Bulk current injection in twisted wire pairs with not perfectly balanced terminations

In this paper, common mode (CM) and differential mode (DM) voltages induced by bulk current injection at the terminations of a differential interconnection are predicted and correlated to the degree of unbalance of the terminal units. This is done by extending a previously derived lumped-parameter representation of the injection probe and by combining modal analysis with a multiconductor transmission line theory. Under nonrestrictive assumptions on termination unbalance, it is shown that CM and DM voltages can be readily predicted by two equivalent circuits. The circuit for CM prediction is independent on termination unbalance and directly excited by the injection probe. The circuit for DM prediction does not involve any of the probe model parameters, and is driven by two voltage sources related to the induced CM voltages through the common-mode rejection ratio of each termination. Model accuracy was proven by measurements carried out on a test bench setup according to the BCI Standards.