I have two comments regarding this post. 1) Regarding this: "(It's good to emphasize that the current limits will only protect radio reception . An excess of a few dB will ONLY interfere with radio reception. No other interference problems can be identified in my opinion)."
Thank you, that apparently cannot be emphasized enough. 2) Regarding this: "The radio protection limits were created with sufficient margins so as that even 10 dB excess will not create excessive interference problems. A margin was built-in." Disagree. All radio reception is matter of signal-to-noise ratio. Agreed, if the incoming signal is 40 dB higher than the limit, you will not notice an emission 10 dB above the limit. But if the incoming broadcast signal is 20 dB above the limit and the limit is exceeded by 10 dB, you will notice definite degradation (on AM, if not FM). The limits were placed (at least in the USA) in order to protect most radio reception, but not trying to protect weak reception is a fringe area - it was not deemed economical to impose a stringent limit across an entire product line to protect a minority of users. Before you say it is different in Europe, look at CISPR B limits vs. original FCC limits (Title 47, Part 15, Sub-part J). If the limits are close, the same rationale holds. > From: "Gert Gremmen" <[email protected]> > Date: Wed, 4 Jul 2007 09:41:42 +0200 > To: <[email protected]> > Subject: RE: Internal Margins for Emissions Testing > > I have been reading this thread for a while, but missed an important > aspect. > > Engineers in our profession have been looking at limits as absolute > LIMITS, not to be exceeded by any device under any circumstances. > To meet this in practical situations, many approaches have been > suggested, most suggesting extra severe limits, where the discussion is > about how much more severe. > > Well in Europe, and this shows the strength of the EMC directive, the > bottom line is that equipment should not interfere. (simplified > essential requirement) > > The fact that a piece of equipment exceeds a limit does not mean that it > will interfere, just that chances are better that it will. The same for > equipment below the limit, chances are only slightly better that it > won't interfere. > A interference peak at 102 MHz, will not create interference until > someone within a certain radius will decide to listen to 102 MHz on his > portable radio. > > (It's good to emphasize that the current limits will only protect radio > reception . An excess of a few dB will ONLY interfere with radio > reception. No other interference problems can be identified in my > opinion) > > It's up to the manufacturer to show that a particular negative margin > will not cause problems in terms of interference. For this to happen he > may choose to create a technical construction file route, and from July > the 20th on he does not need a competent body approval anymore. An > approach showing that an emissions excess of i.e. 2.5 dB at a certain > frequency for a certain percentage of equipment could be well defended > with appropriate arguments, in terms of applied environment, statistics > (i.e. number of equipment in use) and arguments of technical nature. > (i.e. will not be used near in radio receiving equipment) . > Limits are not absolute. > > > Each of us knows that respecting a standard's limits will not guarantee > that no interference will happen, nor can this standard guarantee that > the particular equipment, if deviously used/installed, will not exceed > the limits anyway. > > > Of course, maintaining the limits, following harmonized standards is an > easy (!!?!) legal way to obtain presumption of conformity. > Keeping your emission profile below this limit however, under all > circumstances in all operating modes and all configurations and at the > same time keeping track of manufacturing variations and measurement > uncertainty would lead to unacceptable low values of interference > in-house limits. > This problem may be somewhat relaxed by applying the CISPR 80/80 > limits, but this approach requires multiple samples to be tested which > can be a problem for other reasons. This would lead to unacceptable > costs for EMC while not justifying a reduced interference profile. > > Therefore, I suggest to just maintain the limits, do not add extra > margins, unless your measurement uncertainty exceeds the CISPR16 > suggested values. > > The radio protection limits were created with sufficient margins so as > that even 10 dB excess will not create excessive interference problems. > A margin was build-in. > > In terms of interference capabilities, the amplitude criterion is not > the holey grail. The applied QP detector will not also accurately > describe interference capabilities for FM, PM or digital communication > systems. > > In general, when speaking about the chance of a piece of equipment > causing interference , it also makes sense to qualify the type of > interference. > A data over power line solution, creating full continuous interference > all over the band while remaining below the limit will certainly do more > harm in terms of interference as one clock harmonics at 107.6 MHz. > > I believe that the a limit should be based on the integral of frequency > / amplitude characteristics of a type of equipment. The more > interference components in the band, the lesser the amplitude may be. A > single component would be allowed to exceed the current limits, where a > large number of peaks need to remain substantially below the current > limits. This approach would do justice to the chances any interference > could be created. > Of course these new limits could be detailed out, creating stronger > protection in certain bands, while relaxing in other bands, but I wanted > to discuss the principle. > > Gentlemen and Ladies (yes there are) , > > Let me know your opinions... > > I know I opened (re-opened) a can of worms....... > > > Regards, > > Ing. Gert Gremmen > Approvals manager > ------------------------------------------------------------------------ > ------------ > > ce-test, qualified testing bv > + ce marking > + Independent Consultancy Services > + Compliance Testing and Design for CE > according to: > - Electro magnetic Compatibility 89/336/EC > - Electrical Safety 73/23/EC > - Medical Devices 93/42/EC > - Radio & Telecommunication Terminal equipment 99/5/EC > > + Improvement of Product Quality and Reliability testing > > Web: www.cetest.nl (English) or www.ce-test.nl (Dutch) > > Phone : +31 10 415 24 26 > Fax : +31 10 415 49 53 > ------------------------------------------------------------------------ > ------------ > This e-mail and any attachments thereto may contain information that is > confidential and/or protected by intellectual property rights and are > intended for the sole use of the recipient(s) named above. Any use of > the > information contained herein (including, but not limited to, total or > partial reproduction, communication or distribution in any form) by > persons other than > the designated recipient(s) is prohibited. If you have received this > e-mail in error, please notify the sender either by telephone or by > e-mail and delete the material from any computer. Thank you for your > co-operation. > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf Of John > Woodgate > Sent: Wednesday, July 04, 2007 6:53 AM > To: [email protected] > Subject: Re: Internal Margins for Emissions Testing > > In message > <f38c1e8a75404347a126608a8f48af5503097...@mer2-excha2.echostar.com>, > dated Tue, 3 Jul 2007, "Grasso, Charles" <[email protected]> > writes: > >> In the absense of any data, we decided to *strive* for 6dB of margin. >> That allows for measurement uncertainty >> and a couple of db for manufacturing varience. > > The proof of the pudding... > > It is most unlikely that a practicable margin could be determined by > reasoning alone. You make a reasonable guess, such as 6 dB indeed, and > see whether in practice it eliminates problems, from failures when new > products are first sent to the test house, through rejections in > production testing, to complaints from regulatory authorities. > > With this approach, as with EMC limits, you can't tell if the > requirement is too severe, i.e. you can't tell (usually) if 5 dB margin > would still keep you out of trouble. Quality assurance statistics may > give a steer. > -- > OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk > There are benefits from being irrational - just ask the square root of > 2. > John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. Website: http://www.ieee-pses.org/ > > To post a message to the list, send your e-mail to [email protected] > > 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] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. Website: http://www.ieee-pses.org/ > > To post a message to the list, send your e-mail to [email protected] > > 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] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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