Visualising Diff-Pair Skew

A long time ago, a colleague described the challenge of getting the widest data eye possible and some of the subtle things that he had to consider. He described what happens as a differential pair turned a corner; one of the two traces would always have to take a longer path. Here’s an example, below:

The problem here is that the outer trace is slightly longer than the inner one. What happens to the edge transitions as they propagate down the transmission line when 1 edge is slightly ahead of the other? Yuriy Shlepnev of Simberian Inc posted a video which shown an animated example of exactly that.

If the transition on one trace gets too far ahead of the edge on the other trace, you will get cross-coupling from one to the other which will degrade both the rising and falling edges of the pair. You can correct the edge phase mismatch by adding a small jog-out in the trace with the shorter path close to the source (as shown below), but that causes an impedance discontinuity (and a small reflection). This may or may not be a problem, depending on the frequency that you are transmitting down the diff-pair, but it is worth being aware of.

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