Alex, If ever the RJ cable between your unit A an B carries control or data signals that do not have a dedicated current return in addition to the power and ground lines, it may be possible that adding a CMC on the power and ground lines alone may have broken the ground return path for these signals. This could explain your EFT issue if these signals are only used when unit A is connected to unit B. Note also that even differential signals, when not transformer isolated, normally need a current return to avoid excessive common mode voltage swing. You may need to provide more detail about your I/Os and EFT failure mode so the group can better identify the nature of your problem.
If the above control/data scheme is representative of your setup, here are some possible fixes, which may not be applicable depending on your specific configuration, but I hope this can help you solve your issue or get an idea of the kind of additional information to provide: 1. Use a CMC that can carry all of the signals through the same core. 2. Use shunt filtering to chassis ground on your ground and power lines instead of a CMC. If there is too much common mode noise from your power circuits, and control/data is slow or differential and associated logic return can be tied to chassis ground, you may still use a CMC on your power circuit input to segregate its noise, this, assuming your shunt filtering provides adequate HF current return for control/data from the RJ connector to logic ground via the chassis. You may also try to improve the power circuit design or layout to avoid such issues. 3. Try to eliminate the need for a CMC or shunt filtering by optimizing your power, I/O and chassis grounding scheme to minimize common mode noise between these points. Define current return paths and minimize associated return impedance and HF current levels. 4. If your control/data is low speed, you may try RC or RLC, filtering on the signals a the receive end, or at both ends if bi-directional. The "L" should be a ferrite bead, and the "C" could in some cases simply be the parasitic capacitance of your receive IC if a high value resistor or ferrite can be placed very close to the IC pin. 5. If your signals are differential, a second dedicated very high impedance CMC may also help, but unless you add a dedicated signal return wire in your RJ cable flowing through that same CMC the performance will be limited by the combined common mode impedance of the signals going through the CMC. The common mode impedance is basically the parallel sum of all sources and terminations referred to logic ground (Of unit A or B, whichever is worst; Not A and B in parallel) on the signals going through the CMC. If your signals are single ended, you may also add a dedicated CMC but a dedicated signal return flowing through the CMC would be of even greater importance. Without a dedicated return, excessive crosstalk will likely result between the signals and that case option #4 would be more suitable. 6. If your control/data signals can be more effectively referred to chassis ground than your DC input, you may add a dedicated clean return ground wire in your RJ cable for control/data, which would be referred to the chassis at both ends, and keep the CMC for the power and "power return" lines. There would certainly be other options to consider if you can provide more detail. Best regards, Eric -----Original Message----- From: Alex McNeil [mailto:alex.mcn...@ingenicofortronic.com] Sent: May 27, 2002 10:54 AM To: emc-p...@majordomo.ieee.org Subject: EMC Emissions vs EFT warning! Hi Group, I am working on a small Class II product (A) i.e. no earth. The product is powered by 12Vdc via a 1M cable with RJ type connections from another similar Class II product (B) which itself is powered via an external in-line AC/AC power block (basically an AC/ac transformer) and rectified to 12Vdc internally. It has a CMC on the supply lines. The system (A+B) EMC tests were fine. However, since product A may be used in future as a stand alone product i.e. powered from it's own 12Vdc power supply, I had to add a small common mode choke to the input supply lines, the same lines as the RJ connection method, to enable me to meet the EMC requirements. Since I had added a major EMC component I decided to re-test the EMC of the system. To my horror the system (A+B) readily failed the mains EFT tests on product B. Removing the CMC on product A resolved the EFT problem. I now have to create 2 product A builds depending on the option of usage, one with CMC (Stand alone usgae) and one without a CMC (system usage). I am sure I have had a similar problem in the past but at that time I did not realise that the addition of the CMC (to resolve conducted radiated emissions) was the source of the EFT failures. I think the EFT problem must have something to do with high/low impedance mis-match due to the CMC but I am not sure. I would appreciate a more experienced member of the group explaining as to why this scenario can occur? and more importantly as a warning to others! As always I look forward to your valuable comments Kind Regards Alex McNeil Principal Engineer Tel: +44 (0)131 479 8375 Fax: +44 (0)131 479 8321 email: alex.mcn...@ingenicofortronic.com ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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