I am working on a project where i will be laying out some traces that will be handling Gigabit Ethernet traffic. I know that putting these traces on the outside layers (Top or Bottom) is a bad idea, with noise and other EMI issues, but what other issues should i be concerned with?

I'm assuming that the trace width will be a factor, the board in question is currently 4 layers, standard 1.6mm thick FR4 with 1oz copper thickness on top/bottom with 0.5oz copper thickness for the internal traces.

There are two pairs of TX+ TX- and RX+ RX- for gigabit, should all four of these pairs be routed on the same internal layer, or should i route them on separate layers, for example TX1+ TX1- and RX1+ RX1- on layer 2 and TX2+ TX2- and RX2+ RX2- on layer 3?

How should i run grounds around these signals, between each signal, around the block of signals, and should i put ground planes above and below these traces?

Also, i probably shouldn't run the traces directly from the MCU to the connector, but I'm not really sure about what sort of inductors or coils i should use. By the way, the connector is about 10cm away from the MCU.

  • 1
    \$\begingroup\$ Trace length will be an issue at that frequency. You'll need to impedence-match each line so you don't get phase differences. Easiest way to do that is to ensure that you keep the same copper width / thickness across the entire trace, and make sure that the traces are of equal length. I'm no expert though, hence why I've posted this as a comment rather than an answer. \$\endgroup\$
    – Polynomial
    Jun 20, 2013 at 20:19

1 Answer 1

  1. Start with the vendor's data sheet. They have lots of details.
  2. Make sure each leg of each pair is the same length.
  3. Since the signals are differential, EMI, etc. tends to effect each of + and - the same, so it isn't as bad as you expect.
  4. Keep in mind gigabit Ethernet uses all 4 pairs bidirectionally, so don't read too much into the RX and TX names.
  5. You need a transformer between the MCU and the connector.
  6. You can put the pairs on the same layer. If so, just keep the pair to pair spacing more that the spacing within the pairs.
  7. Do run over a ground plane.
  8. Don't run over a split in a plane.
  • \$\begingroup\$ The connector i am using has built in transformers, i forgot to mention that. Now that you mention it, the naming convention did seem a bit odd, but i would rather be safe than sorry. I don't need any more coasters. \$\endgroup\$ Jun 20, 2013 at 20:29
  • \$\begingroup\$ Do you think the OP can get away with four layers Brian? Did you mean "4 pairs"? \$\endgroup\$
    – Andy aka
    Jun 20, 2013 at 21:32

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