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Guard Traces

White Paper-Issue 02

Lambert Simonovich

5/28/2012

Abstract:

To guard or not to guard? That is the question often asked by digital hardware design engineers. As bit

rates continue to climb, there is increased debate on whether to use guard traces to control crosstalk in

high-speed digital signaling. By doing so, it is believed the guard trace will act as a shield between the

aggressor and victim traces. On the other hand, the argument is that merely separating the victim trace to

at least three times the line width from the aggressor is good enough. This paper studies the application

of guard traces and quantifies the results against non guarded scenarios.

©LAMSIM Enterprises Inc.

GUARD TRACES

Introduction:

As bit rates continue to climb, there is increased debate on whether to use guard traces to control crosstalk

in high-speed digital signaling. By definition, a guard trace is a trace routed coplanar between an

aggressor line and a victim line. Often the guard trace is terminated at each end in its characteristic

impedance or shorted to ground. To be most effective, the guard trace should be shorted to ground at

regular intervals along its length using stitching vias spaced at 1/10th 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 of the debate, the argument is that merely separating the victim trace to at least three

times the line width from the aggressor is good enough. The reasoning here is that crosstalk falls off

rapidly with increased spacing anyways, and by adding a guard trace, you will already have at least three

times the trace separation to fit it in. Furthermore, the added ground stitching will severely restrict routing

of other signals on the board. Of course the only way to settle these kinds of debates is to put in the

numbers.

In order to answer the question, “To guard or not to guard?”, this white paper studies the effect of

applying guard traces, and quantifies the results against the non guarded scenario.

Discussion

When two coplanar parallel traces running in close proximity, as shown in Figure 1, there are two types of

crosstalk generated; Near-End (NEXT), or backwards crosstalk, and Far-End (FEXT), or forward

crosstalk.

Figure 1 Illustration of NEXT and FEXT. As the aggressor signal propagates from port 3 to port 4, Near-End XTalk

appears on port 1 and Far-End XTalk appears on port 2 after one Time Delay (TD) of the interconnect.

NEXT voltage is correlated to the coupled current through a terminating resistor (not shown) at port 1.

The backward crosstalk coefficient, Kb, is equal to the ratio of Vb/Va, as defined by Equation 1; where Vb

is the voltage at port 1; and Va is the peak voltage of the aggressor at port 3.

Mode of Propagation

Victim

NEXT FEXT

Aggressor

Mode of Propagation

Victim

NEXT FEXT

Aggressor

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