[SI-LIST] Re: Understanding S-Parameters
- From: "ryansatrom" <ryan.satrom@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 02 May 2007 13:45:59 -0000
Ralph-
1) On both the input and the output (microstrip and stripline),
almost all of the return path will be on the ground planes.
Therefore, my opinion is that you need to use a four-port model.
I don't have MWS (I use HFSS), but in HFSS there are two ways to
setup a differential port - what they call Driven Terminal and Driven
Modal. In Driven Terminal, you specify each signal and its reference
to ground. In Driven Model, you specify the modes of the signal
(Mode1 - Signal to ground; Mode2 - Signal to Signal). I suspect that
MWS has a similar operation.
2) If you are using differential ports, each port (two inputs)
should be 100-ohms, therefore each signal should be specified as 50-
ohms (again, not sure how MWS does this)
3) S11 is not always positive. S11 should be a complex number, and
both parts must be used to determine Impedance.
S11 is the best way to determine best performance through your
system. You can either look at minimizing return loss (frequency
domain) or look at the TDR in the time domain.
I hope this helps.
--- In si-list@xxxxxxxxxxxxxxx, Ralph A Wilson III <wilson@...> wrote:
>
> 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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