[SI-LIST] Re: 50 Ohm Via?

The question is, what is your nearest return via ?
Are you surrounding your single ended trace via with ground vias to =
ensure true common mode behavior or the nearest via is actually the =
other signal of the differential pair.
You need to consider the ground via placement also to define your common =
mode via impedance at multi-giga speed.=20
Indepedent on whether you believe you have common or differential mode =
vias, what you really want is not just 50ohm single ended or 100ohm =
differential impedance at 1-5GHz. You really want to know :
1) What is the S11 (reflect)
2) What is the S21 (thru)
3) What is 1-|s11|^2-|s21|^2 (loss to the plane cavities)

Common mode vs. differential mode makes a huge difference in all of the =
above. And I assure you differential mode vias win hands down.


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Joel Brown
Sent: Wednesday, January 09, 2008 8:30 PM
To: wolfgang.maichen@xxxxxxxxxxxx; luant@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: 50 Ohm Via?

To Chris:

I have been reading several places that recommend using loosely coupled
differential pairs, that is why I mentioned 50 ohms. I know there are
religious beliefs about tightly coupled vs loosely coupled pairs. The
material I read regarding loosely coupled pairs mentioned advantages =
such as
wider trace widths for a given impedance and avoiding degradation of =
rise
time caused by coupling between signals within a pair.

Thanks - Joel


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