Abstract
Static charge pump mismatch can cause fractional-N frequency synthesizers to exhibit both an elevated in-band noise floor and in-band spurious tones (spurs). To first order, the transfer characteristic of the phase-frequency detector/charge pump (PFD/CP) is piecewise-linear with two segments. The mismatch-related nonidealities can be alleviated by offsetting the operating point so that the phase excursions at the input to the PFD/CP lie completely within one segment. By considering a simplified two-segment piecewise-linear model of the PFD/CP, we show how a combination of delay and/or current offset can be used to combat mismatch. We illustrate our analysis using a simplified Matlab model and confirm its predictions by simulation using the CppSim behavioral simulator.
