A Method for Measuring the Characteristics of an EMI Suppression Ferrite Core

Almost all digital equipment has electromagnetic interference (EMI) suppression ferrite cores to encircle cables, such as mains and peripheral cables. However, their EMI suppression capability is hardly evaluated from the impedance characteristics provided by the manufacturers. Hence, this paper proposes to characterize the ferrite cores in terms of the insertion loss and the reflection coefficient. Theoretical and experimental investigations are carried out to develop a measurement jig for these parameters in the frequency range from 30 to 1000 MHz. It consists of a cylindrical metal rod placed above a metal ground plane, yielding a characteristic impedance of 270 Omega. In addition, a simple method is proposed for measuring the insertion loss and the reflection coefficient of a ferrite core under test. It is found that the proposed method can provide insertion loss data with an error of less than plusmn1.2 dB by using commercially available matching networks (baluns). The reflection coefficient of a ferrite core can be measured with an error less than plusmn1.0 dB. The matching characteristics of the adapters are critical factors that deteriorate measurement accuracy