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.
The coupling between traces is often likened to transformer action where the transformer coils are single turns. This approach does not consider the character of the forward and reverse coupled waves.
Guard traces are sometimes used in an attempt to reduce cross coupling. The approach assumes that added grounds act as shields. The approach is flawed because these traces might function as resonant couplers. The wide spectrum associated with logic transmission means that logic signals can cause ringing in any nearby unterminated transmission lines. The result is added coupling rather than shielding.
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