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An in-depth analysis is performed on the third-order intermodulation distortions (IMD3) in the switching pair of active CMOS mixers. The nonlinear time-varying switching pair is described by a hypothetical circuit composed of a nonlinear time-invariant circuit cascaded with a linear time-varying circuit. This allows us to apply power and Volterra series for nonlinearity analysis. The analysis reveals that the impedance in the common-source node of the switching pair along with the fractions of the LO period in which both transistors are simultaneously ON are responsible for IMD3 generation. Besides it is noted that the IMD3 is composed of several terms which may in some cases cancel out each other. The analysis is validated with simulations.