Issue link: https://iconnect007.uberflip.com/i/1494346
20 DESIGN007 MAGAZINE I MARCH 2023 between lines can be a problem. As seen in the graph of Figure 4, with a rise time of 200 ps, backward crosstalk reaches its maximum amplitude in 12.5 mm (1/2 in) of parallel- ism between lines. e absolute amplitude is determined by the distance between the aggressor and victim lines relative to their height above the reference plane (ground plane). For this reason, it is extremely important to route all traces on a layer that is one dielectric distance above or below a ground plane and to keep the dielectric thickness as thin as reasonably possible, within the constraints of good manufactur- ability rules. Having a wide uniform plane as the return path is the configuration for the lowest radia- tion and crosstalk. Anything that changes the return path from a wide uniform plane will increase the amount of coupled noise between transmission lines. Usually this happens when the signal goes through a connector, and the return paths for more than one signal path are now shared by one of the pins rather than by a plane—the inductively coupled noise increases much more than the capacitively coupled noise. Simultaneous switching noise occurs mostly in connectors, IC packages, and vias, where the return path conductor is not a wide, uniform plane. Ground bounce is really a special case of crosstalk caused when the fields from several lines overlap and couple into one another. As the fields all attempt to couple to a single reference line in the connector or IC package, or into a single ground via, the return currents overlap, creating very high mutual inductance. Engineers oen employ guard traces to help lower crosstalk. In a well-designed PCB with the ideal number of ground planes, guard traces offer little, if any, value unless properly attached to the ground plane(s) of the board. An ungrounded guard trace (Figure 5) or one attached to the ground plane(s) at one or both ends only (middle) can create a bandpass filter—an LC network— that radiates at one-quarter wavelength of its resonant frequency. Whereas, a guard trace that is grounded every 1/12th wavelength (bottom) prevents radiation and drastically lowers cross-coupling. On one - and t wo -layer PCBs, w ith no ground planes, even guard traces that are con- nected to the 0V reference points at both ends only, are oen better than no return reference. at said, employing one- and two-layer PCBs without planes is a very poor design practice. e traces have an impedance of 376 ohms Figure 5: Ground guard trace routing.