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Danish physicist Hans Christian Oersted made great strides in electromagnetic research. During a lecture in 1820, he discovered that electric currents create magnetic fields. He later found that a current-carrying wire created a circular magnetic field around that wire, and that this circular field was strongest closer to the wire.
Traces in a multilayer PCB act in much the same way. A current loop produces a field similar to that of Earth, although much smaller of course. Electromagnetic fields are produced when a logic driver delivers a high-speed, fast rise time pulse into a trace. The electromagnetic wave propagates down the length of the trace radiating into the surrounding dielectric material and coupling energy to nearby trace segments. These electromagnetic fields are not restricted to the multilayer substrate and if proper care is not taken, they may emit radiation causing electromagnetic interference.
Editor's Note: This column originally appeared in the March 2013 issue of The PCB Design Magazine.