Diversity Pertaining to the Attenuation of the RF Disturbance Suppression Power Line Filters

Abstract

Filters for the suppression of the RF disturbances in power lines are passive; wideband stop filters attenuating common as well as differential disturbances should be insensitive to the variation in the impedance at the input and the output. The design and verification process of such filters differs entirely from that used for the signal RF filters. In the document CISPR 17, entitled “Methods of measurement of the suppression characteristics of passive EMC filtering devices”, two definitions of the attenuation of the common and differential modes are presented. The first is simply direct measurement, and the second is based on a calculation using the single-ended S-parameters of the multiport. These two definitions are the source of diversity. In this paper, we hypothesize that the attenuations achieved with these two methods are identical only if the filter is in perfect balance. Consequently, different attenuations can be achieved. The requirement of balance can be violated due to the tolerances of the components used in the filter. The paper begins with a simple unbalanced four-port, for which analytical formulas are derived. It is also shown that an approach with single-ended S-parameters enables the calculation of the mode conversion between the input and the output, which is called the conversion loss. This is the measure of the grade of the imbalance. Thereafter, the common-mode attenuation of an exemplary RF suppression filter is derived with the presented methods. Additionally, the conversion loss of the filter is calculated. The comparison of the conversion loss with both attenuations is satisfactory, proving the hypothesis formulated in the paper.