Observations of the differences between closed-loop behavioral and feed-forward model simulations of fractional-N frequency synthesizers

Conventionally, a fractional-N synthesizer is simulated using a closed-loop behavioral model and the simulations can be very time-consuming. With the analysis by Perrott et al., a more straightforward model, namely the feed-forward model, for the fractional-N frequency synthesizer is available. The feedforward model simulations provide quick, relatively accurate evaluation of fractional-N synthesizer divider controller designs. In this work, we point out several differences in the closed loop behavioral model simulations performed in the CppSim environment and the feed-forward model simulations and discuss their significance on the base of analysis and simulation results.