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104 I-CONNECT007 MAGAZINE I FEBRUARY 2026 DEMYSTIFYING COMMON-MODE RADIATION BY BARRY OLNE Y, IN - CIRCUIT DESIGN PT Y LTD C ommon-mode radiation is a major contrib- utor to unwanted electromagnetic inter- ference (EMI). It arises when equal-phase currents flow on conductors without an oppos- ing return current to cancel their fields. The result- ing imbalance causes those conductors, especially attached cables, to behave as unintended anten- nas. Grasping how common-mode radiation is gen- erated, the problems it creates, and the methods available to control it is essential for designing reliable electronic systems that meet regulatory requirements. Electromagnetic radiation from digital circuits generally appears in either differential mode or common mode operations. In differential mode operation, currents flow in equal and opposite directions, and (at least in theory) their electromag- netic fields cancel, keeping radiation to a minimum. On the other hand, common mode currents are a very different beast. When two coupled conduc- tors carry identical currents, the fields no longer oppose each other; they reinforce, thus turning cables, traces, and even chassis structures into surprisingly efficient antennas. The real trouble begins when perfectly good differential mode signals are inadvertently converted into common mode currents. This mode conversion can arise from parasitic capacitance, geometric imbalances between traces, timing skew, inadequate return paths, or asymmetries anywhere in the channel. Even small discontinui- ties in the return path can create large common mode loop areas, increasing series inductance and dramatically boosting radiated emissions. B E YO N D D E S I G N