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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.
Yuriy Shlepnev, Simberian
A typical PCB design usually starts with the material selection and stackup definition—the stackup planning or design exploration stage. How reliable are the data provided by the material vendors and PCB manufacturers? Can we use these data to predict trace width and spacing for the target trace impedance or to calculate delays or evaluate the loss budget?
Chang Fei Yee, Keysight Technologies
In electronic systems, signal transmission exists in a closed-loop form. The forward current propagates from transmitter to receiver through the signal trace. Meanwhile, the return current travels backward from receiver to transmitter through the power or ground plane directly underneath the signal trace that serves as the reference or return path. The path of forward current and return current forms a loop inductance. It is important to route the high-speed signal on a continuous reference plane so that the return current can propagate on the desired path beneath the signal trace.
Ralf Bruening, Zuken
Using powerful constraint techniques can be a double-edged sword. While the design process is made much safer by including constraints, it is all too easy to over-constrain the design and make it impossible to complete routing and placement. Even paper design guidelines can make products uneconomic to produce unless a great deal of engineering knowledge is applied during the design.