Bruce, DB has identified a very likely source of the problem. From your description, the high value of surface transfer impedance of the shield of the cable assembly occurs at the connectors, rather than along the braid, itself.
An experiment to verify this would be to sacrifice a cable assembly by peeling back the insulating jacket next to the connectors and to strip away the insulation from the connectors. Then construct a conducting tent from copper tape (or copper tape and solder) from the cable braid shield to either the shell of the connector, or the panel to which the connector mounts. This creates an EMI backshell for the connector. If your problem goes away when you use this home shielded connector, then you need a higher quality cable assembly that has a better shield termination at the connector. Jim Dr. Jim Knighten e-mail: [email protected] Senior Consulting Engineer NCR 17095 Via del Campo San Diego, CA 92127 http://www.ncr.com <http://www.ncr.com> Tel: 619-485-2537 Fax: 619-485-3788 -----Original Message----- From: Brumbaugh, David [SMTP:[email protected]] Sent: Thursday, May 27, 1999 7:21 PM To: '[email protected]'; [email protected] Subject: RE: CAN Bus EMC The poor temination is resulting in a high transfer impedance, i.e., currents on the cable result in a relatively high voltage at your circuits. I(shield) x Z(transfer) = V(circuit). You've got to lower the shield termination impedance, preferably with a good (i.e., low Z) circumferential termination to the chassis. DB > -----Original Message----- > From: [email protected] [SMTP:[email protected]] > Sent: Thursday, May 27, 1999 7:15 PM > To: [email protected] > Subject: CAN Bus EMC > > > Hello group: > > > I'm working on a susceptability problem with a CAN bus industrial control > system. We are performing a 30v IEC 1000-4-6 conducted immunity test on > the bus cable and get data loss and system shutdown. We have added common > mode chokes in the bus leads, tranceiver power decoupling etc. The bus > cable is twisted pair shielded with an overall braided shield that is > terminated poorly by connector pin and lead to chassis. This shield > termination seems to be dictated by the standard design of the cables. The > failures occur at 50-70mhz. > > Does anyone have any experience with this? We have been seeing more > products with this type of control bus and I though this experience might > be useful to the group. > > Bruce Fagley > TUV Rheinland > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
