A Curious Result concerning Spur Generation in MASH 1-1-1 based Fractional-N CP-PLLs with a Second-Order Nonlinearity

摘要

Spurious tones appear in the output phase noise spectrum of a fractional-N frequency synthesizer when nonlinearities are present in the system. Significant design efforts have been made for minimizing the nonlinearities and, therefore, the spurious effects on the phase noise performance. However, little attention has been given to the connection between the type of nonlinearity and the spur pattern it produces. In this paper we observe the reaction of a MASH 1-1-1 based fractional-N CP-PLL to different nonlinearities, focusing on an interesting effect that occurs when the system presents a second-order nonlinearity.