I have a main board with a 10/100 MAC and PHY. This board also includes a discrete magnetics. The board is connected via a suitable high speed board-to-board connector to another board, that contains the RJ-45 port. This port also contains magnetics.

Is there a problem with the signal being "magnetized" twice? The RJ-45 port on the second board is preferred to be with magnetics, because it will be used with other projects that won't necessarily have a discrete magnetic element.

Thanks, Gabriel


1 Answer 1


There is no "problem" because that is how EVERY 10BASE-T and 100BASE-T Ethernet connection operates. And you could make the argument that operating over such a short distance will make operation quality higher than average.

An advantage of using "conventional" magnetics is that the modules at either end of the connection can be tested or exercised with conventional Ethernet equipment.

  • \$\begingroup\$ Thank you for your fast reply. I wasn't aware that the usual magnetics is doubled, good to know. \$\endgroup\$
    – Gabriel Mi
    Nov 13, 2016 at 15:47
  • \$\begingroup\$ The CM range of rejection has limited BW so two are used iin series \$\endgroup\$ Nov 13, 2016 at 16:02
  • 1
    \$\begingroup\$ The way I read the question is that there will ultimately be 3 pulse transformers in the system; one on the main board (mentioned by the OP), one in the RJ-45 connector on another board (mentioned by the OP), and then another in the remote ethernet device as usual (not mentioned by the OP). Would that change your answer? \$\endgroup\$
    – marcelm
    Nov 13, 2016 at 16:24
  • \$\begingroup\$ I could be wrong in my interpretation of the question though, but as currently stated, it's a valid interpretation. Really underlines the importance of diagrams in questions... \$\endgroup\$
    – marcelm
    Nov 13, 2016 at 16:25
  • \$\begingroup\$ Hi marcelm, Yes you are right in your interpretation . \$\endgroup\$
    – Gabriel Mi
    Nov 14, 2016 at 7:03

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