John, thanks for the explanation to David, you are right in what you wrote. I was out the whole day so I am catching up now in the evening. However, the issue is not the imbalance specification looking into the DUT connector (test port, diff pairs....). The problem that I see here is that CISPR22, 2005 is trying to emulate the imbalance of the "Typical installation", and that I do not find any measurement data on that, at least not for the CAT3-7 cabling. I do find the LCL (i.e. balance) vs. frequency information in standards, but absolutely no published data on LCL of "typical installations" to support them. So, I don't know where these values in the standards are coming from. Again, I am not talking here about imbalance at the DUT connector, but imbalance that appears looking into the cabling from the DUT. The required level of imbalance for the ISN, combined with the max possible (although not always used) amplitude of 10 BaseT (5.6 Vpk-pk) makes me uncomfortable. On the other hand, if one uses the method with current and voltage clamp, then there is no requirement for LCL, since the measurement is done on the cable without any ISN. In my opinion, manufacturers of Class B products that use Ethernet, e.g. laptops, PCs, SOHO routers/switches) should be concerned by the still (in my opinion) not well defined requirements. Neven ------------- Original message -------------- From: John Woodgate <j...@jmwa.demon.co.uk>
> I'm not sure what you mean by 'an impedance imbalance between the two > ports'. We were not discussing impedance mismatch but mode conversion > from differential to common (leading potentially to emissions from the > cable) or vice versa (leading to a lack of immunity). > > The functioning of balanced circuits in not radiating the signal and > rejecting common-mode disturbances depends on the impedances at each end > of the cable, from each conductor to a common reference point, being > closely equal. Any voltage between the common reference points is then > the input to a balanced bridge, and none of that voltage appears > between the signal conductors. And vice versa, of course. > > While transformers CAN be made with excellent symmetry and very closely > equ! al impe dances to the common point, they are not always so designed, > and it would be appropriate for the system standard to specify the > permitted unbalance, in one of the many ways that can be done. I think > this is what the original enquirer is asking about. > -- > OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk > 2006 is YMMVI- Your mileage may vary immensely. > > John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
