I bought a couple of "cheap" $2-ish USB Ethernet adapter from China. The come in the usual generic cheap white plastic case.
Here is what's inside.
They cheap out on the usual Ethernet transformer and instead on your luck that the other device it connects to is implemented correctly.
The back side isn't better either. The soldering is sloppy as there is a thread of solder at the connector pins. The layout person obviously doesn't understand the Ethernet 1000V isolation nor how to route differential signals.
The two terminals of those 0603 jumper are supposed to be isolated to1000V RMS in the IEEE specs. I supposed I could have improved somewhat on the isolation by cutting and peeling off the now unconnected traces.
All I can hope for is to at least have some minimal ground isolation - probably good for 50V. There is a lot to be said for Ethernet jack with integrated magnetic as it remove some beginner level incompetence from the equation.
I ordered the cheapest 1:1 Ethernet transformer in SOIC16 (Pulse H1102NL) I could find for $3.17 at QTY 10. Not sure what I am going to do with the other spare ones.
After removing the four 0 ohms resistors, I reflowed the transformer. Here is the PCB looks like.
I accidentally pushed R7 during reflow and the PC had some difficulty recognizing the dongle. Upon resoldering R7, the dongle works fine.
BTW here is what's inside the transformer:
There are the two center tapped 1:1 for Tx and Rx side and their common mode filters (4 toroids below).