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50 DESIGN007 MAGAZINE I JULY 2021 sheets usually have a solid knowledge of "cir- cuit theory" (the behavior of voltage and cur- rent). Unfortunately, PC boards' behavior does not revolve around circuit theory. PC boards function using field theory, the movement of energy through the dielectric of the board. When that is understood, it becomes obvious that splitting ground generally leads to trouble. In the witty words of James Pawson, a UK- based EMI consultant: Q: What do you call an engineer who splits the ground plane? A: A customer! DESIGN007 Eric Bogatin: Route your differential pair pitch on the same pitch as the glass weave, between 16-20 mils. Use an L-glass that matches its Dk closer to the resin. Use a low glass content laminate. Heidi Barnes: If you can handle a little more loss, there is the option of zig-zagging the traces so that they end up with an average skew rather than bi-modal. Another option is if you actu- ally know the spacing of the fiber weave, then one can pick a trace width such that a bus of parallel traces will always land on the same percentage of fiber weave and thus have the same relative skew. Lee Ritchey: With the very high data rate in modern designs, controlling skew is oen harder than controlling loss. ere are no cheap solutions that are also safe. Rigid control over the type of glass weave is the only solution I have found that works. Just like with cars, if you want to go fast, it costs more money. Carl Schattke: Obviously, you are talking about a high frequency of operations and a desire to reduce costs. Sometimes designers will rotate designs on a production panel by 10 or 20 degrees to not have the traces ride over an all- glass area or an all-resin area as it makes its way across the substrate. is can help, but will it help you in your application? I don't know; you'll have to run some tests. Rick Hartley: If your boards are sized such that you can rotate them slightly (10 to 15 degrees) in the fabrication panel, and still get the same number of boards per panel, you will likely lower the fiber weave skew issues dramatically. ere is no guarantee that this will solve the problem, but it usually does. DESIGN007 The skew from fiber weave effect is cutting into our jitter unit interval budget. Aside from using much more expensive laminates, what are our options? Q

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