This is a mathematical construct. The reality is that cm and dm flow in the same direction on one conductor and in the opposite direction on the other conductor. Aside from that any additional phasing doesn¹t seem physically real.
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 23:08:43 +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 Sorry for all the typos in that last one and the repeat. When writing equations, typos are pretty confusing to a reader, so I updated the important part below with corrected maths. Basically, if |L| and |N| are almost equal on your SA or EMI receiver (not L and N, |L| and |N|), this tells you absolutely nothing about DM and CM relative to each other. We¹ll define L = DM + CM and N = DM - CM To prove the point above Let¹s suppose CM = jDM then |L |= |DM + CM| = |DM + jDM| = sqrt2 *|DM| |N| = |DM CM| = |DM jDM| = sqrt2 *|DM| Therefore |L| = |N| even though |DM| = |CM| Elliott Martinson From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Tuesday, March 29, 2016 5:14 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN Back in the 1990s I presented a paper where I used LISNMATE to isolate cm emissions using an o¹scope and using an FFT routine on the scope data, could get decent frequency domain info on the cm data, and built a filter this way. Without the DMRN (differential mode rejection network generic name) this was impossible. Because cm is related to the waveform rise time, the output on the scope was clearly visible for what it was, and the filter design by modes went very smoothly. Not saying this is the way to go, but it is/was possible. 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 21:59:10 +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 I understand what you are saying. And if we¹re using scopes to view the time-domain waveform and measuring simultaneously, MATH functions will make this easy. But noise is chaotic, and subsequent measurements of the L conductor only won¹t even be exactly the same. The phase relationships of different noise signals from different sources in the device are constantly changing depending on when the measurement was made as well. Again, I¹m assuming an SA or EMI receiver here. I¹m probably way overthinking this K From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Tuesday, March 29, 2016 4:51 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN On one conductor, cm and dm sum, and on the other, they subtract. If a sum and difference of two quantities is equal, it can only be because one of them is zero (or much less than the other). Ken Javor Ph. (256) 650-5261 From: Elliott Martinson <elliott.martin...@etcconnect.com> Date: Tue, 29 Mar 2016 21:37:42 +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 The LISNMARK (LISN MATE) is exactly the same device as what I linked to. As for the current probes, comparing L and N to determine if one of cm or dm predominates we are talking about noise signals, and measuring magnitudes (no phase info) in frequency domain, right? I know that on one conductor, you can say CM/DM add while they subtract on the other (mathematically correct no matter which you call L and which you call N). I¹m confused at how you know it cannot be similar amounts CM and DM noise when the noise magnitude is the same between L and N. I¹d appreciate it if you could explain. Obviously running L+N simultaneously through the same probe gives common-mode noise, I¹m not sure if I worded my response badly because I meant to say bullet #3 is completely correct (as is 1, along with 4 assuming no ³LISNMATE²) 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: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Tuesday, March 29, 2016 4:14 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: 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 can¹t 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 Nave¹s 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 it¹s trying to say what I think it is. DM and CM each show up on L, but the same is true for N. It¹s a linear combination of both, so even if L and N are almost equal, you can¹t 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 energy¹s 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] 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 can¹t 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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