Evaluation of Insertion Losses of Power EMC Filters Operated in Actual Impedance Environment

Insertion losses (ILs) of power electromagnetic compatibility (EMC) filters are measured often up to the relatively high GHz region, usually by standard microwave 50-
$\Omega $
vector network analyzers (VNAs) in accordance with related standards. However, the obtained
$S$
-parameters do not correspond to conditions when the filters are connected to impedances of actual power sources and loads. Evaluated examples show that actual operational ILs can be significantly lower than values obtained directly from 50-
$\Omega $
measurements. The presented virtual measurement method uses 50-
$\Omega S$
-parameter measurements and subsequently processes the data using the newly derived evaluation formulas applicable for any filter sources and loads at any frequencies. Parameters of three different particular power cables employed as practical sources and loads are measured in the frequency band from 0.1 to 10 GHz. These data are used for the proposed advanced insertion loss calculations. The results demonstrate the influences of practical cable input impedances on power filter ILs for common-mode (CM) and differential-mode (DM) transmissions, and their conversions. Related to common 50-
$\Omega $
measurements, the operational IL drops can reach almost 40 dB for CM–CM transmissions, 5 dB for DM–DM transmissions, and 20 dB for CM–DM conversions. This proves that the practical information capabilities of the standard 50-
$\Omega $
evaluation are really limited.