The Role of Charge Pump Mismatch in the Generation of Integer Boundary Spurs in Fractional-N Frequency Synthesizers: Why Worse Can Be Better

Fractional-N frequency synthesizers exhibit spurious tones (spurs). Spurs outside the loop bandwidth can be attenuated by the loop filter. By contrast, in-band spurs cannot be removed by filtering and are therefore a major cause for concern. The frequency-to-phase conversion of the output of a digital sigma-delta modulator followed by charge-pump mismatch is known to cause in-band spurs; larger mismatch is thought to make the problem worse. By considering a simplified nonlinear model of the mechanism, we show how larger mismatch can paradoxically make the spur problem disappear. We illustrate our predictions using a simplified MATLAB model and confirm them by simulation using the CppSim behavioral simulator.