Observations concerning noise floor and spurs caused by static charge pump mismatch in fractional-N frequency synthesizers

Static charge pump mismatch causes fractional-N frequency synthesizers to exhibit an elevated in-band noise floor and in-band spurious tones (spurs). When the PFD-CP includes a reset delay, the noise floor and spurs first increase with the mismatch, reach a maximum value, and then decrease. By considering a simplified piecewise-linear model of the spur production mechanism, we derive an analytical expression for the scaling of the noise and spur as a function of mismatch. We provide estimates for (i) the value of mismatch at which the noise and spur heights are worst and (ii) the value beyond which they disappear. We illustrate our predictions using a simplified Matlab model and confirm them by simulation using the CppSim behavioral simulator.