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Routing Differential pairs

SteveDeeds

I could use a tutorial, or some pointers on routing differential pairs for ethernet using Allegro PCB Designer 17.2

What I have done so far:

Logic -> Assign Differential Pair -> Auto Generate...  (To assign the pairs)

Setup -> Constraints -> Spacing

Objects -> Create -> Spacing C-set (to make a 100 ohm differential C-set)

Set all the values to 6mil because I want the gap between the traces to be 6 mils.

The problem is that when I route the traces, there is no space between them.

Did I miss a step?

  • Hi you have to specify the width of the actual traces too.

    There are a few different ways to create dif pairs. After you created the pairs under the logic menu, open the constraint manager and go to > Electrical > Net > Differential Pair worksheet. You should see the pairs in the spreadsheet,
    they will have a + to the left of their name indicating this particular diff pair has 2 nets.

    Specify the minimum line spacing, Primary Gap & Primary width.

    On the primary width this will really depend on your layer stackup and how far a ground plane is below where you are routing the diff pair. If you specify an impedance value your trace width might never work out ok because it depends on the physical stackup. It is a good idea in advance to verify your stackup can support the diff-pair you are trying to achieve. There are many what if calculators available on-line so you can try out what if ideas. A great tool for such things is the saturn pcb tool kit.

    One other thing when creating dif pairs under the logic menu, auto generate will work fine, but entering a physical name for the diff pair may well be an easier read in the spreadsheet. For example USB_DIFF_PAIR etc..

    All the best.

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