Analysis of mode conversion using modal equivalent circuits for immunity evaluation of high-speed communication equipment

In this report, the mechanism of mode conversion of interfering electromagnetic waves injected into communication equipment from coaxial cables to in-vehicle communication modules is evaluated and clarified from the viewpoint of immunity. A mode conversion equivalent circuit was used for this purpose. Considering radiated immunity, the applied electromagnetic field induces a common-mode current in the coaxial cable, which propagates through the cable to the signal line inside the enclosure via a connector. When microstrip lines are used within a printed circuit board, the imbalance difference between the two transmission lines causes a conversion from common-mode current to normal-mode voltage. Furthermore, when the imbalance difference is small, the parasitic inductance between the ground connections at the connectors is dominant in the mode conversion. To evaluate these phenomena, a mode transformation circuit with ideal transformers was created. The circuit simulations were conducted using the model and a modal transmission line. The obtained results showed the same trend as that of the 3D electromagnetic field simulation.