Hi Ken and the group,
I always thought that the FCC measures the wrong quantity for conducted
emmissions. What shoud be measured is all conductors (two or three
including phase/neutral/protective earth) together as common mode
current as that is what radiates from the long power lines causing
problems for shortwave receivers and Amateur Radio Operators. The old
demonstration of conducted EMI into AM radios is not so useful,
especially today.
Below 30 MHz, most devices are not large enough to radiate efficiently,
but the power wiring is long enough to radiate. I have a case of EMI in
my house from two Feit Electric LED floodlights that meet conducted
emissions, but I can't use a hand held, battery powered, shortwave
receiver when the two are on except to walk quite a distance from that
part of the house. The FCC test may catch this case, but apparently not
for me, as there are only two wires but that is not the case for other
devices.
So phase+neutral could be noisy with respect to protective earth but as
long as it is balanced by an opposite current on protective earth,
radiation should be low.
Any other Amateur Radio operators want to weigh in on this?
Doug (K4OAP, since 1959)
On Tue, 29 Mar 2016 16:13:58 -0500, Ken Javor wrote:
Re: [PSES] Commom mode current vs. differential mode current and
LISN Disagree. Westin had it right. A current probe can be used to
isolate either cm or dm current. If at any frequency the signal
amplitude on individual line and neutral conductors are very close,
then all you can say is that at that frequency either cm or dm
predominates, but you can’t say which. With a LISN, a separate
device must be used. Mark Nave of EMC Services designed a three port
device (connects to each LISN port and to the EMI receiver) trademarked
LISNMATE in the 1980s to isolate common mode, and sometime later he
produced LISNMARK, which isolated DM. Within the past decade, Ray
Adams while at Fischer Custom Communications packed both functions in
one piece of equipment, which if memory serves was named LISNUP.
EMC Services, Mark Nave’s company, is no longer producing his
products, but I believe the FCC product is still available.
Ken Javor
Ph. (256) 650-5261
-------------------------
From: Elliott Martinson
Reply-To: Elliott Martinson
Date: Tue, 29 Mar 2016 20:36:18 +0000
To:
Conversation: [PSES] Commom mode current vs. differential mode current
and LISN
Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN
http://www.hottconsultants.com/techtips/CM_vs_DM%20Conducted_Emission.html
This is a great resource for your question.
Your second point kind of contradicts your first, if it’s trying to
say what I think it is. DM and CM each show up on L, but the same is
true for N. It’s a linear combination of both, so even if L and N are
almost equal, you can’t say anything about the proportion of DM to CM
currents.
If they are not equal, then this implies current is travelling back via
the ground conductor and/or energy’s being lost to radiated emissions.
What you need is a physical circuit to do the adding/subtracting of the
LISN outputs. (otherwise your 3rd bullet point is correct)
Your 4th bullet, well I refer you to the link above.
Elliott Martinson
Product Assurance Specialist I
Electronic Theatre Controls
3031 N PLEASANT VIEW RD
MIDDLETON WI 53562-4809
Work: 608.824.5696 / Cell: 608.209.9897
[email protected]
From: Amund Westin [mailto:[email protected]]
Sent: Tuesday, March 29, 2016 1:23 PM
To: [email protected]
Subject: [PSES] Commom mode current vs. differential mode current and LISN
Please correct me, if I am wrong (that happens quite often …):
· Let one wire (L) pass through a current clamp, and you
measure the combination of current mode and differential mode currents
· Do the same with wire N. If L and N are (almost)
equal, you either have major part of DM currents or major part of CM
current
· Let both wire (L and N) pass through a current clamp,
and you measure the only CM current (DM is canceled)
· When doing conducted emission test by LISN, you
actually get what you get. LISN do not see the difference between CM or
DM. From LISN measurements, you can’t say if noise is CM or DM.
B.regards
Amund
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