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Design007-Dec2020

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20 DESIGN007 MAGAZINE I DECEMBER 2020 As I write this column, I just completed revising and updating one of Polar's oldest documents on impedance controlled PCBs: "An Introduction to the Design and Manufacture of Impedance Controlled PCBs with Insights into Insertion Loss." While preparing to update it, I noticed the date on the last revision was 20 years ago, in 2000. Time flies! But the laws of physics don't. What struck me as I updated the document was how the core principles are the same, but as geometries shrink and speeds increase, the signal comes under the influence of differ- ent physical characteristics of both the copper and the laminate. This column will focus on how important it is becoming to take DC trace resistance into account when measuring and specifying thin copper traces. A good understanding of this topic will lead to fewer spins and also reduce the risk of going down blind alleys when searching for Don't Ignore DC Trace Resistance correlation. I have noted in previous columns how, sometimes, fabricators attempt to corre- late impedance before removing the effects of DC resistance from their measurements. This can lead to erroneous conclusions about the dielectric constant (Dk) if the Dk value is set by goal-seeking to make a high reading trace correlate. All this reminds me of a conversation at IBM in France many years ago, where the engineer in charge of signal integrity joked that many SI software packages were "Logieciels comme une usine de Gaz," which literally translates to, "Software like a gas refinery." Lots of gauges and levers and pipes—and intimate knowledge of the software—is needed to get a sensible result. Fortunately, the software has become simpler to use over the years, but it remains important to remember to feed the software with good input data in order to get a sensible output. The Pulse by Martyn Gaudion, POLAR INSTRUMENTS

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