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Digital Circuit Boards: Mach 1 GHz by Ralph Morrison

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3.12 Cross Coupling between Parallel Transmission Lines of Equal Length

Parallel transmission lines can be series or parallel terminated with the same or opposite signal directions. Culprit and victim logic transitions can be in the same direction or in opposite direction. The culprit and victim can be unterminated at either end. There can be a culprit on both sides of one victim. A complete analysis of signal integrity requires an analysis of each case. In the following discussion, we will assume that the culprit wave is positive and goes to the right.

Consider the case where the culprit line is series terminated and the culprit makes a round trip during the rise time. The cross talk is composed of reverse coupling for half the round trip time and forward coupling during the return time. The coupled signal is maximum when the culprit signal has made a round trip. This line length is called the critical length.

The distance a wave travels in a rise time is called a TEL that stands for transitional electrical length. Lines longer than one-fourth TEL that are not terminated are apt to fail. In the example above, the critical length is half of TEL.

The following cases are a sample of the cross talk problem. A treatment of every case would be of little value, as there are just too many variables.

Case 1. Victim line is series terminated at the left.
At the point of first coupling, the reverse wave is absorbed in the victims matching impedance. There is no reflection. The forward ...

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