The PCB Design Magazine

PCBD-Oct2017

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14 The PCB Design Magazine • October 2017 SIGNAL INTEGRITY: THE EXPERTS WEIGH IN Eventually, accurate estimation of error-cor- recting code performance on high-speed serial channels is going to become a thing in itself. I ran into someone from Cisco at DesignCon who was ready to have that conversation. "A smaller thing, but also important: Sup- pose you have a transmission line that's going over a ground plane. 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 some place. Well, now, the return currents aren't going to flow all the way around this dis- continuity to meet up at the other side. It turns out what the return currents do is transition to whatever the closest plane is, regardless wheth- er you have a return via or not. 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. No, you're not going to get it from a 3-D field solver; you get it from closed-form equations. "One other thing: We may be approaching a brick wall when it comes to throughput on serial channels. Back in 1995, I told a room full of people that they needed to be designing faster and faster high-speed serial channels and history supported me on that one. 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." Mark Thompson replied, "That's funny, because we've been spending so much time over the last 10 years beating up the material manufacturers, saying, 'Look, we need specific dielectric constants. We need specific loss tangents.' Now we're at a time that we're beat- ing up the copper foil manu- facturers for surface roughness because we're literally at a stage where 25 gig is the key, and at that stage any surface tooth or surface rough- ness is an issue. It's strange that we've gone from a situation where we beat up material manufacturers to the point where we're beating up copper manufacturers to get surface rough- ness down to a minimum." "I think the conversation's going to change again," said Steinberger. "There was a paper giv- en by Rogers at DesignCon two years ago that I found to be a mind-blower. Rogers observed that when they measured transmission losses for two different kinds of copper, rolled copper and electrodeposited copper, they found that all these roughness models that we've been us- ing modeled the electrodeposited copper quite well and over-predicted the losses for the rolled copper, by a lot. "So I wonder, going up to these higher frequencies, if people are really going to take this data from Rogers seriously, as I think they should. Maybe they'll start asking you for rolled copper instead of electrodeposited copper." "We've had some customers ask for rolled annealed copper, as opposed to the electrode- posited variety," Thompson added. "We see less of that stuff than we did in the old days. Re- member when people were asking for things like an 11-degree offset on a panel to minimize hav- ing to deal with having a structure sitting over the top of a knuckle or weave of the material? Or things like skin effect due to differential pairs that were dipped in deep gold that didn't have any sol- der mask over the top of them. We see less of that stuff." Steinberger had an idea about why that might be. "Part of that is that people end up routing it at different angles anyway so you don't have these really long straight lines, or you shouldn't have these long straight lines, that they had on the test boards where they were measuring these effects to begin with. I never was really happy with Mark Thompson

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