5.11: Common Impedance Coupling

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So far coupling has been discussed in terms of the EM fields shared by two transmission lines. This is not the only way coupling of signals occurs. Sharing of a return path results in coupling, often called common impedance coupling, as there is a circuit element common to two or more transmission lines. The simplest situation is a shared impedance rather than a shared transmission line return, so that the return current attributed to one interconnect induces a voltage across the common impedance element. This signal then appears as though it is on the victim line. The common impedance could be the inductance or resistance of the ground conductor in the case of microstrip lines. In general, however, common impedance coupling will occur whenever the current return path is common.