I try to re-send this today to the emc-pstc group. Yesterday it got kicked back
for unknown reasons.
Paolo Roncone
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Hello Martin,
I read a number of excellent replies to your question. I would just like to add
my 0.02 cents based on my hands-on experience. I have been doing radiated
emissions testing and troubleshooting for 11+ years now. Up to date I was lucky
to have a full compliant test site (10m and 3m) available for both
qualification testing and for troubleshooting. I managed to solve quite a
number of emissions problems by using both near-field probes ("sniffers") and
antennas in the far field.
As others have already explained, there are many variables (both related to the
EUT and the test setup) that influence readings in the far field. That's the
reason why in many cases you may not get a correlation between relative
readings with a near field probe and in the test site. Plus (I would add) near
field measurements are more "tricky" and less easily repeatable than far field
measurements (very sensitive to probe position, test operator, external sources
etc.).
Any emissions (or immunity) process has three essential elements (or categories
of elements):
1. Source (electronic components, ICs..)
2. Coupling path (PCB traces, connectors, chassis elements ...)
3. Antenna (PCB traces at high enough frequencies, cables, slots in metal
enclosures...).
All these elements usually play a part in the overall readings in the far
field.
Many times the best (and cheapest) solutions are implemented at the source
level because you block the emissions as much "upstream" as possible. On the
other hand in many other cases the effects of the other 2 elements (coupling
paths and antennas) can dominate (especially when they trigger resonances at
some frequencies).
In my opinion, as a general rule near field probes should only be used to help
locate the most critical element for your specific problem, NOT for
measurements, because the "sniffer" (by definition) cannot pick-up the overall
picture. It usually works best for locating sources and coupling paths (noisy
circuit components and PCB traces). When you have located the "hottest" areas
(either by correlating frequencies of emissions with known signals or by
choking off cables or with "sniffers" or with anything else that works in your
case) you try fixes - one at a time as already mentioned - then after each
modification take readings ALWAYS in the far field, either in a full compliance
test site or in other locations where you can put an antenna far enough from
the EUT and get a repeatable setup. In this way you keep checking all the
elements (sources, copupling paths, antennas) at every step of your
troubleshooting work.
Hope this helps...
Paolo
-----Messaggio originale-----
Da: [email protected] [SMTP:[email protected]]
Inviato: giovedì 14 settembre 2000 19.08
A: [email protected]
Oggetto: Near Field Versus Far Field
I am having a difficult time answering the following question for a
non-technical person. Hopefully, someone can put the answer into a language
that a non-technical person can understand.
We have a 400 MHz clock and are failing radiated emissions at 10 meters by 10 dB
at 400 MHz. We bring the product back to our lab and start making modifications
on the clock circuit and taking measurements with a near field probe. With
these modifications and measuring with a near field probe, we realize a 10 dB
reduction in emissions at 400 MHz. Why would we not see the same reduction when
taking the product back to a 10 meter site?
Your help is appreciated.
Regards
Joe Martin
[email protected]
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