In addition to the responses from Doug, Ghery and Brian, I will note that margin protects you from unexpected or unknown changes from component suppliers. To some extent, this falls under the manufacturing variance Doug mentioned, but component changes is just another area that can be hard to control.
I've had IC vendors do a die shrink on a part resulting in sharper edge rates on the outputs. At a previous employer, I was running emissions testing on a number of samples where Motorola did a die shrink on the microcontroller we were using. Some of my test samples had the old part and some had the new. It took a long time to figure out why some samples were significantly worse than others after controlling for all other variables. Having 6 dB margin to begin with provides some protection against this type of change. Ted Eckert Compliance Engineer Microsoft Corporation ted.eck...@microsoft.com<mailto:ted.eck...@microsoft.com> The opinions expressed are my own and do not necessarily reflect those of my employer. From: Itzenheiser, Jerry (GE Healthcare) [mailto:gerald.itzenhei...@med.ge.com] Sent: Monday, October 12, 2015 12:26 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where? Hello EMCers, I would like to ask... Is there anyone out there that tests to stricter limits than the legal (CISPR, IEC, etc.) limits? If so, what was the rationale behind selecting the stricter limits? Our engineering teams are curious as to where the stricter recommended limits come from, such as the 6dB margin for emissions testing. Thanks, Jerry Itzenheiser Jr EMC Technician - Waukesha GE Healthcare Global Engineering Technologies EMC Laboratory Waukesha T + 262-548-2217 M + 262-720-8846 gerald.itzenhei...@med.ge.com<mailto:gerald.itzenhei...@med.ge.com> www.ge.com<https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fwww.ge.com%2f&data=01%7c01%7cted.eckert%40MICROSOFT.COM%7c1f785e71fc11409e76c408d2d33bbe0a%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=WGwl3c%2bUU7WAC%2bg6Bk9SLRF9h6fEB5snjR84zIbqoVY%3d> 3000 N. 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