Rick, I don't see that you have a problem. Your differential video signals are not susceptible to ground loops, indeed,they do not require ground at all. You can safely rely on the shield to function only as a shield, and not as a signal return. This makes your job easier. So I would answer your questions as follows:
1. No, in this case it is not reasonable to terminate only one end of the shield -- as you have discovered yourself, doing so increases radiated EMI. 2. No, differential returns can not be successfully grounded to chassis return -- that would upset the balance -- and should not be. 3. Given the distance between your devices, you may have to use capacitive decoupling on the shield at one end. There are coaxial connectors made which include planar, low-inductance capacitors between the shell and actual ground. This might do for your application. 4. I always wanted to terminate the signal returns at the device which generated the signal, so the return currents flowed only in the signal return wire and not in the chassis. This seemed to reduce common-mode problems. Regards, Cortland ====================== Original Message Follows ==================== >> Date: 04-Nov-98 11:13:23 MsgID: 1065-13982 ToID: 72146,373 From: rbus...@es.com >INTERNET:rbus...@es.com Subj: Cable Shielding Chrg: $0.00 Imp: Norm Sens: Std Receipt: No Parts: 1 My company manufacturers image generation equipment (large mainframe computer) which is typically placed at a significant distance from the display device. These may be as much as 50 to 150 feet apart. In my EMC tests, I have determined that due to common mode, harmonic noise, both ends of this shielded cable must be terminated to the respective device chassis. Unfortunately, this has the potential to create significant ground loops which severely compromise the quality of the video signal. To add insult to injury, the signals within the cable are twisted pairs, differentially driven. This eliminates (as I understand it) the possibility of terminating the signal return to chassis ground at the point at which the cable exits the enclosure. So, on the surface it appears that I have the choice of meeting EMC regulations or meeting equipment performance. My questions are as follows; 1. Is it reasonable to provide single end termination of the overall (outside) cable shield? 2. Can differentially driven signal grounds be successfully grounded to chassis return? Should they be? 3. Is it reasonable to request or demand that both devices are sourced from the same power AND/OR bonded to a common power grid? 4. Is it desirable for signal returns to terminate at the chassis ground at the point at which the cable exits the device. Thanks in advance Rick Busche Evans & Sutherland rbus...@es.com --------- --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
