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In this paper, the electromagnetic interference (EMI) filter is first characterized using Scattering parameters (S-parameters). Based on this S-parameters model, the insertion voltage gains are derived for EMI filter with arbitrary levels of source and load impedances. Experiments are carried out to verify this approach. Based on the network theory, S-parameters are then utilized to extract the parasitic couplings in both one-stage and two-stage EMI filters. EMI filter models are constructed. Experiments finally verify the proposed methods. The approaches are very useful for the prediction of EMI filter performance, and for the design and optimization of EMI filters.