireless communications applications is degraded by the presence of spurious tones. While the Digital Delta-Sigma Modulator (DDSM) can be directly responsible for the production of such tones, a range of deterministic and stochastic techniques have been invented to eliminate the principal causes associated with the architecture of the DDSM. A second source of spurs, when the spectrum of the DDSM iteself is spur-free, is (analogue) nonlinearities in the synthesizer. Recent work has predicted that specific nonlinearities will produce tones at well-defined frequencies; this paper presents simulation and experimental verification of the prediction.