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I know that Bob Smith termination is recommended for Ethernet differential pairs. Please refer the below image for Bob Smith termination resistor placement. Ethernet Magnetics

As far as I know, Bob Smith terminations are used for EMI reduction. As it is used to reduce the common mode noise by providing a low impedance path for common mode noise.

But I can't get why some of them are referring to this as termination resistors. If I change the value of termination resistor, will there be any change in effects?

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  • \$\begingroup\$ Please don't shout. \$\endgroup\$ – Hearth Jun 21 at 13:39
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Although it is better than nothing, the 75Ω resistors don't really match the common-mode impedance of nearly any cable in use. This value was used in Bob Smith's patent where he was using an unspecified cable measured in an unspecified way that happened to have ~145Ω of common-mode impedance across pairs. This value is commonly used everywhere, even though you can do better in most applications.

A value of 52.3Ω more closely matches CAT5 cable while 66Ω more closely matches CAT6 cable. For most applications an intermediate value between those two would work best. This eetimes article goes through the details, which I believe is the same as this Teltest Electronics Laboratory publication (PDF).

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  • \$\begingroup\$ interest only. Lonnnnng ago we has an ethernet LAN with coax cable throughout. I had a large box of coax cable lengths purchased at auction. We rapidly found that one wss 50 Ohms and the other 75 Ohms (from memory). The system used (standard approach) short T stubs off the main coax to serve each PC. Replacing even one of those stubs with an incorrect impedance length of coax was enough to bring the whole system down :-). \$\endgroup\$ – Russell McMahon Jun 22 at 8:21
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The 75 ohm resistors approximately match the common mode impedance of the Ethernet cabling. This reduces reflections of common mode noise. So it is a common mode termination. There is no reason to change the value of the resistors unless it is determined the Ethernet cable has a common mode impedance other than 75 ohms.

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