Hi All' Another thing that needs to be said about the comparison of DM and CM measurements with a current probe is that, unless you are very careful, the measurement of DM will be inaccurate unless the frequency concerned is quite low.
CISPR16-1-2 clause b5 suggests restricting DM measurement to below 20MHz, whereas figure B3 suggests CM measurement is OK up to 200MHz. The reason for this (not explained in CISPR16) is the conductor loopiness needed to get the two wires going through the hole in opposite directions to enable DM measurement. The commonplace current probe has an outside diameter of some 80mm and is 26mm thick, so a casual splitting of an upstream pair of wires to go through its 30mm hole in opposite directions, and then return to twisted pair downstream wiring, puts a couple of big loops in series with the DM current flow, with consequent inductive impedance that reduces DM current flow and so downgrades the measurement. A description of a small current probe that allows this effect to be minimised so that DM and CM current can be measured up to about 300MHz, can be found on pages 27-30 in the July 2013 edition of "The EMC Journal " (issue 106) now archived at www.compliance-club.com Regards, Richard Richard Marshall Laboratories, 30 Ox Lane, Harpenden, Herts., AL5 4HE, UK +44 (0)1582 460815 www.design-emc.co.uk Member of the EMC Industry Association -----Original Message----- From: ce-test, qualified testing bv - Gert Gremmen [mailto:g.grem...@cetest.nl] Sent: 01 April 2016 11:15 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN I needed to add to my previous contribution to that the parameter that controls this is called TCL value. (John et all: = Transferse Conversion Loss ;<)) . The EMC world has been using LISNS for decades now that simulate a LCL (Longitudinal Conversion Loss) value of the connected mains cable of -6 dB. (for a cable LCL and TCL are normally the same) Something to think about... Regards, Ing. Gert Gremmen Approvals manager ---------------------------------------------------------------------------- -------- + ce marking of electrical/electronic equipment + Independent Consultancy Services + Compliance Testing and Design for CE marking according to EC-directives: - Electro Magnetic Compatibility 2004/108/EC - Electrical Safety 2006/95/EC - Medical Devices 93/42/EC - Radio & Telecommunication Terminal Equipment 99/5/EC + Improvement of Product Quality and Reliability testing + Education Web: www.cetest.nl (English) Phone : +31 10 415 24 26 ------------------------------------------------------------------- This e-mail and any attachments thereto may contain information that is confidential and/or protected by intellectual property rights and are intended for the sole use of the recipient(s) named above. Any use of the information contained herein (including, but not limited to, total or partial reproduction, communication or distribution in any form) by persons other than the designated recipient(s) is prohibited. If you have received this e-mail in error, please notify the sender either by telephone or by e-mail and delete the material from any computer. Thank you for your co-operation. From: Richard Marshall [mailto:richard.marshal...@btinternet.com] Sent: Friday 1 April 2016 11:48 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN Ken says "I seem to recall Michel Mardiguian writing that the relaxed dm could transform into cm causing big problems. That seems counter-intuitive at best." Think about it this way. DM is being transmitted along a wire PAIR that we must view as an rf transmission line, which is balanced UNTIL it meets some discontinuity. ( What discontinuity? - A branch cable with a single-pole switch that is open, leaving a branch wire hanging on the other pole. - A device with a single class X capacitor from neutral to ground. - A solid ground connection to the neutral wire. - A wide separation between Line and Neutral conductor (Think 2-way light switching) - - etc ) At a discontinuity that is unbalanced relative to ground (as all the above are), there is conversion of DM power into CM because unequal amounts of the balanced power in each wire are diverted to ground. The maths. are complicated because of the various and different values of Characteristic impedance Zo between wires and from each wire to ground, but the mechanism for power transfer is clear. There have been studies of this in the last 10/15 years suggesting that typical power transfer coupling from DM to CM in a real mains power network at 230 volts ( I do not know of any at 115 volts, perhaps others do...) involves only about 10dB power loss at some frequencies where the wiring is resonant. So whilst there may be a case for allowing a bit more DM than CM, there can be no technical case for allowing 20dB or more of excess DM. Not on ordinary power distribution circuits. Richard Richard Marshall Laboratories, 30 Ox Lane, Harpenden, Herts., AL5 4HE, UK +44 (0)1582 460815 www.design-emc.co.uk Member of the EMC Industry Association From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: 30 March 2016 21:59 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN With the dm limit 20 dB relaxed, the dm filter design would be simplified. As I said, an industry study said it would save $35 million a year for the power supply industry. I'm not sure if it was Art Wall as much as the old guard who were involved in the original work not liking a fresh look at it. I seem to recall Michel Mardiguian writing that the relaxed dm could transform into cm causing big problems. That seems counter-intuitive at best. The opposite is way more likely. So my takeaway was that they didn't want to consider a change to how they had set thing up. Ken Javor Ph. (256) 650-5261 ________________________________________ From: "Grasso, Charles" <charles.gra...@echostar.com> Date: Wed, 30 Mar 2016 19:01:00 +0000 To: Ken Javor <ken.ja...@emccompliance.com>, "EMC-PSTC@LISTSERV.IEEE.ORG" <EMC-PSTC@LISTSERV.IEEE.ORG> Conversation: [PSES] Commom mode current vs. differential mode current and LISN Subject: RE: [PSES] Commom mode current vs. differential mode current and LISN Hello Ken - I remember the setup and demo to Art Wall of the FCC during the 1999 EMC Symposium here in Denver. In spite of your best efforts it was clear that Art was not "persuaded" !! The impact on the industry (as I recall) would be much cheaper filters...(if any). Best Regards Charles Grasso Compliance Engineer Echostar Communications (w) 303-706-5467 (c) 303-204-2974 (t) 3032042...@vtext.com (e) charles.gra...@echostar.com (e2) chasgra...@gmail.com From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Tuesday, March 29, 2016 3:48 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN The report on which the old 48 dBuV class B CE limit was based did develop that based on the conducted susceptibility of AM and shortwave radios, but it also noted that limit functioned as an adequate control for common mode noise that would radiate as per Doug's observation. Back in the late '90s I presented a detailed test report to TC77 showing that if instead of controlling conducted emissions at each LISN port, they instead controlled by modes, that dm could be relaxed 20 dB to 68 dBuV and the committee took an action item and back then said if that were implemented, it would save the power supply industry $35 million a year. The work I did was based on using the LISNMATE / LISNMARK technology as injection tools injecting pure cm and dm and showing that the susceptibility to dm was 20 dB less, because of bulk filter caps on the secondary of power supplies, whereas there were no Y-caps and no cm filtering. The reason this was missed back in 1977/78 when the report was w! ritten was they used a single LISN with neutral through the case. CM and DM flowed in the same path. Ken Javor Ph. (256) 650-5261 ________________________________________ From: Doug Smith <d...@emcesd.com> Reply-To: <d...@emcesd.com> Date: Tue, 29 Mar 2016 14:31:20 -0700 To: <EMC-PSTC@LISTSERV.IEEE.ORG>, Ken Javor <ken.ja...@emccompliance.com> Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN 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 <ken.ja...@emccompliance.com> 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 <elliott.martin...@etcconnect.com> Reply-To: Elliott Martinson <elliott.martin...@etcconnect.com> Date: Tue, 29 Mar 2016 20:36:18 +0000 To: <EMC-PSTC@LISTSERV.IEEE.ORG> 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 elliott.martin...@etcconnect.com <elliott.martin...@etcconnect.com> From: Amund Westin [mailto:am...@westin-emission.no] <mailto:am...@westin-emission.no%5D> Sent: Tuesday, March 29, 2016 1:23 PM To: EMC-PSTC@LISTSERV.IEEE.ORG 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 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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