Hi Derek, I've seen a couple of posts with good advice. I'm wondering if you've already taken care of bonding the shield; and you're still having issues.
Forgive me for repeating your question...but I'm thinking out loud (which is difficult for email :-)...for some reason when you just have the Ethernet cable plugged into the transmit end (The DUT) in the chamber... the Ethernet signal conductors don't change potential at 125Mhz with enough magnitude to couple to the measurement antenna...but when you terminate the receive end, outside the chamber..they do. That could be due to the symbol rate of the actual ethernet traffic...which would probably increase when the receive end is plugged in. Could it also be a coincidence...how good does the receive end filter its Ethernet cable? Could the receive end have another, unrelated clock at 12.5 or 25Mhz that's catching a ride out? Now...somehow, the emissions get higher with a shielded cable...is this with or without the receive end plugged in? For the shield to work best...it would need to be bonded at both ends to the same, infinite capacity, zero resistance ground at Earth potential...which would also, hopefully, be the same ground as the shield of the coax for the measurement antenna...this would minimize measured emissions. Any break in that chain...and you have an issue. I saw "ground currents" mentioned. This would be due to some potential difference between the shield's end at the DUT, the shield's end at the receiver and or the ground of the measurement antenna system. If you've already messed with shield bonding...Have you also tried the following experiments? -Are you sure of the shield integrity within the cable? Is it old? Has it been bent a few times? -What if you try a different device at the receive end? -Have you played with the grounding of the chassis of the device at the receive end? (grounded, ungrounded...grounded back to chamber ground) -What if you leave the cable hooked up at the receive end, leave it running into the chamber and disconnect it from the DUT? What happens? -Are there any power filters that could possibly be breaking the Earth path between the DUT and the device at the receive end of the Ethernet cable? ---What if you just turn it all off, go to the local pub and catch a ball game over some food and spirits...maybe the emissions will still be there; but you won't care :-) I don't know what's at the receive end; or what your DUT is; but many Ethernet devices are powered by Wall Warts and carry no real chassis ground. Many "shielded" ethernet cables have wimpy shielding and poor termination to the connector...to the point where the termination to the connector could be ruined after some hard bending cycles. Running this type of shielded cable between two devices with unknown ground characteristics is always "fun". I don't know if I've ever gotten a device through a suite of EMC testing unscathed if it had an Ethernet cable...seems as though we always had to fiddle with it; either at our end, the peripheral end, shielded, unshielded... very similar to the issues that you're having now...I just don't remember the differences being so pronounced as what you're seeing. The other problem with Ethernet is that it's a high frequency signal, so the ports are usually poorly filtered (for EMC anyway), which means that: if somebody didn't take care of business with other clocks in the system, they can easily catch a ride out on the Ethernet cable. That's why I was wondering if the 125Mhz measurement is only coincidentally related to an Ethernet frequency. Hope this helps and good luck. Chris Maxwell Electrical Engineer Critical Imaging LLC - 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/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: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
