Weston, I set up the shielded parallel line (vertically oriented) with the line driven against an infinite plane (horizontal). With the lines driven perfectly out-of-phase there is very little radiation (-1 volt on wire A, +1 volt on wire B). The radiation does not change with the shield grounded or floating.
With the line driven with a common-mode signal (0.5 volts on wire A, -1.0 volts on wire B) there is a huge difference in the radiation between grounding and not grounding. But, whether grounding helps or harms depends on whether the shield is 1/2 wavelength or 1/4 wavelength long. I would have expected that grounding the shield would help in all cases. I will look into this and see if we are being fooled by the shield to ground connection wires - there are only two wires and they might be radiating. In any case there will be a voltage drop along the length of these wires. This potential is what will drive CM current along the outside of the shield and cause radiation. Note that in the case of the 1/4 or 1/2 wavelength shield the current through these wires changes quite a bit. This brings up the importance of the shield connection as pointed out by Chris Maxwell. I have some refining to do. Any more suggestions? In a bit I can put this into a word document and send it to whoever wants a copy. Dave Cuthbert From: [email protected] [mailto:[email protected]] On Behalf Of drcuthbert Sent: Monday, September 19, 2005 11:42 AM To: [email protected]; [email protected]; [email protected] Cc: [email protected] Subject: RE: cable shield simulation Weston, I will change the simulation to allow a common mode signal. In the present simulation there is no way to introduce a common-mode signal; the shield is floating and there is no reference point to develop a common-mode signal against. However, in a real application there is a shielded enclosure at one or both ends of the cable. I can drive a common-mode signal referenced to an enclosure (in this case I will use a infinite ground). I believe that the shield voltage will be equal to the common-mode voltage of the T-line. Therefore there will be a common-mode voltage between the shield and the enclosure (infinite plane), which leads to common-mode current, which leads to EM radiation. Dave Cuthbert From: Beal, Weston [mailto:[email protected]] Sent: Monday, September 19, 2005 10:57 AM To: drcuthbert; [email protected]; [email protected] Cc: [email protected] Subject: RE: cable shield simulation Dave, This looks like a good experiment. I think the key that most engineers have tried to reinforce is the differential signal. Since you ran this experiment in a simulation, the signal was somewhat ideal and the radiation is as expected. Can you run the same simulation with a small amount of common-mode current, maybe 5%, to simulate the non-ideal circuits of reality? Regards, Weston From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Monday, September 19, 2005 9:05 AM To: [email protected]; [email protected] Cc: [email protected] Subject: cable shield simulation For what it's worth I ran a NEC-2 model of a differential transmission line under these conditions: 1) free space 2) covered with a mesh shield open at each end 3) covered with a mesh shield closed at each end The radiation under condition 2 is the same as condition 3. The radiation in condition 2 and condition 3 is 13dB below condition 1. >From this I infer that closing the ends makes no difference, as several here have stated. I can improve the model in various ways but I will wait for more input from all of you. NEC-2 Model: 7.0" differential line spaced 0.1" 7.5" mesh shield Mesh shield: Six sides 0.5" diameter 0.5" distance between "rings" Dave Cuthbert Micron Technology - 2005 IEEE Symposium on Product Safety Engineering 3-4 October Schaumburg, IL http://www.ieee-pses.org/symposium 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 [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html 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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - 2005 IEEE Symposium on Product Safety Engineering 3-4 October Schaumburg, IL http://www.ieee-pses.org/symposium 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 [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html 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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - 2005 IEEE Symposium on Product Safety Engineering 3-4 October Schaumburg, IL http://www.ieee-pses.org/symposium 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 [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html 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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
