It's all in the cables and perhaps mode of operation. If you set up the system
in your lab you will no doubt have some biases as to how it should be set up.
If the equipment is sent to a lab without support the lab technician will not
share these same biases and the set up differences may cause some changes in
measured emissions. I have to disagree with 20db variations as suggested. In
my 30 years plus I must say , if you are seeing that kind of variation someone
is doing something wrong and not following the guidance of the standard. The
second contributor is in the S.A site attenuation variation. OATS can give
easily 3 dB variation +/-. This by itself can create some 6 - 8 dB variation
between test labs (exact same configuration). I have equipment tested in
Country in China, Brazil, in third party accredited labs and also in my own
labs. I find that as long as you test worst case configuration , and this will
take some time to determine and add 3 dB for margin. This me!
thodology typically would consistently pass in all other labs.
The trick to less margin is simple.... be honest with your self ...and make
sure you are in fact testing worst case ....and not best case for the sake of
passing..... re-test ALL non EMC benign product changes.( ITE TTE industry)
Rodney Davis
________________________________________
From: gdstuyvenb...@yahoo.com <0000058ee1229c70-dmarc-requ...@ieee.org>
Sent: Tuesday, October 13, 2015 12:02 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] SV: [PSES] Stricter limits than legal (CISPR11, IEC, etc,)
Where?
Thank you Gert! I was beginning to wonder if anyone would say it.
Gary Stuyvenberg
Sr. Electrical/EMC Engineer
thompsonconsultingservi...@yahoo.com
_______________________________________________
Note that for some frequencies and measurements taking 6 dB of margin will
bring you dangerously close to the system noise level (automotive, marine , GHz)
In addition, emission non-compliances, caused by components and build
(enclosure, mounting, wiring) changes
come more often the in the 10-12 dB range than within the 6 dB range.
Measurement errors of the test equipment are well in control nowadays,
and keeping a 4-6 dB margin for that makes sense, but the test set up is not
just
about the measurement equipment used. A much bigger test error (>20dB) is
easily obtained by varying (or not varying depending
on you test attitude), the operation modus of the test sample , EUT rotation
issues (degree step values?) and cable lay-out and -termination errors (both CM
and DM)
If for having peace of mind a margin is taken, than the 6 dB value is fully
arbitrary.
The definition of a meaningful margin requires statistical analysis of your
production samples, something we just wanted to avoid by defining a margin
;<(((((
Regards,
Ing. Gert Gremmen
Approvals manager
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From: Amund Westin [mailto:am...@westin-emission.no]
Sent: Tuesday 13 October 2015 07:19
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] SV: [PSES] Stricter limits than legal (CISPR11, IEC, etc,)
Where?
5-6dB margin.
Bring two samples to the test lab, and make a quick check that sample no.2 also
is within 6dB (yes, additional test costs may apply)
Repeat testing (quick scanning) after 2 years to check compliance. More often
if modified in some way.
This is not a 100% good approach, but in some way, you have some sort of
tracking the emission.
Best regards
Amund
Fra: Brent DeWitt [mailto:bdew...@ix.netcom.com]
Sendt: 13. oktober 2015 01:56
Til: EMC-PSTC@LISTSERV.IEEE.ORG
Emne: Re: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where?
Nobody seems to be asking _why_ margin is important or not. I expect no one
believes airliners are going to fall out of the sky or grandma’s pacemaker to
stop if a product is 10 dB over the FCC/CISPR Class B limits for radiated
emissions. So why? For emissions (not immunity, that’s a different question)
it’s probably so you can be guaranteed of legally selling the product in the
light of a third party audit or an internal whistle blower (just kidding). So
now the _probabilities_ start stacking up and your tolerance for risk comes
into play. All of the factors brought up by this list are valid for one
product scenario or another. Simply put, if you take the maximum measurement
uncertainty allowed for a lab under the law, multiply it by 1.414, add the
observed statistical variation of a large enough sample of your product that
the standard deviation from all causes starts settling down, and use that as
your required margin you should have a
guaranteed peaceful sleep. The bad news is that nobody really wants to design
for a number that huge (or should they?).
All that is a long winded way of saying “there is no universal right answer”.
Examine _your_ product, _your_ lab and _your_ anxiety level and act accordingly.
Respectfully,
Brent G DeWitt, AB1LF
Happily employed in the EMC thing since 1978
From: Douglas Powell [mailto:doug...@gmail.com]
Sent: Monday, October 12, 2015 7:23 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where?
Patrick,
In your story, what you did was a small statistical study. Multiple samples
allow you to do this. It is similar to the problems of EMC. One possibility is
to test multiple units and record the results as a probability or simply
record the worst case. When dealing with large expensive equipment that can
take days to evaluate, it is far simpler to build confidence by simply adding,
pause for effect, margin.
Doug
Douglas E Powell
doug...@gmail.com
http://www.linkedin.com/in/dougp01
From: Patrick
Sent: Monday, October 12, 2015 5:13 PM
To: doug...@gmail.com
Cc: EMC-PSTC@listserv.ieee.org
Subject: Re: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where?
Ahhh, our old friend: "Margin".
Margin is one of those timeless EMC topics.
IMHO- there is simply no evidence that margin is helpful.
In a practical sense we all know that a "single-measurement-plus-margin" is not
a confidence builder.
As an example, think about the last time you worked on your home wiring.
How many times did you read that multi-meter before you touched the wires?
Did you read it just once? Then add some margin?
Nope- I'll bet you did like me: I read that meter, and double-read it.
Then I read it again.
One measurement is simply not enough to build confidence.
Next time someone asks you for "reasonable margin" ask them about their last
wiring project....
-Patrick
OOO.
On Mon, Oct 12, 2015 at 2:54 PM, Douglas Powell <doug...@gmail.com> wrote:
Ted,
Very good points. If minimum passing margins are a result of edge rates on
transistors, diodes or ICs, then a second source or "upgrade" to a faster
device can be counterproductive with regard to the emissions profile.
If such engineering changes or supplier changes have occurred, then a retest is
often the best policy. I was also aware that several cumulative engineering
changes over time can result in a non-compliance. Each change, when
evaluated by itself was inconsequential. But in aggregate, result was a
failure. This is often the reason I would require a retest after some number of
engineering changes had been applied to a product. Of course, keeping full
data test reports on each passing result is really the only way to do this well.
Doug
Douglas E Powell
doug...@gmail.com
http://www.linkedin.com/in/dougp01
From: Ted Eckert
Sent: Monday, October 12, 2015 2:44 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Reply To: Ted Eckert
Subject: Re: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where?
In addition to the responses from Doug, Ghery and Brian, I will note that
margin protects you from unexpected or unknown changes from component
suppliers. To some extent, this falls under the manufacturing variance Doug
mentioned, but component changes is just another area that can be hard to
control.
I’ve had IC vendors do a die shrink on a part resulting in sharper edge rates
on the outputs. At a previous employer, I was running emissions testing on a
number of samples where Motorola did a die shrink on the microcontroller we
were using. Some of my test samples had the old part and some had the new. It
took a long time to figure out why some samples were significantly worse than
others after controlling for all other variables. Having 6 dB margin to begin
with provides some protection against this type of change.
Ted Eckert
Compliance Engineer
Microsoft Corporation
ted.eck...@microsoft.com
The opinions expressed are my own and do not necessarily reflect those of my
employer.
From: Itzenheiser, Jerry (GE Healthcare) [mailto:gerald.itzenhei...@med.ge.com]
Sent: Monday, October 12, 2015 12:26 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where?
Hello EMCers,
I would like to ask…
Is there anyone out there that tests to stricter limits than the legal (CISPR,
IEC, etc.) limits? If so, what was the rationale behind selecting the stricter
limits? Our engineering teams are curious as to where the stricter recommended
limits come from, such as the 6dB margin for emissions testing.
Thanks,
Jerry Itzenheiser Jr
EMC Technician - Waukesha
GE Healthcare
Global Engineering Technologies
EMC Laboratory Waukesha
T + 262-548-2217
M + 262-720-8846
gerald.itzenhei...@med.ge.com
www.ge.com
3000 N. Grandview Blvd.
Mail Code W618
Waukesha, WI 53188
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