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Using two sets of Ethernet magnetics

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mho_logos

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Hello All,

I have a design where an embedded computer is put inside a case. The CPU board has an Ethernet port with integrated magnetics.

I need to run a length of cable (CAT-5) from the CPU board to an interface board that connects to the outside. In order to avoid EMI problems with the cable picking up noise inside the enclosure and bringing it outside the enclosure, I would like to add some filtering on the interface board.

I have searched for a common mode filter suitable for ethernet (10/100Mbps), but have not come up with much. In fact the only possibility I found was Stewart Connector SI-60063-F (Digi-key part number 380-1115-ND), but this part is rather expensive, does not have LEDs (which is desired in this application), and I'm not even sure if it will do what I expect it to (i.e. if the common mode filter is appropriate).

Ethernet magnetics contain a common mode filter, as well as the isolation transformer. Would it be possible to use a standard Ethernet connector with magnetics on my interface board? This would give me the common mode choke right at the port, for the most effective filtering. This also gives galvanic isolation, which is already present on the CPU board. Will a second transformer cause problems with signal degradation? Should I do anything special with the center taps on the inside of the magnetics?

What I am thinking is this simple chain:

[CPU Board ETH] ---- (CAT5) ---- [plastic RJ45] ---- (PCB traces) ---- [RJ45 with magnetics and LEDs]

In other words I would put a CAT5 cable from the CPU board Ethernet port to a simple RJ45 connector (no magnetics) on the inside of the interface board. Then short PCB traces to a standard RJ45 with magnetics and LEDs (the LEDs will be controlled by a driver on the interface board).

Any suggestions will be greatly appreciated.

/Mikkel Holm Olsen
 

mho_logos

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I have made a simple test with a connector with magnetics (Tyco 6605759-1), soldered directly to a length of CAT-5 cable with an RJ45 at the other end. This seems to work fine.

At this point I have left the center tabs of the isolation transformers floating, but I suppose these should be terminated with 75 ohms and capacitively coupled to ground (as it is also done on the LAN-side of the magnetics). Can someone confirm this?
 

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