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Based on a two-probe measurement approach, the noise source and noise termination impedances of a switched-mode power supply (SMPS) under its normal operating condition are measured. With the accurate noise source and noise termination impedances, an electromagnetic interference (EMI) filter can be optimally designed. A practical example of the design of an EMI filter to comply with a regulatory conducted EMI limit using the proposed procedure is demonstrated and compared with the cases, where the noise source and noise termination impedances are not taken into account, or coarse estimates of them are considered. Although all approaches allow filtered SMPS to pass the regulation limits, designing EMI filters with the accurate noise source and termination impedances leads to optimal component values and avoids overdesign.