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.
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."
Someone at IEC thinks its possible. And many labs do, in fact, test at 3
meters.
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.
_\\|//_
(' O-O ')
ooO-(_)-Ooo
Mark Carter
AM Communications, Inc.
car...@amcomm.com <mailto:car...@amcomm.com>
Voice: 215-538-8710
Fax: 215-538-8779
-----Original Message-----
From: Ken Javor [mailto:ken.ja...@emccompliance.com]
Sent: Friday, July 28, 2000 2:51 PM
To: CARTER; 'jim_bac...@mail.monarch.com'; Franz Gisin;
emc-p...@ieee.org
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 <car...@amcomm.com>
>To: "'jim_bac...@mail.monarch.com'" <jim_bac...@mail.monarch.com>, Franz
Gisin
<emc_...@yahoo.com>, emc-p...@ieee.org
>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: jim_bac...@mail.monarch.com [mailto:jim_bac...@mail.monarch.com]
> Sent: Friday, July 28, 2000 10:36 AM
> To: Franz Gisin; emc-p...@ieee.org
> Subject: Re:RE: Rack populating??-Rationalize it!!
>
>
>
> forwarding for Franz
>
> ____________________Reply Separator____________________
> Subject: RE: Rack populating??-Rationalize it!!
> Author: Franz Gisin <emc_...@yahoo.com>
> 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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>
> -------------------------------------------
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