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A frequency-domain lumped-nonlinear-source behavioral model is presented. This generalized two-port nonlinear model is used to characterize the linearity of either RF devices or circuits. Similar to two-port ac behavioral models (i.e., using S-, Y-, Z-, or H-parameters), this nonlinear model supports combinations such as shunt, series, and cascade, and thus is suitable for systematic analysis. This model is then applied to analyze the impact of harmonic impedance on the second-order intermodulation and third-order intermodulation of a state-of-the-art SiGe HBTs and circuits. Simple and general linearity expressions are then derived. Harmonic load-pull simulations and measurements are used to demonstrate the usefulness of the proposed analysis technique.