This is the unfortunate case of ideal world vs.. real world. A manufacturer of a subassembly will not design for the EMC behavior of other components in the target system unless he is really aware and involved in the system integration or forced to do so by other means. A manufacturer of a card for, let's say, a PC can use a (current) model of his choice for EMC testing and it is unlikely that the worst case will be used in this case for many reasons. The principle of due diligence applies, but is often victim to other principles... One way out of this dilemma could be a standard which uses for example your findings to define the requirements and an environment for verification, but the only applicable field I see are relatively simple systems. With more complex systems, you get a mix of shielded and unshielded cables depending on the required interfaces, making a standardized approach difficult. The best approach -IMHO- to control EMI is to minimize potential emissions at the source, reducing the need for filtering and shielding. This must happen during the design and must involve EMC experts. An approach which is not generalized yet - unfortunately. Best regards, Michael Nagel Michael Nagel Senior Staff EMC Test Engineer Embedded Computing
Emerson Network Power T +49-89-9608-0 F +49-89-9608-2376 [email protected] www.emersonnetworkpower.com/embeddedcomputing Emerson Network Power - Embedded Computing GmbH, Lilienthalstr. 15, D-85579 Neubiberg/Landkreis München, Deutschland / Germany. Geschäftsführer Josef Wenzl, Amtsgericht München HRB 171431, VAT/USt.-ID: DE 127472241 ________________________________ From: ce-test, qualified testing bv - Gert Gremmen [mailto:[email protected]] Sent: Montag, 27. April 2009 19:57 To: Nagel, Michael [NETPWR/EMBED/DE]; [email protected] Subject: RE: Compliant plus compliant does not equal compliant! All of these cases happen when a device or apparatus passes by sheer luck instead of design! For example: The port in question was not radiating at a resonant frequency because the WAS no such frequency in the EUT . Connecting a peripheral having such a frequency on board would connect it’s lead to this port and make it resonate. Or the lead length would be doubled in length compared to the test setup, allowing more efficient radiation. Or the connected EUT was transparent for RF and now Its other leads are radiating too, creating a antenna system with longer leads and radiating more efficient at lower F. It all happens when EMI is NOT under control. I/O ports should always be considered as an open port needing to be RF “firewalled” Allowed signals should pass, unwanted signals should be blocked. There are a lot of problems with software design but here we can learn something of those guys. No open ports! Good EMC design does not let one port open, unfiltered or unshielded and keeps it’s CM input impedance nor a short to enclosure (but for shielded cables), nor an infinite impedance, but ideally 150 ohm to enclosure. This effectively reduces resonant behavior. CM signals should meet a high ( >300 ohm) input impedance between lead and circuit. A peripheral should “end” a EUT’s cable, not extend it ! BTW that is why I do not like shielded cables to prevent EMC problems. Their functionality depends heavily on (very) low impedance paths to ground, which is acceptable in laboratory conditions, but difficult to maintain in industrial , automotive, dirty and corrosive environments. Gert Gremmen Van: [email protected] [mailto:[email protected]] Namens [email protected] Verzonden: woensdag 15 april 2009 17:41 Aan: [email protected] Onderwerp: Compliant plus compliant does not equal compliant, was: "Quiet" Laptop Hello! As pointed out below (and during the thread), the whole issue shows, that two devices, being measured compliant, are not automatically compliant as a system when measured together. A 'quiet' USB device connected to a 'quiet' host can pass with a poorly shielded cable. The same device with the same cable will most likely fail when connected to a 'loud' host. <insert finger pointing between manufacturer of USB device and host here> Given the number of different product/designs in domain of personal computers that can be plugged together, I cannot imagine an easy way to cope with that. Currently it is cost vs. pressure of the government agencies which establishes some kind of balance. System integrators try to cope with that by imposing lower emission limits to their suppliers. This works, but not in every case. I have seen a computer fail radiated emissions testing thanks to a printer connected to it. The physical arrangement of the I/O ports helped to build an antenna. The same printer must have passed in another configuration. The same computer passed radiated emissions testing with a different printer. Any similar experiences? Best regards, Michael Nagel Michael Nagel Senior Staff EMC Test Engineer Embedded Computing Emerson Network Power T +49-89-9608-0 F +49-89-9608-2376 [email protected] www.emersonnetworkpower.com/embeddedcomputing Emerson Network Power - Embedded Computing GmbH, Lilienthalstr. 15, D-85579 Neubiberg/Landkreis München, Deutschland / Germany. Geschäftsführer Josef Wenzl, Amtsgericht München HRB 171431, VAT/USt.-ID: DE 127472241 ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Bill Owsley Sent: Mittwoch, 15. April 2009 15:57 To: EMC-PSTC; [email protected]; Piotr Galka Subject: Re: SV: "Quiet" Laptop "...if all devices were tested separately the number of observed problems during using them in sets will grow by zero, zero, nothing." Alas, The problems seem to grow by the cube of the permutations of geometric sum of triple integral of number of connections. <;-) The symptom that empty ports don't radiate much, nor do poorly terminated cables that are not plugged into previously mentioned ports, seems to manifest when those two parts are plugged together, thus the reason for "system" testing. - Bill Indecision may or may not be the problem. - 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 <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - 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 <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]>
