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I have an issue with an Ethernet cable causing a ground loop between my router and a raspberry pi, causing noise on the ground of the DC power of the Raspberry. The router/power supply of the RPi are plugged into different wall sockets.

Since to my knowledge Ethernet itself is decoupled, I assume it is the shielding of the Ethernet cable that causes the problem. How can I avoid this?

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  • \$\begingroup\$ Why do you think the cause is a ground loop? Ground loops are very specific in producing low frequency AC power frequency noise so I'm interested in how you went from "symptoms" to "wiring problem". \$\endgroup\$
    – Andy aka
    Sep 18, 2019 at 13:39
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    \$\begingroup\$ Plugging in the ethernet cable causes significant 50Hz noise to appear on a laser error signal that shares ground with the RPi \$\endgroup\$
    – Whitecold
    Sep 18, 2019 at 14:16
  • \$\begingroup\$ Unplug all the gear from power and see if the Ethernet cable is actually creating a path of continuity between signals beyond the magnetics. It should not be, if it is those seem somehow bypassed. Don't rule out some periodic data/software behavior linked to the existence of the network. If you line trigger a scope can you see that the noise is mains locked? \$\endgroup\$ Sep 18, 2019 at 14:28
  • \$\begingroup\$ Most Ethernet cable is UTP - Unshielded Twisted Pair, containing 8 wires and no cable shield. Are you using STP - Shielded Twisted Pair cable? \$\endgroup\$
    – rdtsc
    Sep 18, 2019 at 14:31
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    \$\begingroup\$ @ChrisStratton I did not linetrigger it, but it is a sinusoidal 50Hz signal appearing on the scope, immediately appearing/disappearing with plugging in the cable \$\endgroup\$
    – Whitecold
    Sep 18, 2019 at 14:36

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Don't use a shielded cable? Cut the shield at the raspberry pi end? Put a 10 ohm resistor in the shield at the raspberry pi end? Make a cat 6 isolation transformer?

Mostly stolen from here

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