Influence of $S_{\text{cd}21}$ Characteristics of Common-Mode Filter on Radiated Emission from UTP Cable

The common-mode filter (CMF) is a key device for suppressing emissions radiated from telecommunication cables. Recent serial interfaces deliver the necessity of considering that common-mode (CM) disturbances are converted from the differential mode (DM) telecommunication signals by a CMF. To examine this phenomenon, this study investigated the Scd21 characteristic, which is one of the mixed-mode S-parameters. Two types of CMFs whose Scd21 were different were examined. A signal was supplied to an unshielded twisted pair (UTP) cable through the CMF. The emission from the cable was measured using a setup complying with CISPR 25. Then, the relationship between the ratio of CM to DM signals and the emission level was investigated. The results showed that the emission level decreased as a result of employing the lower Scd21 CMF and that the emission level was linearly proportional to the ratio of the CM to DM signals. The results indicate that Scd21 is an essential parameter with which to evaluate CMFs used in telecommunication ports of serial interfaces.