Agreed Ken.
It's certainly possible to make CM-DM measurements with two lines of
LISNs using 0 degree/180 degree splitter combiners (also sold as the
"LISN Mate"). Great troubleshooting method that I've kept in my
bag-o-tricks for many years. The best thing about the discrimination is
how it leads to filter design and even pcb alterations.
I will stand my belief that if the noise "termination impedance" isn't
known, it's anybody's guess what will happen. In my previous life
(recently retired), I seldom went the extra step of attempting to
quantify the noise source impedance. As you know, an LISN is simply a
capacitive voltage probe with a well defined termination Z. My thought
is that attempting to translate that to current without knowledge of
either source or load Z is not really possible. Again, if a person is
troubleshooting a known failing device, relative is good enough 😉
Thank you for catching the EM vs Absorbing error!
On 1/2/2025 4:41 PM, Ken Javor wrote:
You all are ignoring the by far the most salient point that I made
last year. The poster said he measured cm and dm with a current
probe, and this did not compare well with the LISN measurements, which
are line-to-ground. Â Â Had he made line-to-ground measurements with a
current probe, or cm and dm measurements off the LISN, there would
have been much more agreement. Not complete, because of impedance
differences with and without a LISN, but the degree of agreement would
have been much greater than what would be achieved controlling line
impedance alone.
And it is an absorbing clamp. Not an EM clamp: the EM clamp is
designed for injection.
--
Ken Javor
Ph: (256) 650-5261
*From: *Brent DeWitt <bdew...@ix.netcom.com>
*Reply-To: *Brent DeWitt <bdew...@ix.netcom.com>
*Date: *Thursday, January 2, 2025 at 8:57 AM
*To: *<EMC-PSTC@LISTSERV.IEEE.ORG>
*Subject: *Re: [PSES] Conducted emission - LISN vs. current clamp
Good point James. The EM clamp is well behaved and understood. My
comment was only meant to address a simple device like an FCC F-33
used on a cord plugged into the wall mains.
Thanks for the clarification!
- Brent
On 1/2/2025 3:50 AM, James Pawson (U3C) wrote:
Following up on Brent’s reply, the CM current measured by the
probe will be partially determined by the CM termination impedance
which is 150 ohms in the CISPR setups. You can use an IEC
61000-4-6 EM clamp to approximate this with reasonably good results.
All the best
James
James Pawson
Managing Director & EMC Problem Solver
*Unit 3 Compliance Ltd*
*EMC : Environmental & Vibration : Electrical Safety : CE & UKCA :
Consultancy*
www.unit3compliance.co.uk <http://www.unit3compliance.co.uk/> |
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Registered in England and Wales # 10574298
/For inquiries, bookings, and testing updates please send us an
email on he...@unit3compliance.co.uk or call 01274 911747. Our
lead times for testing and consultancy are typically 4-5 weeks./
*From:*Brent DeWitt <bdew...@ix.netcom.com>
<mailto:bdew...@ix.netcom.com>
*Sent:* 17 December 2024 21:54
*To:* EMC-PSTC@LISTSERV.IEEE.ORG
*Subject:* Re: [PSES] Conducted emission - LISN vs. current clamp
From my experience, I would seldom expect the current probe and
the LISN to show the same answer. The big difference is that the
current probe is looking at a fairly random termination impedance
compared to the "Line Impedance Stabilization Network", which is
reasonably stable in its design. An interesting experiment is to
use the CM/DM choke on a power cord plugged into a LISN and
examine the differences,
On 12/17/2024 4:27 PM, Ken Wyatt wrote:
We’ll need a lot more information on what you’re testing, the
test setup configuration and how you’re measuring DM and CM.
Can you share some photos or screen captures?
Usually there will be a difference between DM and CM current
spectral response. Now Line and Neutral from a LISN will like
look the same typically.
Kenneth Wyatt
Windsor, CO
On Dec 17, 2024, at 13:46, am...@westin-emission.no wrote:

Clamp is ok for investigate DM and CM currents. LISN for
formal testing at lab.
Right now, the plots of CM and DM currents are almost
equal over the whole spectrum (10k – 30MHz). Surprise?
In the same setup, using a LISN, the spectrum is quite
different regarding peaks and waveform.
I know the information from this test is defective, but do
you have a first thought anyway?
BR
Amund
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