HI James,
This problem child isn't my design, just one that came in my lab for debugging and testing: my life revolves around fixing other peoples problems :-) I am not a proponent of having all planes tied together, only ever seen disasters that way. If you allow circuit currents to deliberately on inadvertently flow in the chassis it becomes a nightmare to control them. Much better to separate and segregate so you have control over outbound ( usually on a trace ) and return current ( usually on a plane ). This technique also allows you to keep outside threats like ESD and HIRF restricted to the connector and prevents them progressing into your ( metal ) box. If your box is plastic, you have to me more careful. Does this help? Cheers, Derek. L F Research -----Original Message----- From: Pawson, James <james.paw...@echostar.com> To: EMC-PSTC <EMC-PSTC@LISTSERV.IEEE.ORG> Sent: Thu, Dec 19, 2013 11:00 am Subject: Re: [PSES] EMC Puzzle update Hi Derek, I’m glad you managed to find out what had been causing that problem. Why did you elect to have a separate “chassis plane” in the first place that (presumably) wasn’t tied into the PCB ground planes? My (limited) understanding is that it is better to have all ground planes connected together to provide a good quality RF return path and minimise the chance of traces crossing plane splits. Many thanks James From: Derek Walton [mailto:lfresea...@aol.com] Sent: 19 December 2013 16:24 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] EMC Puzzle update First off, many thanks to all who replied on and off list. Most of the replies we had already addressed, but here is an update. To re-cap, we had a sealed metal box with just a power cord, and that power cord had feed through type caps. In theory this should not radiate. To save the agony that we went through what was finally identified was on a 6 layer board, 6 traces left the sanctuary of a ground or power plane and were routed directly over a thin chassis plane that was incorporated to increase the number of chassis connections that we really needed on this board. Even though internal, we were able to break that connection and emissions reduced close to 30 dB. A board re-layout is needed to really fix this issue. As part of the debugging process, we also found that since the board chassis layer was "backfeeding" the Ethernet connector housing metal and the "grounding tabs" were just not adequate for bonding. The result was current on the outside of the case that cause the enclosure to radiate pretty much equally in all directions... Can I patent this? :-) The lesson here is that if you do bring chassis onto your PWB, and we do as a means to divert ESD and some RF current, that you keep it well away from internal high speed or sensitive traces and ideally confined to the region around where it's used. After our mods, we now have dropped from 20 dB over class A to about 13 dB under Class B. A big thank you once more to everyone that replied!!! Seasons Greatings to all. Sincerely, Derek. L F Research - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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