An accurate estimation of Electromagnetic interference (EMI) reduction is realized by a theoretical analysis. The proposed analysis allows to estimating EMI reduction in a straightforward manner from the predetermined modulation index and noise injection. The degradation in EMI reduction is directly correlated with the amount of injected noise in a SSCG. A statistic analysis of the state-of-the-art Fractional-N Phase Locked Loop (PLL) based SSCGs is presented to prove the validity of the proposed analysis. The estimation is matched to measurement results taken from the state-of-the-art Fractional-N PLL based SSCGs. Furthermore, a design example of N-fractional Phase Locked Loop (PLL) based SSCG is presented to prove the validity of the proposed analysis. The estimation of EMI reduction in the designed N-fractional PLL based SSCG is finalized by the loop bandwidth trade-off of noise injection. The measured EMI reduction of a designed N-fractional PLL based SSCG is matched to the proposed estimation value. For further study on the bandwidth trade-off issue, a new architecture of N-fractional PLL based SSCG without bandwidth trade-off is proposed.