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 not just that the outer trace is slightly longer than the inner one. What happens to the edge transitions as they propagate down the transmission line? Yuriy Shlepnev of Simberian Inc just 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.