Comparison of Suppression Effect by CMF for UTP and STP Lines Using Chain Parameter Matrix

Common-mode filter (CMF) is a vital device to improve electromagnetic compatibility (EMC), and the analysis of the CMF effect is essential to improve the design quality of EMC. This paper analyzes the suppression effect of CMF used for telecommunication lines using a chain parameter matrix. The chain parameter matrix represented the CMF and the interface cable, and the $S_{21}$ at the far end of the cable was calculated. The analytical method was validated by comparing measurements using unshielded twisted pair (UTP) and shielded twisted pair (STP) cables. The results showed that the analysis method was appropriate for analyzing the suppression effect. Using the analysis method, the suppression effect was evaluated by the common mode (CM) current at the cable input and the radiated electric field. The result showed that the characteristic impedance and the transmission loss of the CM transmission line affect the suppression effect. The result also showed that the suppression effect by the radiated electric field could be estimated from the suppression effect by the CM current at the cable input.