Investigation of common mode (CM) impedance : Comparison of pure CM and bulk current injection

The common mode (CM) signal appears equally on both lines of a 2-wire cable not connected to ground. It is expressed mathematically as the average of the two signal voltages with respect to ground. In this paper, two different methods to inject CM signals are presented. The first meets with the mathematical expression of CM whereas the second follow the setup of a bulk current injection (BCI) test. Mathematical relations have been developed to achieve to a CM impedance definition. The analytical values have been validated with mixed-mode S-parameters. It has been shown that mathematical CM impedance term can be used more accurately rather in the first method than the BCI injection.