All PCB designers should be aware of the impact of crosstalk on signal integrity. As signal traces come into close proximity to an aggressor signal, part of that signal is unintentionally 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 copper? 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 current takes the path of least inductance in the nearest plane(s). Returning signal currents tend to stay in close proximity to their signal conductors, 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 increasing loss leaving a section of unused copper in the center of the plane. This redistribution of current causes the resistance per length to increase and the loop inductance per length to decrease. As frequency increases beyond 1 GHz, the resistance continues to increase while the loop inductance reaches a limiting value. The higher the frequency, the greater the tendency for current to flow in the outer surface of the conductor.
To read this entire column, which appeared in the May 2017 issue of The PCB Design Magazine, click here.