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42 The PCB Design Magazine • May 2017 All PCB designers should be aware of the impact of crosstalk on signal integrity. As sig- nal traces come into close proximity, of an ag- gressor signal, part of that signal is uninten- tionally electromagnetically coupled into the victim trace as noise. I have mentioned before that current flow is a round trip—the current must return to the source to complete the loop. What about crosstalk in the return path, of the reference planes, as the current weaves its way back through the expansive wasteland of cop- per? This month's column follows on from my April column, "Return Path Discontinuities," and elaborates on crosstalk in the unseen "dark side" of the signal. I guess we all think of a copper plane as a thick, solid plate of copper that can basically handle any amount of current we sink into it. It also serves to make the circuit layout easier, allowing the PCB designer to ground anything, anywhere without having to run multiple tracks. That may well be the case with DC or very low- frequency analog circuits, but certainly not in the case of high-speed design. The return cur- rent takes the path of least inductance in the nearest plane(s). Returning signal currents tend to stay in close proximity to their signal con- ductors, falling off in intensity with the square of increasing distance. However, as the frequency increases, the current is forced into the outer surface of the copper, due to the skin effect, dramatically in- creasing loss leaving a section of unused copper in the center of the plane. This redistribution by Barry Olney IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA The Dark Side – Return of the Signal BEYOND DESIGN Figure 1: See you on the dark side of the moon (Courtesy of NASA).

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