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Design007-Oct2021

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82 DESIGN007 MAGAZINE I OCTOBER 2021 professional term would be to apply "goal seek- ing" or "empirical" correction. But this comes with a warning, as oen noted by Polar's signal integrity product manager Neil Chamberlain, "Occasionally a little fudge tastes very good, but too much fudge can make you feel quite ill." e same is true with measurement cor- relation. Goal seeking—"fudging"—the results can actually be a very powerful tool, provided you have a good enough knowledge of the physics and the range over which goal seeking is reasonable. How Much is Reasonable? Sometimes a process engineer with an impedance correlation problem will microsec- tion and find an inexplicable variation between the modelled and measured value of character- istic impedance. A microsection or two is all it takes to confirm geometries of the line and the dielectric separation of the planes. So, the temptation to enforce correlation is to take the only electrical characteristic that's not easy to measure and "goal seek it" to the correct value; that mystery characteristic is the dielectric constant. Our technical staff has uncovered situations where the "goal seeking" of dielectric constant, commonly called Er, has gone way beyond the realistic limits of Er for a given material. Why should that be? Well, Er value could be part of the correlation but more oen there are sev- eral parameters, each having leverage over the gap between measured and modelled. If a PCB fabricator goal-seeks outside of these limits, their customer may well raise eyebrows that the figure landed on is unachievable. I have some personal experience with fabri- cators who are maybe new to the fabrication of transmission lines, goal seeking an Er of less than 2.0 on an FR-4 stripline. Given that the resin has an Er of 3.0 or thereabouts, and the glass of 6.0, then no amount of excess resin or resin starvation in FR-4 will get anywhere near 2, which is the Er of a pure PTFE material. It is also worth recalling that Z O varies as 1/sq. root Er, so relatively large changes are needed in Er to effect small changes in modelled imped- ance. Line width and dielectric separation have a much larger effect. Whilst on the subject of dielectric constant, many PCB engineers who may not have an electrical background obsess over Er at fre- quency. It's a bit trickier to measure Er than Z O , but once you have a suitable coupon and the correct tools (in this case, a short line, long Figure 1: Effective Er from 100MHz to 43GHz. (Source: Polar Atlas for Anritsu VectorStar)

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