Immunity of ENOP-based Fractional-N Frequency Synthesizer to Wandering and Horn Spurs

The divider controllers in fractional-N frequency synthesizers are typically digital $\triangle\Sigma$ modulators (DDSMs). The DDSM can contribute significantly to the phase noise and spur pattern in the output of a nonlinear synthesizer. A type of time-varying spurs and another type of fixed sub-fractional frequency spurs, denoted wandering spurs and horn spurs, respectively, have been observed in simulations and measurements of fractional-N frequency synthesizers with a MASH-based divider controller. Different families of DDSMs have been presented in the past which represent an alternative to MASHDDSMs. Among these, the ENOP DDSMs are worthy of notice because of their performance in terms of nonlinearity-induced noise and spurs. In this work we show that ENOP-based divider controllers offer strong mitigation of both wandering and horn spurs.