Comparison of a feed-forward phase domain model and a time domain behavioral model for predicting mismatch-related noise floor and spurs in fractional-N frequency synthesizers

Static mismatch between the charge pump up and down currents in a fractional-N frequency synthesizer is known to elevate the in-band noise floor and to produce spurs. Offset current can be used to reduce nonlinear effects by moving the operating point of the charge-pump to a more linear region of its transfer characteristic. While both positive and negative offset current will work, one direction requires a smaller offset current and therefore adds less noise to the system. In this paper, a simplified feed-forward phase domain model is compared with a full time-domain behavioral model. We show that the feedforward model accurately predicts both the mismatch-related noise floor and the spurs. We also note a limitation of the feedforward model, namely that it fails to predict the 20 dB/decade slope due to non-uniform sampling.