I agree that the extrapolations are not, shall we say, extremely precise. Nor are most of the measurements we do - regardless of our confidence in our equipment and our expertise. All the more reason to try to make test environments repeatable, and EUT's configured for maximized (worst case?) emissions.
-----Original Message----- From: Ken Javor [mailto:[email protected]] Sent: Friday, July 28, 2000 3:40 PM To: CARTER; carter; '[email protected]'; franz gisin; [email protected] Subject: Re: Rack populating??-Rationalize it!! ---------- >From: CARTER <[email protected]> >To: "'Ken Javor'" <[email protected]>, CARTER <[email protected]>, "'[email protected]'" <[email protected]>, Franz Gisin <[email protected]>, [email protected] >Subject: RE: Rack populating??-Rationalize it!! >Date: Fri, Jul 28, 2000, 10:14 AM > > I have long forgotten the equation for computing free space transmission > loss, but it seems to me that when radiated power, frequency, and distance > are known, one can certainly predict field intensity at any other distance. > If that is not true, we have made a large number of terrestrial microwave > transmission paths work purely by accident. MICROWAVE LINKS WORK IN THE FAR FIELD. > > Also, CISPR 22 11.2.1: > "NOTE - If the field-strength measurement at 10 m cannot be made because of > high ambient noise levels or for other reasons, measurements of Class B > EUT's may be made at a closer distance, for example 3 m. An inverse > proportionality factor of 20 dB per decade should be used to normalize the > measured data to the specified distance for determining compliance. Card > should be taken in the measurement of large EUT's at 3 m at frequencies near > 30 MHz due to near field effects." 10 METER TO 3 METER SCALING WORKS ONLY WHEN THE EUT IS SMALL WITH RESPECT TO 3 METERS, AND WHEN THE ANTENNA DIMESNIOS ARE SMALL WITH RESPECT TO 3 M. IF YOU TRIED TO DO A 3 METER SITE ATTENUATION WITH TWO 30 MHz TUNED DIPOLES, YOU WOULD FIND YOU NEEDED A CORRECTION FACTOR TO ACCOUNT FOR THE NEAR FIELD EFECTS. > > Someone at IEC thinks its possible. And many labs do, in fact, test at 3 > meters. THERE HAVE BEEN A LOT OF PAPERS WRITTEN SAYING THE EXTRAPOLATION IS FAR FROM ACCURATE. > > I was not suggesting that it would be wise or meaningful to do these things. > My point is simply that it is wise to maximize the emissions from an EUT to > insure that we are not missing emissions which may be above the limits, and > that the end user of the equipment is not likely to inadvertently create > such a situation. UNDERSTAND. > > _\\|//_ > (' O-O ') > ooO-(_)-Ooo > > Mark Carter > AM Communications, Inc. > [email protected] <mailto:[email protected]> > Voice: 215-538-8710 > Fax: 215-538-8779 > > > -----Original Message----- > From: Ken Javor [mailto:[email protected]] > Sent: Friday, July 28, 2000 2:51 PM > To: CARTER; '[email protected]'; Franz Gisin; > [email protected] > Subject: Re: Rack populating??-Rationalize it!! > > > There is a very big, very important error in Mr. Carter's point number 2. > You absolutely CANNOT extrapolate from 10 m to any other distance, unless > that other distance also happens to be both in the far field of the EUT AND > the far field of the measurement antenna. That is the reason for a 10 m > measurement in the first place. Everyone would be doing three meter > measurements if it weren't for issues related to three meters not being far > enough away. > > Extrapolating a 10 m measurement to one airline seat away is totally > impossible. And completely unnecessary: the victim protected by the 10 m > measurement is not some arbitrary "gizmoid," it is a radio receiver. You > aren't supposed to be operating a radio receiver on the airplane. More to > the point, the problem with laptops and other personal electronics on a > commercial transport is not EMI to other personal electronics, but > interference with aircraft antenna-connected receivers, whose antennas are > mounted external to the aircraft. > > ---------- >>From: CARTER <[email protected]> >>To: "'[email protected]'" <[email protected]>, Franz > Gisin > <[email protected]>, [email protected] >>Subject: RE: RE: Rack populating??-Rationalize it!! >>Date: Fri, Jul 28, 2000, 7:35 AM >> > >> >> Jim, >> >> I differ in opinion on at least two counts: >> >> 1. CISPR 22 9.1 states "An attempt shall be made to maximize the > disturbance >> consistent with typical applications . . . etc." This can and should be >> construed as creating a worst-case scenario. Worst case is always assumed > to >> be within the range of operating conditions reasonably expected. If a > laptop >> will have higher emissions when it is in flames, no one (at least no one I >> know) is suggesting that it be tested that way. Setting a laptop ablaze is >> not within the range of reason. >> >> 2. The intent of making measurements at 10 meters, or 3 meters, or with an >> absorbing clamp, or any of the other requirements is not to insure that no >> interference will occur at 10 meters, 3 meters, or in a clamp, but that > test >> results are repeatable. If the limit is 40 dBuV/m at 10 meters, and I test >> at 20 meters, will the EUT pass? Of course not. If I have test results > from >> a 10 meter set-up, I can compute the effect of an emission at any other >> distance from the EUT to the next airline seat, or whatever. If I wanted > to >> know that. The point is that we're all on the same page that way. >> >> If you do the math, you can compute the interfering field strength of an >> emission from a device in the next airline seat, and if you know the >> immunity of the gizmoid in that seat, you can pretty much be certain that >> will not be any disruptive interference. >> >> My 2 cents >> >> >> -----Original Message----- >> From: [email protected] [mailto:[email protected]] >> Sent: Friday, July 28, 2000 10:36 AM >> To: Franz Gisin; [email protected] >> Subject: Re:RE: Rack populating??-Rationalize it!! >> >> >> >> forwarding for Franz >> >> ____________________Reply Separator____________________ >> Subject: RE: Rack populating??-Rationalize it!! >> Author: Franz Gisin <[email protected]> >> Date: 7/27/00 6:48 PM >> >> Worst case is not relevant when it comes to defining >> EMC test configurations for ITE equipment. I do not >> know of any ITE EMC test standard that specifically >> states "worst case" must be used. Eveywhere I look I >> see words like "typical" or "representative" or >> "minimum". >> >> I am willing to bet that 99.9% of you, when you bought >> your last car, did not insist the car be at least 6 dB >> under the smog limits under worst case conditions >> before you bought it (e.g. ask the car manufacturer to >> load the car down with bricks, take it to a very steep >> hill, facing the front of the car up the hill instead >> of down, and then pushed the accelerator all the way >> to the floor - with the engine running but still cold >> - before they measured the peak emission levels rather >> than quasi-peak). Whenever I ask EMC engineers if >> they do this when they buy a car, they think I am >> crazy, and yet they see nothing wrong with doing >> exactly the same thing themselves when it comes to >> defining EMC test configurations and test methods. >> >> Claiming that a configuration more than what the >> regulations ask for is "failing", is as ridiculous as >> claiming that a configuration less than what the >> regulations ask for is "passing". Not a day goes by >> that I don't hear one or the other being carelessly >> brandished about. >> >> If you want to bring reality into the picture (usually >> a bad idea when it comes to law and regulations so >> forgive me for going there anyway), we should make >> sure all products which are tested at a 10 meter >> distance are spaced at least 10 meters apart when >> installed at our customer's sites. Laptops on >> airplanes should be spaced at least 20 rows apart >> (this assumes the flight attendants verify each laptop >> comes from a reputable manufacturer they can >> personally vouch for as having an EMC department >> populated by people of integrity). >> >> It is worthwhile to note that the ITE EMC emission >> standards in the US and most of the rest of the world >> are designed to reduce the probability of >> interference, not completely eliminate it - just as >> smog standards are not designed to prevent death >> should some frustrated EMC engineer decide to take his >> life by running his smog-compliant car inside a closed >> garage. >> >> How many EMC engineers do you know who are >> uncomfortable with the "probability" aspects of the >> regulations and decide to take it upon themselves to >> get as close as possible to eliminating all potential >> for interference by distorting the test configuration >> sections of the standards until they fit under their >> personal definition of quality? There are a lot of >> places where quality (mean time between failures, for >> example) means more to a customer (including car >> owners) than whether the unit was overdesigned to meet >> EMC and/or smog standards. I don't feel I am any less >> of an EMC engineer or manager for making that >> statement. >> >> I would like to close by saying all the comments above >> are meant to act as a catalyst, food for thought so to >> speak, rather than be an accurate rendition of how I >> (or any of the companies I have worked for, am working >> for, or will work for) perceive what this crazy EMC >> discipline is all about. Feel free to discuss it as >> you wish. If any of you want to dialog eyeball to >> eyeball, look me up at the International EMC symposium >> in Washington DC next month. I prefer cold beers >> (domestic or imported) over wine. >> >> Franz Gisin >> Sometimes an EMC Engineer >> Sometimes an EMC Manager >> Always Opinionated as .... >> >> >> ------------------------------------------- >> This message is from the IEEE EMC Society Product Safety >> Technical Committee emc-pstc discussion list. >> >> To cancel your subscription, send mail to: >> [email protected] >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Jim Bacher: [email protected] >> Michael Garretson: [email protected] >> >> For policy questions, send mail to: >> Richard Nute: [email protected] >> >> >> ------------------------------------------- >> This message is from the IEEE EMC Society Product Safety >> Technical Committee emc-pstc discussion list. >> >> To cancel your subscription, send mail to: >> [email protected] >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Jim Bacher: [email protected] >> Michael Garretson: [email protected] >> >> For policy questions, send mail to: >> Richard Nute: [email protected] >> >> > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
