©LAMSIM Enterprises Inc.
3
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.
3
1
2
4
Mode of Propagation
Victim
NEXT FEXT
Aggressor
3
1
2
4
Mode of Propagation
Victim
NEXT FEXT
Aggressor
