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The nonlinearity behavior of CMOS current-switching mixers is investigated. By treating the mixer as a periodically-time-varying weakly nonlinear circuit, we study the distortion-causing mechanisms and we predict the mixer distortion performance. Normalized graphs are provided from which the designer can readily estimate the mixer nonlinearity for particular process and design parameters. A simple CMOS transistor model appropriate for our calculations, which also takes into account deviation from the square law is adopted. The significance of a physical transistor model for reliable distortion simulation is demonstrated. The prediction of our analysis is compared with simulation results and with experimental data.