Spurs in Fractional-N Frequency Synthesizers: Investigation of Mechanisms for the Spur Generation in Digital - Modulator Based Fractional-N Frequency Synthesizers

The performance of a fractional-N frequency synthesizer is degraded due to the presence of unwanted spurious tones (spurs) in the output spectrum. The Digital Delta-Sigma Modulator can be directly responsible for the generation of spurs because of its inherent nonlinearity and periodicity. In this booklet we have predicted that specific nonlinearities in a fractional-N frequency synthesizer produce spurs at well-defined frequencies even if the output of the DDSM is spur free. Different spur free DDSM architectures have been investigated for the analysis of spurious tones in the output spectrum of fractional-N frequency synthesizers.