Reference spur suppression technique using ratioed current charge pump

Abstract

Reference spurs are one of the main problems in integer-N phaselocked loops (PLLs). A ratioed current charge pump is proposed to suppress reference spur magnitude in the PLL output. The ratioed current charge pump can be implemented by resizing the source and drain network of the charge pump. A formula to calculate the ratioed current and transistor size is presented. The reference spur magnitude for a PLL with a ratioed current charge pump is compared to a conventional charge pump PLL, resulting in about 4 dBc/Hz improvement in the reference spur magnitude.