I have to admit that I used an absorbing clamp as a current probe, but only up to 200 MHz. I compared it to a conventional probe at multiple locations along the cable, and I got very repeatable results fairly independent of line length related standing waves (3 meter coax terminated in N connectors to L-brackets bonded to a ground plane). This was a laboratory experiment where I used an injection clamp to drive rf current on to the shield and monitored peaks and nulls with the conventional probe and the absorbing clamp. I can supply test data to interested parties (I adhere to the prohibition against mass mailings with attachments for this service). The test data shows that the absorbing clamp smoothes out the standing wave problem and there is no reason in my mind to expect performance deterioration above 200 MHz. I was able to get the absorbing clamp arbitrarily close to the injection clamp, and as Ing. Gremmen astutely points out, this would not always be possible with real cables connected to real EUTs. Nevertheless it seems a promising technique, especially as, again as Ing. Gremmen points out, only the first fractional wavelength of cable is important to radiation, and therefore at higher frequencies the cable lay should be easier to optimize for maximum radiation (or if not optimized, then it should be able to be standardized).
Ken Javor ---------- >From: "CE-test - Ing. Gert Gremmen - ce-marking and more..." <[email protected]> >To: "Ken Javor" <[email protected]>, "David Heald" <[email protected]>, "Tudor, Allen" <[email protected]> >Cc: "EMC-PCST \(E-mail\)" <[email protected]> >Subject: RE: Site Correlation >Date: Sat, Jan 13, 2001, 9:08 AM > > > You analysis of the situation is correct but for one thing: > > In real life you cannot measure the current from a cable using a > current clamp above approx 300 Mhz for several reasons. > - physical limitations between current clamp size, coil inductance and > parasitic capacitance > make the behavior off all but very small (<10 mm) clamps unpredictable > above 300 Mhz (rule of thumb) > - most of the emission of a cable can be achieved in the first 1/4 lambda > of the cable > at 1000 Mhz this is 7.5 cm. ( just dipole equation) > To be able to get at least 90% of the current that initially > flows on to the cable, one needs to approach the cable exit to 7.5 mm. > This is physically > impossible but for laboratory measurements. > Of course on longer cables this current will have many peaks and nulls on > the cable length, > but some energy has already left the cable... > > This approach is therefore limited to lower frequencies. The immunity test > standard EN 61000-4-6 follows this approach using CDN's up to 80 Mhz and > 230 Mhz for > some equipment. The same approach would be very practical for small EUTs > and emission. > This standard in coordination with EN 61000-4-3 affectively presumes that > most immunity problems > are caused by CM current son cables below 80 MHz and by Cabinet coupling AND > CM cable current above 80 Mhz. > > As both emission and immunity behavior is determined by passive networks, > the whole system is > reciprocal. Therefore if one accepts immunity tests using this presumption, > there is no reason to > use the same reasoning for emission measurements. > > I personally want to emphasize that my personal contribution in the IEC EMC > standardization groups > follows this approach. > > As for 80 Mhz and up shielded rooms behave much more consistent and > reproducible, test costs using this > approach are cheaper (smaller rooms) and quicker. > > > Regards, > > Gert Gremmen, (Ing) > > ce-test, qualified testing > > =============================================== > Web presence http://www.cetest.nl > CE-shop http://www.cetest.nl/ce_shop.htm > /-/ Compliance testing is our core business /-/ > =============================================== > > >>>-----Original Message----- >>>From: [email protected] [mailto:[email protected]]On Behalf >>>Of Ken Javor >>>Sent: Saturday, January 13, 2001 9:44 AM >>>To: David Heald; Tudor, Allen >>>Cc: EMC-PCST (E-mail) >>>Subject: Re: Site Correlation >>> >>> >>> >>>I must say that this thread has been a refreshing alternative to the >>>EMC-law/regulations questions that typically occupy this service. Not >>>complaining either, because If I suddenly found myself working commercial >>>EMC issues I would likely be flooding this line with those self-same >>>questions. >>> >>>Almost as an aside, Mr. Heald raises an issue of enduring >>>interest to myself >>>and others. >>> >>>"Another important factor... is to manipulate the cables during >>>testing (oh, >>>how much easier our job would be without cables)." >>> >>>The same issue was raised parenthetically in my answer to the >>>question about >>>GTEM polarization. The issue is control of cable-sourced radiated >>>emissions. I am now about to allegorically take a baseball bat to a >>>hornets' nest... >>> >>>Bela Szentkuti pointed out almost twenty years ago that it would be much >>>more efficient and accurate to analytically/experimentally determine the >>>relationship between cable common mode currents and the resultant radiated >>>field based on the maximum possible radiation efficiency of that >>>cable, and >>>use that relationship to derive a common mode current limit for >>>cables from >>>30 MHz to 1 GHz, using the absorbing clamp as a measuring tool. >>>This would >>>speed up OATS or any other kind of RE testing by deleting the >>>requirement to >>>maximize cable radiation. >>> >>>So this question is a poll. How do the subscribers to this service feel >>>about cable common mode current control in lieu of direct measurement of >>>cable-sourced RE measurement? The idea being that first you would measure >>>and bring cable cm CE into compliance with a cable-type limit and >>>only then >>>would you make the RE measurement. The cables would only be support >>>equipment which did not contribute to the RE profile, hence any measured >>>emissions at or near the limit would be guaranteed EUT enclosure-related. >>> >>>Polite responses only, please!!! >>> >>>Ken Javor >>> >>> >>> >>>---------- >>>>From: David Heald <[email protected]> >>>>To: "Tudor, Allen" <[email protected]> >>>>Cc: "EMC-PCST (E-mail)" <[email protected]> >>>>Subject: Re: Site Correlation >>>>Date: Fri, Jan 12, 2001, 9:36 AM >>>> >>> >>>> >>>> Greetings again. >>>> I received some questions about this off list and there has been more >>>> discussion in this direction, so I thought I would throw my other two >>>> cents in. >>>> For small fully anechoic chambers with little room for antenna height >>>> adjustment, you should be able to have uncertainty of about 6dB or so >>>> (10dB is much safer realistically) when you apply correction factors for >>>> a 10m site. The reason for this is, as John Barnes pointed out, the >>>> absence of reflected waves being received in addition to the direct >>>> waves. The key importance to a fully lined chamber (including the >>>> floor) is that destructive waves are not present. With a reflective >>>> floor, destructive waves can lower your readings by more than 30dB. Add >>>> this to the 6 dB or so of uncertainty for additive waves and your total >>>> error could be enormous. With an absorber lined floor, the influence of >>>> the destructive waves is eliminated or reduced, so a correlation of 6dB >>>> (again 10dB is safer) should be achievable (this simply accounts for the >>>> absence of constructive interference). >>>> Another important factor to ensure you don't have any surprises when >>>> moving from precompliance to a compliance run is to manipulate the >>>> cables during testing (oh, how much easier our job would be without >>>> cables). Large signal strength changes can be achieved just by moving >>>> cables a few inches. >>>> I also have to agree with Gert's and Ken's comments on far field >>>> measurements. I mentioned this in my original message, but didn't >>>> elaborate at all. These are very important considerations that can >>>> greatly affect any expected correlation to a 10m OATS. >>>> >>>> -- >>>> David Heald >>>> Senior EMC Engineer/ >>>> Product Safety Engineer >>>> >>>> Curtis-Straus LLC NRTL >>>> Laboratory for NEBS, EMC, Safety, and Telecom >>>> Voice:978.486.8880x254 Fax:978.486.8828 >>>> www.curtis-straus.com >>>> >>>> >>>> Tudor, Allen wrote: >>>> >>>>> Greetings: >>>>> >>>>> What's the best way to correlate a pre-compliance chamber >>>(smaller than a 3m >>>>> chamber) to a 10m anechoic chamber? Should I use a signal >>>generator and >>>>> antenna or should I use a comb generator? >>>>> >>>>> Would the answer be different if I were correlating the pre-compliance >>>>> chamber to an OATS? >>>>> >>>>> Thanks in advance. >>>>> >>>>> >>>>> Allen Tudor, Compliance Engineer >>>>> ADC DSL Systems Inc. >>>>> 6531 Meridien Dr. >>>>> Raleigh, NC 27616 >>>>> phone: 919.875.3382 >>>>> email: [email protected] >>>>> >>>>> >>>>> ------------------------------------------- >>>>> This message is from the IEEE EMC Society Product Safety >>>>> Technical Committee emc-pstc discussion list. >>>>> >>>>> To cancel your subscription, send mail to: >>>>> [email protected] >>>>> with the single line: >>>>> unsubscribe emc-pstc >>>>> >>>>> For help, send mail to the list administrators: >>>>> Jim Bacher: [email protected] >>>>> Michael Garretson: [email protected] >>>>> >>>>> For policy questions, send mail to: >>>>> Richard Nute: [email protected] >>>>> >>>>> >>>>> >>>>> >>>> >>>> >>>> >>>> ------------------------------------------- >>>> This message is from the IEEE EMC Society Product Safety >>>> Technical Committee emc-pstc discussion list. >>>> >>>> To cancel your subscription, send mail to: >>>> [email protected] >>>> with the single line: >>>> unsubscribe emc-pstc >>>> >>>> For help, send mail to the list administrators: >>>> Jim Bacher: [email protected] >>>> Michael Garretson: [email protected] >>>> >>>> For policy questions, send mail to: >>>> Richard Nute: [email protected] >>>> >>>> >>> >>>------------------------------------------- >>>This message is from the IEEE EMC Society Product Safety >>>Technical Committee emc-pstc discussion list. >>> >>>To cancel your subscription, send mail to: >>> [email protected] >>>with the single line: >>> unsubscribe emc-pstc >>> >>>For help, send mail to the list administrators: >>> Jim Bacher: [email protected] >>> Michael Garretson: [email protected] >>> >>>For policy questions, send mail to: >>> Richard Nute: [email protected] >>> >>> >>> > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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