"Today, the commonplace approach, at least in the designs that I observe, is to suppress the low frequency CM currents with power line filters and directly connect the chassis and digital grounds at practically as many points as possible against radiated emissions." We are more in agreement than you think. I happened to be a simple minded person and simple means having everything in common and tying them up as tight as one can. That said, I am in the business of big iron servers. I don't do communication switches, consumer stuff nor desktop/side boxes. If you step back and think about what sticks out of a server, it will most likely be some hba or hub cards from PCIX/e slots or peripherals like cdrom that I have no control or even have an idea what's going on inside. What I do on the motherboard or at the centerplane will unlikely to have significant impact because they are buried inside the chassis. Hence the fatalistic view below. -----Original Message----- From: Ihsan Erdin [mailto:erdinih@xxxxxxxxx] Sent: Wednesday, August 16, 2006 5:45 AM To: Chris Cheng Cc: si-list@xxxxxxxxxxxxx Subject: Re: [SI-LIST] Re: Cable grounding scheme Ethernet isolation is more of a safety issue than EMI/SI, which overrides the design considerations here; hence beyond the scope of the discussion. I think I made myself clear in the previous discussion. For any specific application, each designer can use his common sense within the framework of the general picture I drew there. Regards, Ihsan On 8/16/06, Chris Cheng < Chris.Cheng@xxxxxxxxxxxx> wrote: We can argue about tying chassis to logic ground here or there till everyone faces turn blue but specs like Ethernet calls out specifically for isolated chassis from logic ground on RJ45. FC-PI CHANGES from DC short between logic and chassis (in optical modules) to complete isolation (in quad lane connector). So how do you dictate a system that support iSCSI and FCAL at the same time ? To make things more fun, what are you going to do with peripherals like disk drives and cdrom ? Have you ohm out your disk drive chassis and your disk cable ground lately ? Do you even see them being consistent between different vendors ? -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto: si-list-bounce@xxxxxxxxxxxxx]On Behalf Of chen_jinhua@xxxxxxx Sent: Tuesday, August 15, 2006 9:13 AM To: erdinih@xxxxxxxxx; xileil@xxxxxxxxxxx <mailto:xileil@xxxxxxxxxxx> Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Cable grounding scheme Ihsan If I understand your email correctly, you still use separate chassis and logic grounds. But you use many many stitching points to connect them together. If you consider the high speed cable application. It will impact SI. If the cable does not have separate logic and chassis grounds. Cable reference is chassis ground when it connects to the board connector. From connector to semiconductor chips, there will be a reference interruption because chip references to logic ground. Depends on how bad of the reference interruption, the SI impacts will vary. If the signal-point connection is used, I would guess the SI impact is huge. This brings an old question: single-point connection vs. many many points of connections, which one we prefer for high speed SI and EMI? Or it depends ... Thanks! Jinhua ________________________________ From: Ihsan Erdin [mailto: erdinih@xxxxxxxxx] Sent: Tuesday, August 15, 2006 9:40 AM To: Xilei Liu Cc: chen, jinhua; si-list@xxxxxxxxxxxxx Subject: Re: [SI-LIST] Re: Cable grounding scheme Celine, First of all, I want to express my apologies to Jinhua for -kind of- hijacking his topic for a potentially flaring issue. I think this issue goes as far back as the debate over single-point vs.multi-point connection between chassis and digital grounds. As such, it is more of an conducted/radiated emission problem than SI. The dilemma is while single-point connection could be justified by the fact that it avoids very low frequency common mode (noise) currents from creeping into the power line, in order to cut down on the radiated emissions at high frequencies, multi-point connection is strictly required between the two reference systems. In his "EMC and printed Circuit board design theory and layout made simple" book, for example, M. Montrose suggests stitching the two reference systems at a distance of lambda/20, with lambda being the wavelength of the highest frequency component of the spectrum of the system. The book was published in 1999. With today's multi-gigahertz systems, it's impossible to achieve such a design goal and it's an overkill at any rate. But the necessity of multi-point connection is not a debate any more. Some designers try to find a mid-way by connecting the reference systems with high frequency caps but the boards are already overly-populated by the same type of caps used for decoupling and there's the issue of parasitic inductances that defeat the purpose. Today, the commonplace approach, at least in the designs that I observe, is to suppress the low frequency CM currents with power line filters and directly connect the chassis and digital grounds at practically as many points as possible against radiated emissions. If you want to see some numbers and charts to support these ideas, in "EMI and Troubleshooting Techniques" book, M. Mardiguian gives a very good example that compares the two grounding strategies. Regards, Ihsan On 8/15/06, Xilei Liu < <mailto:xileil@xxxxxxxxxxx> xileil@xxxxxxxxxxx> wrote: Hey, Ihsan, I've ever seen notes that published in the past in 2002, saying that"we learned from NRAO engineers that it is both feasible and advisable to physically separate digital circuits from analog systems, and to interpose a minimum of two levels of Faraday shielding acting in series." From my point of view, it should be easier to employ different EMI solutions for power line and signal line separately when the digital/analog grounds are separated and connected somehow at a single-point. What will be the problems in terms of SI? Welcome your 'fight back' so that I can learn more ;) Regards, Celine >From: "Ihsan Erdin" < erdinih@xxxxxxxxx> >Reply-To: erdinih@xxxxxxxxx >To: chen_jinhua@xxxxxxx >CC: si-list@xxxxxxxxxxxxx >Subject: [SI-LIST] Re: Cable grounding scheme >Date: Tue, 15 Aug 2006 06:29:32 -0400 > >This question makes me wonder if there're any designers left who still >separate logic ground from the chassis ground in high-speed digital circuit >design -and on what basis? I thought this whole issue of chassis vs. logic >ground was something of the past. >Regards, > >Ihsan > >On 8/14/06, chen_jinhua@xxxxxxx < chen_jinhua@xxxxxxx > wrote: > > > > Hi, > > > > I have a few general questions about the high speed cable grounding > > scheme. It could impact both SI and EMI. I would like to have your > > inputs about this issue. > > > > Scheme 1: cable does not separate logic ground and chassis ground. But > > when it connects the system/boards, the system/boards have separate > > logic ground and chassis ground. How do you separate/connect the logic > > ground to chassis ground in boards? What is the pros and cons for SI > > and/or EMI?=20 > > > > Scheme 2: Cable keeps separate logic ground and chassis ground. > > System/boards also keep the separate logic and chassis ground. Cable > > logic ground and board logic ground connects, and chassis connects the > > chassis ground. How do you separate/connect the logic ground to chassis > > ground in boards? What is the pros and cons for SI and/or EMI? > > > > Do you prefer scheme 1 or scheme 2? What is the pros and cons of scheme > > 1 vs. scheme 2 for SI and/or EMI? Does SI and EMI have conflict > > requirements? > > > > Thanks! > > > > Jinhua Chen > > SI of Hardware Engineering > > EMC Corp. > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu