FreeLists / si-list / [SI-LIST] Understanding S-Parameters

[Ralph A Wilson III <wilson@xxxxxxxxxxxxxxxxxx>]

The question was asked, "which of these differential
vias is better?"  Ignoring all the philosophical issues
of the meaning "better", I put together a simple 3D-model
of each of the vias in Microwave Studio and took a
look at S11 and S21 (among other things).  I have
several questions regarding the setup of the simulation
and the interpretation of the results:

1) Is a differential via (pair) a two-port or four-port
   model?  The model I put together has two vias, with
   pads/connections (a "P" and an "N" pair) on the top layer
   and on an inner layer.  The model includes a number of
   reference planes tied together with additional "ground
   vias".  In modeling this, do I attach a single port
   between the "P" and "N" leads at the micro-strip, and
   a second port between the "P" and "N" striplines (hence
   a two-port model)?  Or, should I attach a port from
   a reference plane to each "P", and from a reference
   plane to each "N", hence having a four-port model?
   Exciting the two-port model in MWS is straight
   forward - I haven't figured out how to stimulate the
   four port simulation to give me "differential"
   operation - any hints?

2) Assuming I get #1 figured out, do I want to normalize
   the resulting S-parameter to 50 Ohms?  If a 4-port model
   is the right one, I'm assuming normalizing to 50 Ohms is
   the "right thing to do".  However, if the answer to #1 is
   a 2-port model, shouldn't the normalization be done to
   100 Ohms?

3) Finally, regarding S11 - I'm assuming I can use this to
   determine how "transparent" my vias are - that is, how
   close each via is to matching the characteristic
   impedance of the rest of my transmission line
   (normalized to 50 or 100 Ohms).  True or not true?
   Can I use S11 to calculate the "Z0" of the via?
   I'm intuitively drawn to wanting to equate S11 to the
   reflection coefficient, and therefore use that to calculate
   the Z0 of the via (given that S11 is normalized to 50 or
   100 Ohms).  However, since S11 is always positive,
   this implies I could never have a via whose impedance is
   less than that of the transmission line - and I don't
   believe that.  Or, do I need to look at the phase information
   as well as S11 to determine positive vs. negative reflection?

Thanks in advance for the help!
Ralph Wilson
Alcatel-Lucent

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