----------
>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 ....
>>
>>
>> -------------------------------------------
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>>
>> For policy questions, send mail to:
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>>
>>
>> -------------------------------------------
>> 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]
>>
>>
>
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