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Are Guard Traces Worth It?
By definition, a guard trace is a trace routed coplanar between an aggressor line and a victim line. There has always been an argument about whether to use guard traces in high-speed digital and mixed signal applications to reduce the noise coupled from an aggressor transmission line to a victim transmission line.
On one side of the debate, the argument is that the guard trace should be shorted to ground at regular intervals along its length using stitching vias spaced at 1/10 of a wavelength of the highest frequency component of the aggressor’s signal. By doing so, it is believed the guard trace will act as a shield between the aggressor and victim traces.
On the other side, merely separating the victim trace to at least three times the line width from the aggressor is good enough. The reasoning is that crosstalk falls off rapidly with increased spacing anyway, and by adding a guard trace, you will already have at least three times the trace separation to fit it in.
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Editor's Note: This column originally appeared in the April 2013 issue of The PCB Design Magazine.
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