[SI-LIST] Re: Question on radiation limits
- From: David Heald <dheald@xxxxxxxxxxx>
- To: "'AAnbazhagan@xxxxxxxxxxxxxx'" <AAnbazhagan@xxxxxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
- Date: Mon, 8 Jul 2002 13:28:05 -0400
Anbazhagan,
Where to begin....?
My initial question is what distance did the foreign lab measure to? =
If
your product is indeed Class B and if the foreign lab measured at 10m, =
then
the limits (30dB) are correct. If the differences are in the 3m near =
field
(below 100MHz or so), well, differences can be expected when the =
emissions
are remeasured at the 10m distances. Measurements in the 3m near field =
are
not that reliable and failures could easily result if your margin was
negligible.
Second, if the chamber you used was indeed 'fully' anechoic (walls, =
cieling,
AND FLOOR lined), the results from such a scan can be up to 6dB low due =
to
the absence of floor reflections (and the resultant additive waves). =
Later
scanning in an OATS would have a reflective ground plane and would =
therefore
exhibit the reflective additive waves. =20
If the chamber was Semi-Anechoic (all EXCEPT FLOOR lined - as it should =
be
for EMI), the results should be relatively close, but a few other
difficulties can present themselves. First is the near field issue
mentioned above. Second, cable manipulation is usually nonexistant in
chamber scans and can yield large variances in your emissions results.
Moving a cable just a few inches can have a +10dB effect on the =
emissions
from that cable (=B15dB differences are very common with tabletop =
products
when manipulating cables). =20
Was the anechoic chamber scan done with the antenna height varied from =
1 to
4 meters? A fixed antenna height in a semi-anechoic chamber can have a =
+6
-20 dB uncertainty due to possible constructive (+6dB as described =
above)
and destructive waves (up to -20dB) from floor reflections. Changing =
the
antenna height should negate the uncertainty caused by the reflective =
waves
and would certainly be practiced in an OATS scan. Some anechoic =
chambers,
however, cannot support variable height antennas due to space =
restrictions.
Other factors that can highly affect EMI:
Host/support PC's for EUT PCI cards
Support PC's for cable connectivity (USB/RS-232/Firewire/etc...)
PC side USB interfaces were notorious a few years ago. Not sure if
things have improved.
As for the limits applying only to OATS: Officially, EMI measurements =
are
to be performed in an OATS unless the semi-anechoic chamber has been
demonstrated to meet the site attenuation characteristics in the =
applicable
European standards (the lab will have 'registered' the chamber with the =
EU
or something to that effect). Additionally, 3m testing is allowed only =
for
Class B products.
Finally, check your data from both reports. If it looks like there =
were
many readings taken at the same azimuth(s), you probably got a crappy =
scan.
I have seen this at less experienced labs. Usually every emission will =
have
its peak lobe at a different azimuth and antenna height.
Best Regards and Good Luck!
Dave Heald
-----Original Message-----
From: AAnbazhagan@xxxxxxxxxxxxxx [mailto:AAnbazhagan@xxxxxxxxxxxxxx]
Sent: Monday, July 08, 2002 12:49 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Question on radiation limits
Dear All,
Though it is not strictly related to signal integrity, I have a =
ques=3D
tion
on the radiation limits set by the CISPR (Eurpoean emission standard,
EN55022 for emission and EN55024 for immunity).
We have a got a lab in which we did the radation test in a fully =
anecho=3D
ic
chamber at 3 m distance (Antenna to EUT horizontal distance) to get =
the=3D
CE
certification. The limit was set as 40.35 dB=3DB5V/m from 30MHz to =
230MHz=3D
and
47.35 dB=3DB5V/m from 230 to 1GHz.
The same test was conducted in another lab located at a different =
count=3D
ry.
There they had set the limit as 30 dB=3DB5V/m and 37dB=3DB5V/m and the =
boar=3D
d failed
with that limits.
Which one of the limit is correct? It was argued that the 40dB=3DB5V/m =
li=3D
mit
for 3m distance is valid only for OATS and not for anechoic chambers.
Can someone clarify it.
Thanks
Anbazhagan.
=3D
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