Paolo, Actually, we did give a damn about you. If you just follow for a couple of paragraphs, maybe you'll see how. One of the problems with small pre-compliance chambers is that it is sometimes hard if not impossible to fit the DUT and all of its associated cabling into the chamber. Also, even if you can fit the cables, it may be hard for a small chamber to measure the cable emissions accurately. We use a 0.7m cubic G-Strip (not a G-TEM) cell for pre-compliance emissions measurements and A-B comparisons. The chamber is fully anechoic, so reflections aren't a problem. However, cables are a problem.
Usually, what I end up doing is: After we finish pre-compliance, I take a product to the OATS for compliance testing. I bring the product back to our lab and put it in our chamber. However, in order to fit it in, I usually have to strip the serial cable, keyboard, mouse, parallel cable and VGA cable. All that I can fit in the chamber is the unit and the power supply. I then take a reference measurement of this "stripped" system. I store these readings for future A-B comparisons. When I do these A-B comparisons, it would be nice to supplement them with coupling clamp measurements from the cables before I remove them. I would then possibly have a more complete picture by using the small cell to assess the enclosure radiation and the coupling clamps to assess the cable radiation. I believe that this is how the CE vs. RE measurement methods for cables got mixed into the thread. Using a combination of small chambers for the enclosure emissions and clamp measurements for the cable emissions may provide a more complete picture than using the chamber alone. This may have been where Ken Javor was going when he brought up the possibility of measuring cable emissions with a clamp. I personally don't know of anybody performing this "combination" measurement. It sounds like a good idea, but it would be nice to hear if anyone has real life experience with it. It may add some useful information to the original question that started this thread, which was how well small chamber emissions measurements correlate with OATS measurements. Chris Maxwell Design Engineer GN Nettest 6 Rhoads Drive, Building 4 Utica,NY 13502 email: [email protected] phone: 315-266-5128 fax: 315-797-8024 > -----Original Message----- > From: Paolo Roncone [SMTP:[email protected]] > Sent: Wednesday, January 17, 2001 6:08 AM > To: Lothar Schmidt > Cc: EMC-PCST (E-mail) > Subject: RE: Site Correlation > > > Good point Lothar, > it was about time that the original technical grounds and limitations of > CE > method were brought up. > Just one additional point: with the ever increasing operating frequencies > of many electronic products, box and/or PCB level radiation is getting > more > and more important vs cable radiation (and as a by-product cable layout > should weigh less in measurement uncertainty). > > One last point: I was a bit perplexed by the way this thread shifted from > the original question. I myself stepped in early with a question about > fully-anechoic vs semi-anechoic pre-compliance chambers but then the > subject switched to the CE vs RE issue and nobody gave a damn about me.. > > Paolo > > > At 11:28 AM 1/16/01 -0800, Lothar Schmidt wrote: > > >I have the feeling that different issues are mixed in this discussion. > > > >supposed that CE vs. RE methods is the issue, I can give you some > historical > >information. The CE method is used as a simplified method for the > radiation > >of the tested device. > >The CE method was used for devices which have to met several conditions > >1. the cable length was long compared to the size of the device ( the > >longest side should not be longer than 80 cm) > >2. the number of cable is limited to one or maximum 2 cables. > >3. the frequencies produced in the equipment have to be low due the > >limitation of the method to 300 MHz. > > > >Reasons for > >1. the cable should be the preferred antenna for the emission of the > device > >2. You can only made a correlation between CE and RE if all the radiated > by > >the one cable. You will not be able to calculate the sum of different > cables > >because you don't know the relation. > >3. The method is only specified up to 300 MHz. At higher frequencies the > >cables act different. > > > >This method was used e. g. simple household devices and tools. > > > >I don't know if I got the real point because I didn't followed the whole > >discussion, but perhaps I can put in some more ideas. > > > >Best Regards > > > >Lothar Schmidt > >Technical Manager EMC/Bluetooth, > >BQB, Competent Body > >Cetecom Inc. > >411 Dixon Landing Road > >Milpitas, CA 95035 > >Phone: +1 (408) 586 6214 > >Fax: +1 (408) 586 6299 > > > > > >-----Original Message----- > >From: Ken Javor [mailto:[email protected]] > >Sent: Tuesday, January 16, 2001 7:45 AM > >To: Ralph Cameron; chris maxwell; dan kwok > >Cc: EMC-PCST (E-mail) > >Subject: Re: Site Correlation > > > > > > > >I am getting the distinct (but uncomfortable) feeling that was is being > >discussed by a lot of people on this thread is that cable cm CE need to > be > >controlled to prevent either crosstalk to another bundle, or to prevent > >interference to equipment connected to the same bundle. Am I > interpreting > >these comments correctly? For the record, I don't believe either of > these > >is a real issue. The only traditional, and in my experience, legitimate > >purpose of controlling cable cm CE is to prevent coupling to the antennas > >connected to radio receivers. > > > >Ken Javor > > > >---------- > > >From: "Ralph Cameron" <[email protected]> > > >To: "Chris Maxwell" <[email protected]>, "Ken Javor" > ><[email protected]>, "dan kwok" <[email protected]> > > >Cc: "EMC-PCST \(E-mail\)" <[email protected]> > > >Subject: Re: Site Correlation > > >Date: Tue, Jan 16, 2001, 9:01 AM > > > > > > > > What it boils down to Chris is the lack of immunity of the consumer > > > equipment contributes to degradation of the intended function. Once > the > > > undesired energy reaches the consumer device there's no way to get rid > of > > > it. The rememdy is to prevent it from reaching the device and or > isolating > > > it from the source. > > > > > > At one time injection clamps were used for immunity testing- are they > >still? > > > > > > Ralph Cameron > > > > > > > > > ----- Original Message ----- > > > From: "Chris Maxwell" <[email protected]> > > > To: "'Ralph Cameron'" <[email protected]>; "Ken Javor" > > > <[email protected]>; "dan kwok" <[email protected]> > > > Cc: "EMC-PCST (E-mail)" <[email protected]> > > > Sent: Tuesday, January 16, 2001 8:38 AM > > > Subject: RE: Site Correlation > > > > > > > > >> Seems like this thread has gotten into how to correlate common mode > cable > > >> currents with their expected radiated emissions. > > >> > > >> For those interested, Fischer Custom Communications makes coupling > and > > >> measuring clamps which can measure common mode surface currents on > cables > > >> and surfaces. They used to publish some application notes regarding > the > > >> usage of their clamps to measure surface/cable currents and how to > > > correlate > > >> them to expected radiated emissions. > > >> > > >> I read them a couple of years ago. I never bought the clamps, but it > did > > >> make for some very good technical reading. > > >> > > >> I do know of a table top power supply manufacturer that uses this > method > > >> almost exclusively. They send one power supply to a calibrated OATS. > > > They > > >> get it to pass. Then, when the sample comes back to the factory, > they > > > take > > >> clamp measurements of the common mode currents of the AC input and DC > > > output > > >> cable. > > >> > > >> They then model the power supply as a dipole antenna with the AC > input > > > cable > > >> and DC output cable being the two poles. > > >> > > >> For future power supplies, they then use the clamp method in-house to > > >> measure the cable currents, if the currents pass, they assume the > supply > > >> passes radiated emissions. > > >> > > >> This won't work for every product, but it does fit this application > well. > > >> The power supply company could make more than 10 versions (3.3VDC, > 5VDC, > > >> 9VDC, 12VDC ...) of a power supply with the same case and cabling so > it > > > can > > >> save them a great deal of time and money. The supplies only have two > > >> cables, which is easy to model. The supplies have clock speeds in > the > > >> 100-500Khz range, meaning that most of thier harmonics will be "dead" > >over > > >> 230Mhz, which is the cutoff for most coupling clamps. > > >> > > >> I thought that this method would be difficult to use for our products > > > since > > >> we have higher clock speeds and multiple cables. > > >> > > >> I guess many times the measurement method is somewhat defined by what > > > you're > > >> measuring. > > >> > > >> Chris Maxwell > > >> Design Engineer > > >> GN Nettest > > >> 6 Rhoads Drive, Building 4 > > >> Utica,NY 13502 > > >> email: [email protected] > > >> phone: 315-266-5128 > > >> fax: 315-797-8024 > > >> > > >> > > >> > > >> > > >> > -----Original Message----- > > >> > From: Ralph Cameron [SMTP:[email protected]] > > >> > Sent: Monday, January 15, 2001 10:57 PM > > >> > To: Ken Javor; dan kwok > > >> > Cc: EMC-PCST (E-mail) > > >> > Subject: Re: Site Correlation > > >> > > > >> > > > >> > No, your message is clear, what I am saying is that the emissions > below > > >> > 30Mhz cause the majority of the interference problems to consumer > > >> > electronics and that's not being addressed. > > >> > > > >> > Ralph Cameron > > >> > > > >> > ----- Original Message ----- > > >> > From: "Ken Javor" <[email protected]> > > >> > To: "Ralph Cameron" <[email protected]>; "dan kwok" > <[email protected]> > > >> > Cc: "EMC-PCST (E-mail)" <[email protected]> > > >> > Sent: Monday, January 15, 2001 10:34 PM > > >> > Subject: Re: Site Correlation > > >> > > > >> > > > >> > > I must have been unclear in my previous message. The purpose of > > >> > controlling > > >> > > cable cm CE is to control the resultant cable-induced RE, which > are > > >> > > controlled to protect tunable antenna-connected radio receivers, > > > period. > > >> > > There was never any other purpose for controlling CE or RE. > > >> > > > > >> > > Ken Javor > > >> > > > > >> > > ---------- > > >> > > >From: "Ralph Cameron" <[email protected]> > > >> > > >To: "Ken Javor" <[email protected]>, "Dan Kwok" > > >> > <[email protected]> > > >> > > >Cc: "EMC-PCST \(E-mail\)" <[email protected]> > > >> > > >Subject: Re: Site Correlation > > >> > > >Date: Mon, Jan 15, 2001, 8:51 PM > > >> > > > > > >> > > > > >> > > > Perhaps what you state is correct Ken but there has been a > > > supposition > > >> > that > > >> > > > RE , induced or other wise when converted to conducted current > does > > >> > not > > >> > > > effect other devices connected to those same conductors whether > >they > > >> > be > > >> > > > power, incoming TV or telephone cables etc. All these > conductors > > >> > intercept > > >> > > > RE and their effects have been eliminated in 90% of cases( I > have > > >> > > > personally suppressed ) , by suppresseing the common mode > > > signals.Over > > >> > 300 > > >> > > > successes is a significant statistic. > > >> > > > > > >> > > > Ralph Cameron > > >> > > > > > >> > > > > > >> > > > . > > >> > > > ----- Original Message ----- > > >> > > > From: "Ken Javor" <[email protected]> > > >> > > > To: "Dan Kwok" <[email protected]>; "Ralph Cameron" > > > <[email protected]> > > >> > > > Cc: "EMC-PCST (E-mail)" <[email protected]> > > >> > > > Sent: Monday, January 15, 2001 5:42 PM > > >> > > > Subject: Re: Site Correlation > > >> > > > > > >> > > > > > >> > > >> > > >> > > >> Mr. Kwok's theories are logical and no doubt bear on the > subject, > > > but > > >> > > > there > > >> > > >> is a historical angle that bears inspection. About the time > FCC > > >> > limits > > >> > > > for > > >> > > >> IT equipment were being drawn up (late '70s) PCs were not yet > on > > >> > > > everyone's > > >> > > >> desktop. Most of the business equipment that would have been > > >> > envisioned > > >> > > > to > > >> > > >> be qualified to USC Title 47, Part 15, Subpart J would have > been > > >> > > > stand-alone > > >> > > >> items such a copier, with the only cable connection being ac > >power. > > >> > The > > >> > > >> report which documents the development of the CE and RE > >limits/test > > >> > > > methods > > >> > > >> found in the above mentioned FCC limits specifically states > that > >30 > > >> > MHz > > >> > > > was > > >> > > >> picked as the cutoff between CE and RE for the reason of > radiation > > >> > > >> efficiency per Mr. Kwok's surmise, but also because 30 MHz was > the > > >> > lowest > > >> > > >> frequency at which a 3 m OATS measurement would provide the > >desired > > >> > > >> accuracy. > > >> > > >> > > >> > > >> Ken Javor > > >> > > >> > > >> > > >> P.S. Said report also demonstrated that the CE limit below 30 > MHz > > >> > > > sufficed > > >> > > >> to control RE from the power cable to levels sufficient to > protect > > >> > against > > >> > > >> cable radiation-induced rfi. > > >> > > >> > > >> > > >> ---------- > > >> > > >> >From: Dan Kwok <[email protected]> > > >> > > >> >To: Ralph Cameron <[email protected]> > > >> > > >> >Cc: "EMC-PCST (E-mail)" <[email protected]> > > >> > > >> >Subject: Re: Site Correlation > > >> > > >> >Date: Mon, Jan 15, 2001, 2:49 PM > > >> > > >> > > > >> > > >> > > >> > > >> > > > >> > > >> > Hello Ralph: > > >> > > >> > > > >> > > >> > That's a good question. At one time, I pondered the same > >question > > >> > > >> > myself. There are obviously plenty of communication systems > > >> > operating > > >> > > >> > under 30 MHz. I suppose there are reasons why CISPR or CISPR > 22 > > >> > does > > >> > not > > >> > > >> > specify radiated emissions below 30 MHz. I can suggest one > > >> > possibility. > > >> > > >> > Perhaps others here will come up with more. > > >> > > >> > > > >> > > >> > For a fixed cable of length L, the ratio of L/lambda gets > > >> > progressively > > >> > > >> > small for frequencies much less than 30 MHz with most > commercial > > >> > EUTs. > > >> > > >> > If we consider the cable part of dipole antenna, the > reduction > >in > > >> > > >> > frequency has a diminishing effect on the antenna's > radiation > > >> > > >> > resistance. Given a constant current, the radiated power > would > > >> > decrease > > >> > > >> > with decreasing radiation resistance. At 550 KHz (bottom of > the > > > AM > > >> > > >> > broadcast band in North America), the 1/4 wavelength is 136 > > > meters. > > >> > Even > > >> > > >> > if the antenna's reactance is ignored, one would need very > long > > >> > cables > > >> > > >> > driven by a significant CM noise voltage at this frequency > to > > >> > radiate > > >> > > >> > much energy. > > >> > > >> > > > >> > > >> > -- > > >> > > >> > > >> > > > >> > ------------------------------------------------------------------------ > > >> > > >> > Daniel Kwok > > >> > > >> > Principal EMC Engineer > > >> > > >> > Intetron Consulting, Inc. > > >> > > >> > Vancouver, Canada > > >> > > >> > Phone (604) 432-9874 > > >> > > >> > Email [email protected] > > >> > > >> > Web http://www.intetron.com"; > > >> > > >> > > > >> > > >> > > > >> > > >> > Ralph Cameron wrote: > > >> > > >> >> > > >> > > >> >> Ken: > > >> > > >> >> > > >> > > >> >> I like the idea of setting a limit to common mode currents > on > > >> > attaching > > >> > > >> >> cables but mI wonder why CISPR has chosen to start such > > >> > measurements > > >> > at > > >> > > >> >> 30Mhz when most of the common mode currents are the result > of > > >> > switching > > >> > > >> >> products and are generated harmonically from the > fundamental > > > and > > >> > as > > >> > > > such > > >> > > >> >> propagate from the low Khz range up through 30Mhz. is there > no > > >> > > > consideration > > >> > > >> >> for those who occupy the spectrum below 30Mhz? 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