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? > > >> >> > > >> > > > >> > ------------------------------------------- > > >> > 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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