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The feasibility of two methods for determining the worst-case insertion loss of power-line interference filters is demonstrated. The first method makes use of injection and detection probes (transformers) which introduce an extremely small impedance (L Â¿50 nH, R Â¿ 3 mÂ¿) into the test circuit. The second method does not use injection and detection probes, thus eliminating their problems. Both methods maintain the high Q of the test circuit and permit interfacial resonances to occur between the power-line and filter interface impedances. A conventional filter and a lossy filter were measured by both methods. Large resonances, including a negative insertion loss at 150 kHz, were measured in the stopband of the conventional filter. No resonances were detected in the stopband of the lossy filter. Measurements with the two test methods differed by only a few dB.