S-parameter characterization for EMI filters

MIL-STD-220 is one of the most common methods of EMI filter characterization. Unfortunately, it is based on a 50-ohm source and load impedance, and provides only a simple magnitude of the insertion loss. Most filtering situations involve source and/or load impedances that are different than 50-ohms. Thus MLT-STD-220 characterization data offers little value for system or subsystem analysis involving an EMI filter. This paper presents approaches for using scattering parameters (s-parameters) for EMI filter characterization and analysis. Examples of multi-port filter characterization are presented as well as using the S-parameter characterization data for both common mode and differential mode analysis in the time and frequency domains.