| Electronics > Beginners |
| Grounding ethernet shield |
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| testtube44:
I'm trying to run shielded ethernet cable to my modem downstairs, and the project requires low RF noise. I don't know how to ground the shield, or if grounding of the shield is necessary at all. I think it's going to become an antenna, but grounding it would just transfer the electrical noise to the ground wires in my house. What should I do? |
| T3sl4co1l:
Shield is returned to the RF ground of the device. Galvanic ground is only relevant to safety, and irrelevant for noise purposes (indeed, often worse for noise purposes, considering things like ground loop). If your project requires low noise but a shield will bring noise into it, you're screwed either way, because the Ethernet pair is made of conductive wire, shield or not, and it will act as an antenna regardless. (Mind that Ethernet itself can be a source of noise, since it is digital data, after all.) It would then seem, the project needs to be housed inside a full metal shield, with all shielded cables returned to that first, before penetrating the housing. Tim |
| JS:
The shield should be connected only to one end to prevent ground loops, you usually want the receiving device to have it connected to but not essential and ethernet goes either way so connect the shield to the easier end. If you can choose the better grounded one. The problem with shielded wires is the capacitance from the signal to ground is higher, so longer runs bring new problems. Ethernet is ok for pretty long runs but likely not so ok for such long runs with shielded wires. I never have an issue with ethernet wiring so far, I found myself in the first one not so long ago, I asked around and it was an old wire that was placed in a different place, I made replace the run and all good. Twisted pairs are pretty good rejecting noise, even better with the fully differential and balanced scheme used in ethernet, so the shielding is not as critical as it is in other signals like analog audio where balanced is not so balanced, differential is not so differential and environmental noise inside the band is much higher, like power lines running inside the band of interest. JS |
| testtube44:
I'm trying to keep the digital noise from the ethernet inside the ethernet cable. Is there no way to do this without extensive modification of the cable? I can't let the RF escape into the living space. And what levels of noise do escape, considering what JS said about the ethernet cables being differential? |
| David Hess:
The problem crops up all the time in low frequency instrumentation applications where you want to keep outside noise from getting into the signal. The usual solution is to only ground the shield on the receiving end where connecting both ends would just create a ground loop which would likely make things worse but like T3sl4co1l says, sometimes there is no good solution and you are screwed either way. Ethernet has the great advantage of being galvanically isolated at both ends preventing common mode problems when the grounds are at different levels which can be a real problem over 100 meters or more. While not a complete solution, this suggests tying the shield to chassis ground at both ends through an RF capacitor with a low reactance at the frequencies to be shielded and high reactance at power line frequencies which is easy enough. If that does not work well enough, then there is a sure fire solution which is actually pretty economical; use Ethernet over optical cable like 1000BASE-SX and there will be no emission (and no susceptibility) at all. An Ethernet to optical bridge which is essentially a two port switch can be used at either end with the existing wired Ethernet ports if necessary. |
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