[SI-LIST] Via Placement between two Differential-Pair Signals


Hi, All SI Experts:

For the stripline layout, it comes to my concern for
the placement of a via between two differential-pair
signals routed through the via as the follows:

    Y-axis
     |
     |
     0-----> X-axis
    /
   /
  Z                  Signal on Layer#3
                       /
                      /
                     /
           /    /  (@)         /    /
         P1   N1    |         P2   N2
         /    /     |        /    /
       (O)  (O)     |      (O)  (O)  <----- Upper Layer #5
        |    |      |       |    |
        |    |      |       |    |
        |    |      |       |    |
       (O)  (O)     |      (O)  (O)  <----- Lower Layer #7
       /    /       |      /    /
      P1   N1      (@)    P2   N2
     /    /        /     /    /
    /    /        /     /    /
   /    /      Layer   /    /
  /    /        #9    /    /
/    /              /    /
   S          D        S

where

   Traces P and N: a pair of differential signal
                   routed through the via (O)-(O)

   S: spacing between the intra-pair (P1-N1)

   D: spacing between the inter-pair (N1-P2)

   (@)-(@): the via to be placed through
                the Layer #3 to the Layer #9
                carrying a different power signal

That is, a power signal through the via (@)-(@) from
the Layer #3 to Layer #9; nonetheless, there exist two
differential-pair signals routed on Layers #5 and #7.

In the Z-X plane on the upper layer, one would have
the top view:

    0----> X
    |
    |
    Z

          |   |           |   |
          P1  N1          P2  N2
          |   |           |   |
          |   |    (@)    |   |
          |   |           |   |
         (O) (x)         (O) (O)

To minimize the electromagnetic coupling effects,
what would be the required spacing between the via
(x) and via (@)?  Geometrically, the radius R can
be referred to the distance measured from (x) to
(@), where the via to be placed. What would be the
recommended R for the confident signal integrity?

Appreciate so much for all your comments,

SI_Tek





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