A guard trace is just another trace that runs between two signals in order to isolate them and reduce coupling between the two traces. The ends of the guard traces are often grounded and this helps in grounding the field lines from the high speed traces. Some times guard traces are used to isolate just a single critical high speed signal such as a clock signal. In this case, the guard trace provides a path to terminate the field lines to ground.

Though guard traces are typically routed right next to the critical signals, they do not provide any extra help if other signal integrity techniques such as impedance matching and providing proper ground return are compromised in the layout.

Most designs use regular vias to connect the guard lines to the ground plane. Though via spacing is debatable, many EMC experts suggest placing the vias at 1/20 th of the minimum wavelength of interest.

While routing guard traces next to differential traces, care must be taken to route guard traces on both sides of the differential pair. If this is not followed, this could potentially lead to an impedance imbalance between the differential pair.