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I have task that I need to make adapter board which transforms gigabit ethernet connector RJ45 to another connector (not RJ45). I also have power connector to carry power one side to other.

I need to route 100ohm differential pair without GNd I can't find any PCB calculator for parallel routing on one layer. I found this article though.

And there is calculator in Saturn PCb that calculates differential pair which I think what I need. It shows that pairs must be adjecent layers between PCB.

enter image description here

What is the best practice here ?

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  • \$\begingroup\$ Differential or any impedance match routing needs to have a reference layer in order to work like impedance match \$\endgroup\$
    – Abhishek Parikh
    Jul 2, 2018 at 12:23
  • \$\begingroup\$ So CAT6 doesn't have impedance matching. Do I only need to match lengths? \$\endgroup\$
    – erkan
    Jul 2, 2018 at 13:01
  • \$\begingroup\$ @AbhishekParikh that's incorrect. Erkan, of course your network cable has an impedance. \$\endgroup\$
    – Marcus Müller
    Jul 2, 2018 at 13:17
  • \$\begingroup\$ @erkan is your copper layer buried or on top? \$\endgroup\$
    – Marcus Müller
    Jul 2, 2018 at 13:20
  • \$\begingroup\$ I haven't routed PCB. That is what I am asking. I want to match the impedance but I am not sure how to do it. I can use 4 layer PCB if it is needed. \$\endgroup\$
    – erkan
    Jul 2, 2018 at 13:48

2 Answers 2

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I need to route 100ohm differential pair [...] for parallel routing on one layer. I can't find any PCB calculator.

That is because a one- or two-layer board without prepregs leads to ridiculous track width. 70mil and wider with 10mil spacing. (I had once seen an USB hub with such a stackup. It worked for a while, then it began destroying random devices, obviously because of other design errors.)

You need a ground plane right below the impedance-controlled pair. Rule of thumb: 0.35mm prepreg, 8mil track width, 8mil spacing → 100Ω impedance.

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The pair in PCB is not twisted. Arrange a plane connected to RJ45 shield. You will get both calculated (and controlled) impedance and noise immunity to some grade. If you do stripline- even better.

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  • \$\begingroup\$ Twist isn't needed for controlled impedance. It's benefit is reducing magnetically coupled interference. \$\endgroup\$
    – The Photon
    Jul 2, 2018 at 14:53
  • \$\begingroup\$ Of course! Not just magnetically, electrically (capacitively) as well. But this is the second point why to use a plane, even if it is not GND \$\endgroup\$
    – user76844
    Jul 2, 2018 at 14:58
  • \$\begingroup\$ So you are suggesting I should use shielding like ground. And draw it like standard 100 ohm differential impedance. I don't think I can get shielding from connector. I am basically imitate cable impedance somehow. \$\endgroup\$
    – erkan
    Jul 3, 2018 at 7:32
  • \$\begingroup\$ This is the way. In fact even a floating plane would work. But it's better to connect it to some \$\endgroup\$
    – user76844
    Jul 3, 2018 at 7:36

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