Chapter 7. Taming Signal Integrity Problems When Signals Change Return Planes

7.1 Signal Integrity and Planes

The DC voltage on a return plane has no impact on the characteristic impedance of a transmission line or on the quality of a signal propagating between the signal and return path. This means a 5 v or even a 12 v plane makes just as good a return path as a ground plane.

When the signal line transitions from one layer to another in a multilayer circuit board and its return current changes plane layers, the return current must flow between the two plane layers. Herein lies the problem. It is similar to the problem of someone falling off a building. The falling part is not the problem. The landing is the problem.

When the return current flows ...

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