There was/is a very smart engineer named Balint Szentkuti (a good Hungarian name, just like Javor) who about twenty years ago proposed replacing radiated measurements of cable-sourced emissions with measurement of cable common mode conducted emissions. This seems an eminently sensible idea to me. You base the conducted current emission limits on a reasonable worst case radiating efficiency of a cable of a certain length a certain distance above ground.
Mr. Szentkuti wrote several papers on this subject, to my recollection. Here is one reference: Szentkuti, B., Give Up Radiation Testing In Favour Of Conduction Testing, Proceedings, EMC Zurich 1989. ---------- >From: don_borow...@selinc.com >To: emc-p...@majordomo.ieee.org >Subject: RE: Ferrite clamps >Date: Thu, Nov 21, 2002, 12:59 PM > > > > David- > > I agree with what you say, but it is even more complicated than that. > > As you say, ferrite clamps can be ineffective. Their effectiveness all > depends on the wave impedance at the point where they are attached. Due to > standing waves, the wave impedance varies greatly. If a ferrite clamp is > attached at a high wave impedance point (current minimum point), there will > be minimal effect. > > A ferrite clamp tries to insert a high series impedance in the cable. There > are several problems: > 1. It is difficult to obtain a high series impedance over a broad > range of frequencies. > 2. If all you have is a series impedance, the S21 is highly dependant > on the system impedance. In the limit where the series impedance Z is much > greater than the system impedance Zs, |S21| = |2Zs/Z|. The system impedance > is the wave impedance (which varies greatly) at the clamp. > > One could use clamps that measure a consistent, small value of S21 measured > in a 50 ohm system and still have a quite a bit of variation during > application. On the other hand, if S21 is very small (series impedance is > very high), it probably doesn't make much difference that it varies, since > the signal passed though would always be rather small. Unfortunately, > making such a high impedance over a broad range of frequencies is very > difficult. > > The only way I see to get really good repeatability would be to have > devices with high impedance series element(s) and low impedance shunt > element(s). But then we are talking coupling-decoupling networks (CDNs), > and need to connect them to the ground plane. This is possible to do, more > more costly and complex. > > Just controlling insertion loss (unless it is very large) will not do the > trick. > > Having said all that, while ferrite clamps are not the be-all and end-all, > they certainly do improve the test to some degree, since they do indeed > isolate the EUT from the cable beyond the clamp when the wave impedance is > moderately low (which it is at least some of the time). > > Don Borowski > Schweitzer Engineering Labs > (Ex-HP/Agilent) > > > > > > "Pommerenke, David" <davi...@umr.edu>@majordomo.ieee.org on 11/21/2002 > 07:54:34 AM > > Please respond to "Pommerenke, David" <davi...@umr.edu> > > Sent by: owner-emc-p...@majordomo.ieee.org > > > To: "Pettit, Ghery" <ghery.pet...@intel.com>, "Conway, Patrick R" > <conw...@louisville.stortek.com>, <emc-p...@majordomo.ieee.org> > cc: > Subject: RE: Ferrite clamps > > > > Dear Patrick, > > The aim of the standard was to avoid having standing current waves on > cables between the EUT and the point at which they leave the chamber. > Those resonances will cause highly setup and termination impedance > dependent radiation measurements leading to large site-to-site > correlation problems. > > The ferrite clamps have been added for absorbing the current wave in a > defined fashion. > > The problem is that the standard is fundamentally wrong! > > Why ? > The ferrite clamps are only specified by their insertion loss. Insertion > loss says NOTHING about the reflection. So the ferrite clamps may not > absorb at all, they may just reflect the current wave worsening the > resonance problem or shifting it to a different frequency. > > The overall uncertainty is not a bit reduced by inserting the clamps due > to false specification in the standard. An S11 (reflection coefficient > for the common mode wave on the cable) requirement should have been > taken, not an S21 requirement. > > As is, the standard should have never been accepted. Now people will buy > all kinds of probably useless ferrite clamps. > > David Pommerenke > > > > > > -----Original Message----- > From: Pettit, Ghery [mailto:ghery.pet...@intel.com] > Sent: Wednesday, November 20, 2002 6:30 PM > To: 'Conway, Patrick R'; Pettit, Ghery; 'emc-p...@majordomo.ieee.org' > Subject: RE: Ferrite clamps > > > Patrick, > > The whole reason for A1:2000 to CISPR 22:1997 was to improve > repeatability > between labs. I agree with your concern about it causing double testing > for > radiated emissions from 30 MHz to 1000 MHz until all regulatory bodies > accept the ferrite clamps. Not a good thing. I am working through an > industry association (ITI) to get the FCC to accept them. I've been > working > on this for 2 years. Nothing so far, other than some work in ANSI C63 > that > might result in the clamps being added to C63.4, maybe in 2004. > > Ghery Pettit > Intel > > > -----Original Message----- > From: Conway, Patrick R [mailto:conw...@louisville.stortek.com] > Sent: Tuesday, November 19, 2002 11:42 AM > To: 'Pettit, Ghery'; 'emc-p...@majordomo.ieee.org' > Subject: RE: > > > Hello Ghery- > > Thank you for the information. > > To be honest, I'm not all that familiar with the CISPR voting > process but I do recognize that any election with a 1 vote margin must > be a > bit contentious. Unless of course you live in Florida where every vote > counts AT LEAST once. There wasn't any "hanging chad" during that CISPR > vote, was there? > > > But- back to A1:2000: The data you report indicates that the > emission profile will change with the addition of the ferrite clamps. > This > is bothersome for (at least) three reasons- > > 1st: if the ferrite clamp reduces the emissions from a frequency > or > two then I can achieve compliance but a customer may experience an > interference problem due to the fact that they do not install the > ferrite > clamp at their facility. > > 2nd: if the ferrite clamp increases emissions from a frequency > then > a product that now achieves compliance may have to be redesigned in > order to > pass after the DOW. > > 3rd: since the FCC doesn't presently allow the use of the > ferrite > clamps then I have to test each product one more time- this adds cost > and > time delay- especially if a failure arises due to this test. > > > > This could be a major headache for people who deliver product to > market in Europe. > > > Can anyone tell us the driving reason behind this regulation? > Was > it to increase repeatability at test sites? Was it to reduce the number > of > interference complaints from ITE installations? > > > > Best Regards, > > Patrick Conway NCE > StorageTek > EMC Advisory Engineer > 303.661.6391 > 303.661.6717 (FAX) > > > > -----Original Message----- > From: Pettit, Ghery [mailto:ghery.pet...@intel.com] > Sent: Monday, November 18, 2002 10:09 PM > To: Conway, Patrick R; 'emc-p...@majordomo.ieee.org' > Subject: RE: > > > Patrick, > > I performed some A/B comparison measurements several years ago when this > was > still working its way through CISPR to aid in the determination of the > US > vote. I found that some emissions go down (some by a bunch) and others > may > go up when you add the clamps. You will need to re-test products for > Europe > as you can't predict what the change will by just by inspection. > > BTW, this amendment to CISPR 22 passed by 1 vote. The US voted no as > the > clamps were not adequately defined in the proposal. > > Ghery Pettit > Intel > > > -----Original Message----- > From: Conway, Patrick R [mailto:conw...@louisville.stortek.com] > Sent: Monday, November 18, 2002 2:53 PM > To: 'emc-p...@majordomo.ieee.org' > Subject: > > > > > All- > > I'd like to know if there are any opinions about... > > > It is my understanding the CISPR 22 A1:2000 will require > the > use of "ferrite clamps" during RE tests of table-top equipment. > > Has anyone started using these devices during their > testing? > Has anyone seen a difference in their test results with > the > use of these devices? > > > > Best Regards, > > Patrick Conway NCE > StorageTek > EMC Advisory Engineer > 303.661.6391 > 303.661.6717 (FAX) > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: davehe...@attbi.com > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > All emc-pstc postings are archived and searchable on the web at: > http://ieeepstc.mindcruiser.com/ > Click on "browse" and then "emc-pstc mailing list" > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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