Hi Doug, I figure it's the case where you have a high input impedance at the other end (second box), or just open-ended. In that case the filed to shield against is prevalent E-field and in that case the shield works pretty much the same whether it's connected or not at the other end. No need to close a current loop...
Regards, Paolo On Sun, Feb 4, 2018 at 5:06 AM, Doug Smith <d...@emcesd.com> wrote: > Hi All, > > I have another riddle for you, this time a video is not needed. > > We all know that cable shields need to be connected on both ends to work > at high frequencies, but do they? > > Imagine the case of a source in a metal box driving a shielded cable, > let's assume it is shielded twisted pair and we are worried about the > common mode noise on the pair radiating or ESD noise getting in and > saturating the receiver in the box, exceeding its common mode range. We can > also assume that the shielded connector on the first metal box is ra eally > good one with a 360 degree connection to the box. The other end of the > shielded cable goes to another metal box with a passive circuit inside, say > a temperature sensor connected to the twisted pair wires. There is one > special, but not uncommon, case where it does not matter if the shield is > connected to the second box or not (or even if there is a second box at > all), it will work in either case! Do you know what that case is? > > This is another experiment I do during my classes. I prove the conditions > needed for the situation above and what the limits are. > > I am pretty much a lab rat and I have devised experiments to test the > limits of most all EE concepts I have learned. I find there is more than > meets the eye on most every principle. I started doing experiments like > this around age 11 when I duplicated one of Marconi's experiments to see if > it really worked, it did, although I suspect the neighbors may have had a > rough time watching TV that day in 1958. A few years later another > experiment brought the FCC to my house, but that is another story. > > Doug > - > ---------------------------------------------------------------- > > 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 < > emc-p...@ieee.org> > > 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) <http://www.ieee-pses.org/list.html> > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <sdoug...@ieee.org> > Mike Cantwell <mcantw...@ieee.org> > > For policy questions, send mail to: > Jim Bacher <j.bac...@ieee.org> > David Heald <dhe...@gmail.com> > - ---------------------------------------------------------------- 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 <emc-p...@ieee.org> 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 <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
