Just as a point of clarity.....................the +/- 4 dB NSA range
applies to OATS as well so there could be as much as 8 dB difference
between one OATS and another and both could have good NSAs. The +/- 4 dB is
not a parameter just for volumetric measurements but applies to all test
sites. With volumetric measurements it is more difficult to attain the +/-
4 dB.
And I have to agree with an earlier email. A lot 3 meter "chambers" that I
have seen are "home made" and would not pass NSA.
Bob Heller
3M EMC Laboratory, 76-1-01
St. Paul, MN 55107-1208
Tel: 651- 778-6336
Fax: 651-778-6252
=======================================
==========================================================
Bill Owsley
<[email protected]
om> To
Sent by: "Garnier, David S \(GE
[email protected] Healthcare\)"
<[email protected]>
[email protected]
06/19/2007 01:22 cc
PM
Subject
Re: FW: RE Measurement Distance per
EU
A number of years ago, IBM did a chamber correlation around the world. I
think they published it (IEEE). They used a point source, well as close as
one can get in the real world - something from SETH if I remember right.
It looked like a ball with two isolated hemispheres driven from the
internal circuits. Given that the 3 meter and 10 meter chamber are meant
to correlate with the assumption of a point source radiator, the survey
found that the chambers did just that. (again going from old memory)
Now since we do not normally test point sources, not a profitable market
there, what we do test are products that are larger, with multiple parts,
numerous leaks, cables, etc. All of these real world effects conspire to
make a mess of any correlation between 3 and 10 meters. The real world
products do not have a 1/r roll off of a plane wave, until you get so far
away that the product does begin to look like a point source. Humour ON,
the way to tell if far enough away is hold up your thumb at arms length and
if your thumb covers the product, you are in the far field - that called a
rule of thumb. There is an exception (as always) called an EMM - look that
one up. humour off. I've had a number of products pass at 3 meters to
fail at 10 and with nearly the same profile on the spectrum plot, besides
the amplitude that did not drop the 10 dB as expected.
BTW, Isn't the NSA for chambers a volumetric one now? And isn't the NSA a
+/- 4 dB range, so there could be as much as 8 dB from one to another
chamber that both have good NSA's? Could we call "correlate" as something
within 8 dB difference?
"Garnier, David S (GE Healthcare)" <[email protected]> wrote:
Ken,
I agree with most everything you have said concerning 10 meter sites but I
am going to play
the heretic role against 3 meter chamber usage.
We build and test large product here, CT, CT/Pet, X-ray systems, (EUT's
often weight close to
12 tons.) It's very common today to have hospitals order 2 or 3 phase
UPS's with our systems,
data backup during weekly emergency generator testing or riding out power
glitches during
electrical storms.
My negative experience with 3 meter chambers is the following.
Unfortunately, some of our UPS vendors like to certify their products in 3
meter chambers.
I wish I had a 100 dollars for every conversation that has ensued, "No,
your 3 meter radiated
emission test data does not correlate to our 10 meter chamber for 100 MHz
and below." "Why?"
I think UPS's are evil but that's a different subject. ;-)
We also build smaller diagnostic products, such as roll around, plug into
120 Vac 15A outlet
Ultrasound machines. For example, if we test this product in our 3 meter
chamber we will not
see 100 MHz and below board band radiated emissions, same situation as 3
meter RE test
data for UPS's.
For the sake of discussion, I would support 3 meter chamber usage for 100
MHz and below
*IF* the requirement for AC line conducted emission were raised to 100
MHz. I bet then we
would see some correlation to 10 meter RE data.
Regards,
Dave Garnier
David Garnier
e GE HealthCare
___________________________________________
David S. Garnier
Quality Assurance Technician
Quality-DI RA-EMC
3000 N. Grandview Ave - M/S W-1250
Waukesha, Wi. 53188
Tel: 262.312.7246
Cel: 414.899.7580
From: [email protected] [mailto:[email protected]] On Behalf Of Ken Javor
Sent: Monday, June 18, 2007 2:51 PM
To: [email protected]
Subject: Re: RE Measurement Distance per EU
It seems clear to me, from this post and what I know, that the 10 meter
site is the gold-standard. If I were a commercial test facility, or an
in-house test facility for a company making large products, a ten meter
site would be the way to go.
But if I were a PC manufacturer, or if I qualified items that size or
smaller, I can see no justification whatsoever for the huge cost of the
ten meter chamber vs. a three meter chamber. As long as I am already
using a biconical antenna instead of a tuned dipole at the low end, I see
no cost-effective rationale for the larger chamber.
Even if my small test item might have a long cable attached to it, I ought
to be able to control the cable common-mode current emissions using a
current probe or absorbing clamp and be able to generate a limit for those
currents commensurate with the radiated emission limit at three or ten
meters.
This approach is not new, it dates back to at least the 1980s, with a
series of papers given at the Zurich symposia by Mr. Balint Szentkuti
(e.g., Szentkuti, B., Give Up Radiation Testing In Favour Of Conduction
Testing, Proceedings, EMC Zurich 1989).
I'm not advocating elimination of radiated testing, but simply recognizing
that emissions emanating from long cables are more accurately measured and
controlled as currents than as fields, not to mention much less
expensively. For precisely the same reason that 61000-4-6 is performed in
lieu of 61000-4-3 below 80 MHz.
From: "Kunde, Brian" <[email protected]>
Date: Mon, 18 Jun 2007 15:32:32 -0400
To: "Grace Lin" <[email protected]>, <[email protected]>
Subject: RE: RE Measurement Distance per EU
Grace,
Do they make a fully compliant 3 meter semi-anechoic chamber? I
wasn't aware that a 3 meter chamber passed the NSA requirements down
to 30Mhz.
The purpose of having a Test Standard is to standardize on a method
in which testing is performed so the results can be repeated at
other test sites and hopefully have the same or similar results.
When you stray from what the standard calls out, then you take the
risk of your results being brought into question.
I think your biggest problem with testing at 3 meter is going to be
with near field below 100Mhz. Because of the common differences seen
when testing a product at 3 meter verses 10 meter, I think there
will always be a high level of doubt in your test results.
In my opinion, "compliance testing" can only be confidently done at
a 10 meter OATS or equivalent.
BTW, the note in section 10.2.1 is a method to test signals that
fall near or on a high ambient. Most all OATS have at least a few
areas with high ambient and this section gives them a work around at
only those frequencies. It is not intended to allow full testing at
distances than 10 meter. In a chamber you wouldn't have any ambient
signals so you couldn't use this section to justify 3 meter testing.
I am very curious to know the confidence level of 3 meter chambers
are. Maybe some who have such chambers will post.
The Other Brian.
From: [email protected] [mailto:[email protected]] On Behalf Of
Grace Lin
Sent: Monday, June 18, 2007 1:43 PM
To: [email protected]
Subject: RE Measurement Distance per EU
Dear Members,
Is 3m measurement distance for radiated emission not accepted by EU?
I propose a 3m full compliant chamber. A chamber manufacturer
informs me that EU only accepts 10m measurement distance. I checked
EN 55022: 1998 incorprating Corrigendum Nol 1 and A1 and A2. Clause
6 states:
"The EUT shall meet the limits of Table 5 or Table 6 when measured
at the measuring distance R in accordance with the methods described
in Clause 10. ... "
The title of Table 5 is: "Table 5 - Limits for radiated disturbance
of class A ITE at a measuring distance of 10m". The title of Table
6 is: "Table 6 - Limits for radiated disturbanc eof class B ITE at a
measuring distance of 10m". Subclause 10.2.1 states:
"
10.2.1 Antenna-to-EUT distance
Measurements of the radiated field shall be made with the antenna
located at the horizontal distance from the boundary of the EUT as
specified in Clasue 6.
NOTE If the field-strength measurement at 10m cannot be made because
of high ambient noise levels, or fo rother reasons, measurement of
class B EUT's may be made at a closer distance, for example 3m. An
inverse proportionality factor of 20 dB per decade should be used to
normalize the measured data to the specified distance for
determining compliance. Care should be taken in the measurement of
large EUTs at 3m at frequencies near 30 MHz, due to near field
effects.
"
I look forward to your comments.
Best regards,
Grace Lin
Crestron Electronics, Inc.
www.crestron.com <http://www.crestron.com>
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