No argument, and once a cable is electrically long, the probe must be scanned down the length of the cable looking for maxima. Along those lines, the Henry Ott equation referenced by someone else is only valid for electrically short cables. Aschenberg and Grasso extended the equation to the case where the cable is electrically long, treating the cable in that case as a tuned dipole.
http://emcesd.com/tt2006/mat-cmi.pdf Ken Javor Phone: (256) 650-5261 > From: Ed Price <[email protected]> > Reply-To: Ed Price <[email protected]> > Date: Fri, 15 Jan 2016 09:40:22 -0800 > To: <[email protected]> > Subject: Re: [PSES] Fwd: [PSES] Current probe for CM currents > > I would add a caution to Ken's comment about common mode cable currents > creating RE. Yes, the CM currents certainly do create RE, but you need to > probe the cables at several intervals to understand those current paths. It > is not immediately obvious that all CM current flowing on a cable at one end > of a cable does not necessarily flow at the other end of that cable. > Especially where cables are bundled, or where they pass closely along a > chassis or structural member, there are possibilities for that current to > couple off of the cable. The current flow will follow the impedances, both > at the ends and at other fortuitous nodes. This is one of the reasons that a > ferrite absorber might work much better at one position along a cable than > at another position and also why RE might be dependent on something as > obscure as cable bundle tightness. > > Ed Price > WB6WSN > Chula Vista, CA USA > > > > From: Ken Javor [mailto:[email protected]] > Sent: Friday, January 15, 2016 8:53 AM > To: [email protected] > Subject: Re: [PSES] Fwd: [PSES] Current probe for CM currents > > > > I think it is important to not lose sight of the original query that started > this thread. The query was about whether placing a current probe around a > cable perturbed the current to be measured. > > There is no doubt that radiated emissions can originate within an equipment > enclosure separately from driving common mode currents on a cable, but that > wasn't the query. In fact, the poster was probing cables within a large rack > (enclosure) looking for a source within an enclosure. > > Ken Javor > Phone: (256) 650-5261 > > > > _____ > > From: Bill Owsley <[email protected]> > Reply-To: Bill Owsley <[email protected]> > Date: Fri, 15 Jan 2016 07:26:08 +0000 > To: <[email protected]> > Subject: Re: [PSES] Fwd: [PSES] Current probe for CM currents > > If you can measure common mode noise on a cable, you have a problem from the > port !! > Note the world famous Ott's math on this effect in his 1st edition. Might > be in his 2nd too. > > I have used both e-field and h-field (current clamp) at the same time. > We are engineers so figure out how I did that! > And since some of the work is below 30 MHz, I have also added a loop antenna > for a 3rd measurement. > My approach is if I find any emission, locally, near field, bench stuff, > that varies by position over the area of the product, then I have a problem. > E-field scan,using a o'scope probe. H-field scan usually using a personally > built small loop, and any other sort of scan, conducted or radiated, that I > can make up at the moment. > I work for a homogeneous field in the scans over the area of the product. > My assumption is that if I find a homogeneous field, then there are no or > low emission gradients which can equate to a field at a distance. So get > creative, and redundant, by different methods for measuring the emissions. > Ironic, I am good at mashing all emissions, and then they hand me an > intentional radiator and ask that I don't kill the fundamental. What ? You > mean I have to pick what to mash, and what not to mash? > Ok, so I caught on quick enough to keep the job. > > > ps. I suffer from not being able to use a leaky enclosure. I don't get any > shielding for the products. > Cable shielding that is bogus terminated, but at the low frequencies of > interests, it works. > Then I have to deal with the higher frequencies, the harmonics !!! > Plastic covers and pcb and cables up to 15 KW or more of digital BS to make > an analog signal. > And then 'normal' digital signals for the ADC circuits all in the middle of > this. > Management is like, we have done it this way for over 25 years and so we are > not changing it now. > It works (I have to make it work) so don't change anything. Sucks to be me > - but I do like a challenge. > > > > > > > > > > > > > > > _____ > > From: Ken Wyatt <[email protected]> > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > <[email protected]> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used > formats), large files, etc. > > Website: http://www.ieee-pses.org/ > Instructions: http://www.ieee-pses.org/list.html (including how to > unsubscribe) > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <[email protected]> > Mike Cantwell <[email protected]> > > For policy questions, send mail to: > Jim Bacher: <[email protected]> > David Heald: <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
