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PCB-Oct2017

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24 The PCB Magazine • October 2017 by the I-Connect007 Team When the content gathering for an issue on signal integrity and controlled impedance was officially underway, our I-Connect007 ed- itorial team's first stop was an "experts discus- sion" with industry experts: Mike Steinberger of SiSoft, Mark Thompson of Prototron Circuits, and Yogen and Sunny Patel of Candor Indus- tries. This teleconference call was a whirlwind at times, but we captured valuable information that we have distilled here, for our readers. We started the conversation by asking that all important question: What are some of the challenges in signal integrity that are not being addressed right now? Mike Steinberger said that two points came immediately to mind: "The large point is in the analysis of error- correcting code performance on high-speed se- rial channels. …I think that the benefits of er- ror-correcting codes have been oversold, in that the performance analyses for error-correcting codes have been based on assumptions that are appropriate for radio channels, like where you have a lot of added noise—so a satellite channel or something like that. Therefore, the errors are completely uncorrelated. The fact of the matter is that on high-speed serial channels the errors are more correlated than that…I did some simu- lations and was able demonstrate that the corre- lation of errors is due to the inter-symbol inter- ference. It's a very practical problem from a sim- ulation perspective, but people simply haven't been doing that analysis. I predict that eventu- ally error-correcting codes aren't going to give people quite the performance boost they ex- pected. "A smaller thing, but also important, has to do with what happens when you have a cut- out in a reference plane. Suppose you have a transmission line that's going over a ground plane, for example. It gets to a certain point and there's a cutout in the ground plane. What happens when the transmission line hits that discontinuity? There are return currents flow- ing in the reference plane and those currents have to go someplace. Well, now, the return currents aren't going to flow all the way around this discontinuity to meet up at the other side. It turns out what the return currents do is FEATURE

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