I have been reading this thread for a while, but missed an important
aspect.
Engineers in our profession have been looking at limits as absolute
LIMITS, not to be exceeded by any device under any circumstances.
To meet this in practical situations, many approaches have been
suggested, most suggesting extra severe limits, where the discussion is
about how much more severe.
Well in Europe, and this shows the strength of the EMC directive, the
bottom line is that equipment should not interfere. (simplified
essential requirement)
The fact that a piece of equipment exceeds a limit does not mean that it
will interfere, just that chances are better that it will. The same for
equipment below the limit, chances are only slightly better that it
won't interfere.
A interference peak at 102 MHz, will not create interference until
someone within a certain radius will decide to listen to 102 MHz on his
portable radio.
(It's good to emphasize that the current limits will only protect radio
reception . An excess of a few dB will ONLY interfere with radio
reception. No other interference problems can be identified in my
opinion)
It's up to the manufacturer to show that a particular negative margin
will not cause problems in terms of interference. For this to happen he
may choose to create a technical construction file route, and from July
the 20th on he does not need a competent body approval anymore. An
approach showing that an emissions excess of i.e. 2.5 dB at a certain
frequency for a certain percentage of equipment could be well defended
with appropriate arguments, in terms of applied environment, statistics
(i.e. number of equipment in use) and arguments of technical nature.
(i.e. will not be used near in radio receiving equipment) .
Limits are not absolute.
Each of us knows that respecting a standard's limits will not guarantee
that no interference will happen, nor can this standard guarantee that
the particular equipment, if deviously used/installed, will not exceed
the limits anyway.
Of course, maintaining the limits, following harmonized standards is an
easy (!!?!) legal way to obtain presumption of conformity.
Keeping your emission profile below this limit however, under all
circumstances in all operating modes and all configurations and at the
same time keeping track of manufacturing variations and measurement
uncertainty would lead to unacceptable low values of interference
in-house limits.
This problem may be somewhat relaxed by applying the CISPR 80/80
limits, but this approach requires multiple samples to be tested which
can be a problem for other reasons. This would lead to unacceptable
costs for EMC while not justifying a reduced interference profile.
Therefore, I suggest to just maintain the limits, do not add extra
margins, unless your measurement uncertainty exceeds the CISPR16
suggested values.
The radio protection limits were created with sufficient margins so as
that even 10 dB excess will not create excessive interference problems.
A margin was build-in.
In terms of interference capabilities, the amplitude criterion is not
the holey grail. The applied QP detector will not also accurately
describe interference capabilities for FM, PM or digital communication
systems.
In general, when speaking about the chance of a piece of equipment
causing interference , it also makes sense to qualify the type of
interference.
A data over power line solution, creating full continuous interference
all over the band while remaining below the limit will certainly do more
harm in terms of interference as one clock harmonics at 107.6 MHz.
I believe that the a limit should be based on the integral of frequency
/ amplitude characteristics of a type of equipment. The more
interference components in the band, the lesser the amplitude may be. A
single component would be allowed to exceed the current limits, where a
large number of peaks need to remain substantially below the current
limits. This approach would do justice to the chances any interference
could be created.
Of course these new limits could be detailed out, creating stronger
protection in certain bands, while relaxing in other bands, but I wanted
to discuss the principle.
Gentlemen and Ladies (yes there are) ,
Let me know your opinions...
I know I opened (re-opened) a can of worms.......
Regards,
Ing. Gert Gremmen
Approvals manager
ce-test, qualified testing bv
+ ce marking
+ Independent Consultancy Services
+ Compliance Testing and Design for CE
according to:
- Electro magnetic Compatibility 89/336/EC
- Electrical Safety 73/23/EC
- Medical Devices 93/42/EC
- Radio & Telecommunication Terminal equipment 99/5/EC
+ Improvement of Product Quality and Reliability testing
Web: www.cetest.nl (English) or www.ce-test.nl (Dutch)
Phone : +31 10 415 24 26
Fax : +31 10 415 49 53
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From: [email protected] [mailto:[email protected]] On Behalf Of John
Woodgate
Sent: Wednesday, July 04, 2007 6:53 AM
To: [email protected]
Subject: Re: Internal Margins for Emissions Testing
In message
<f38c1e8a75404347a126608a8f48af5503097...@mer2-excha2.echostar.com>,
dated Tue, 3 Jul 2007, "Grasso, Charles" <[email protected]>
writes:
>In the absense of any data, we decided to *strive* for 6dB of margin.
>That allows for measurement uncertainty
>and a couple of db for manufacturing varience.
The proof of the pudding...
It is most unlikely that a practicable margin could be determined by
reasoning alone. You make a reasonable guess, such as 6 dB indeed, and
see whether in practice it eliminates problems, from failures when new
products are first sent to the test house, through rejections in
production testing, to complaints from regulatory authorities.
With this approach, as with EMC limits, you can't tell if the
requirement is too severe, i.e. you can't tell (usually) if 5 dB margin
would still keep you out of trouble. Quality assurance statistics may
give a steer.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of
2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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