Having worked on Ethernet (100 MB UTP) EMC for a few years, I now find myself lacking understanding of Ethernet communications and how this may be affecting radiated emissions.
What is seen on the OATs is broadband noise between 50 and 150 MHz. With two cables connected and linked, broadband is not even out of the noise floor. With the third and subsequent cables linked the broadband increases With all cables attached and not linked there is no broadband. Reducing the resolution shows peaks every 60 kHz. Probing on a linked signal shows peaks every 30 kHz with every other peak (60 kHz) being 20 dB higher. Probed several other types of Ethernet equipment, NICs and switches and this seem consistent across all products and manufactures. I feel the filters and layout is quite good, in that, with cables attached but no link there is no emission. In probing on the board it self the 30 kHz signature is present only on the transmit and receive all other signals and voltages appear clean. Is this the nature of Ethernet 100 MB over Unshielded Twisted Pair? Could this be the fault of the UTP cables? If so any suggestions on the type of UTP cable to purchase? Thanks in advance for your time and responses. Rick Linford And thanks to a marketing department for this opportunity to learn, by believing 6 ports of 100 MB is need in the home.
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