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I want to run a 10 Gigabit Ethernet (GbE) signal over a 15 pin LVDS flex cable like this one: Molex LVDS Flex Cable

My PCB will accept an RJ45 connector, and those traces will run over the PCB to the flex jumper, then through the flex to another PCB.

I want to optimize the layout of the traces through the connector to reduce crosstalk, both between signals and alien. As we only need 8 signals (4 differential pairs) we have plenty of room for grounds. Here are 2 configurations I've come up with as options. My question:

Which configuration is best from a signal integrity perspective?

  1. config 1
  2. config 2
  3. Another configuration I haven't included.
  4. It won't and doesn't matter.
  5. Impossible to say without testing.

Config 1 keeps DPs as close as possible, isolated by GND. Config 2 spaces all traces with GND between them

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I would go with option 1.

The fundamental goal of a differential pair is that any unintended signal which couples on to one wire in the pair also couples on to the other wire in the pair, and since the receiver subtracts the two signals, they cancel. This works better the closer the two wires are coupled.

Another argument is the inductive loop. For single ended signalling, you want every signal running over a ground plane to minimize the inductive loop between a signal and its return current. For a differential signal, the (differential mode) return current flows through the other wire of the pair, rather than through ground - so by adding a ground wire between the two signal wires, you actually increase the size of the loop.

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  • \$\begingroup\$ Thanks a lot Selvek. Marking as answer for now. Going to spin some boards and test myself. \$\endgroup\$ – Jim Aug 3 '18 at 19:04
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This flex cable is designed for 100 ohm differential signals. To maintain that impedance you would want the + / - signals to be adjacent to each other. You could put either one or two grounds between each differential pair. Obviously two grounds will result in less crosstalk between pairs.

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