Experts Discussion: Signal Integrity and Impedance Control

I-Connect007 | 10-11-2017

When the content gathering for an issue on signal integrity and controlled impedance was officially underway, our I-Connect007 editorial team’s first stop was an “experts discussion” with industry experts: Mike Steinberger of SiSoft, Mark Thompson of Prototron Circuits, and Yogen and Sunny Patel of Candor Industries. 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 errorcorrecting code performance on high-speed serial channels. …I think that the benefits of error-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 simulations and was able demonstrate that the correlation of errors is due to the inter-symbol interference.

It’s a very practical problem from a simulation perspective, but people simply haven’t been doing that analysis. I predict that eventually error-correcting codes aren’t going to give people quite the performance boost they expected.

“A smaller thing, but also important, has to do with what happens when you have a cutout 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 flowing 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 transition to whatever the closest plane is, regardless of whether you have a return via. So you don’t need ground vias to have this transition of the return current occur, and it turns out that there is a very simple formula for what amounts to an inductance that the return current goes through when it transitions from one reference plane to another. This is a piece of knowledge that all signal integrity engineers should understand, and basically none of them do…You get it from closed-form equations.

“Those are two immediate topics that I think need to be addressed, and the other thing we had better start thinking about is that we may be approaching a brick wall when it comes to throughput on serial channels. I’m going to take another chance here and say that there is going to come a limit, and we may be starting to get close to that limit.”

To read the full version of this article which appeared in the October 2017 issue of The PCB Magazine, click here.