I don't know much about the commercial side, but there is a really well-written document for the US military standard (MIL-STD-461) called SAE EIA EMCB 1-1 - Historical Rationale for Military EMI Limits (behind a damned paywall, unfortunately, but I can try to send it to you if you'd like). (https://standards.globalspec.com/std/10021073/EIA%20EMCB%201-1) Every test has it's own unique origin story, but I think the answer often tends to be starting with a known problem, doing the analysis work to say what it would take to solve it, throwing on an extra x dB of margin and making minor course corrections based on the feedback you get from the community along the way. Sprinkle in some marketing, bias, and in-fighting and you've got yourself a standard! To be clear, I say this from a perspective of having never served on a standards committee (despite desperately wanting to fill a former colleagues position on the tri-service working group). I'm sure Ken and others in the group will have a much more valuable input here, but I just so happen to be fascinated with the history and finding any old rationale document I can.
I'm always telling newer folks that it really doesn't matter if the limit or test methodology (as written in the standard) is "wrong", so long as it's predictive and everyone does it the same wrong way. And the tests are really good at predicting that you won't have a problem down the road when you pass... and not so great at predicting anything when you fail. Even some of the easiest tests to translate to installation (CE106 and CS104) don't do a great job at predicting actual problems when they've failed the test. I've seen things fail CS104 for broad swathes of frequencies at -40 dBm and surrounded by emitters in that frequency range, and it performs just fine on the aircraft... until one day when it won't... but now I've strayed too far from your original question. On Tue, 3 May 2022 at 04:42, James Pawson (U3C) <ja...@unit3compliance.co.uk> wrote: > Hello all, > > > > A question I get asked by our customers, and that I’ve not found a > satisfactory answer to, is why are there steps in the limit lines for > radiated emissions? > > > > This kind of leads to the question of how do limit lines get specified in > the first place? For emissions and immunity. > > > > I know that it relates to protection of radio receivers – the limits in > CISPR 25 or EN 60945 are good examples – but how are the limits set? > > > > Apologies for the vague expression of the question! > > > > All the best > > James > > > > > > James Pawson > > Managing Director & EMC Problem Solver > > > > *Unit 3 Compliance Ltd* > > *EMC : Environmental & Vibration : Electrical Safety : CE & UKCA : > Consultancy* > > > > www.unit3compliance.co.uk | ja...@unit3compliance.co.uk > > +44(0)1274 911747 | +44(0)7811 139957 > > 2 Wellington Business Park, New Lane, Bradford, BD4 8AL > > Registered in England and Wales # 10574298 > > > > > > > > > ------------------------------ > [image: AVG logo] <https://www.avg.com/internet-security> > > This email has been checked for viruses by AVG antivirus software. > www.avg.com <https://www.avg.com/internet-security> > > <#m_8956265982425463356_DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> > - > ---------------------------------------------------------------- > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to < > emc-p...@ieee.org> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in > well-used formats), large files, etc. > > Website: http://www.ieee-pses.org/ > Instructions: http://www.ieee-pses.org/list.html (including how to > unsubscribe) <http://www.ieee-pses.org/list.html> > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <sdoug...@ieee.org> > Mike Cantwell <mcantw...@ieee.org> > > For policy questions, send mail to: > Jim Bacher <j.bac...@ieee.org> > David Heald <dhe...@gmail.com> > - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
