[SI-LIST] Re: Understanding S-Parameters

Ralph:

There are two ways to setup ports in CST or many other 3D full wave
tool, one is waveguide port and the other discrete port. Waveguide port
is used to excite TEM or quasi-TEM transmission with fields excitation
on a surface area perpendicular to the direction of wave propagation.
Usually a certain length of TL lines is  necessary between port location
and discontinuity (e.g. via) that you want to characterize so that all
higher order modes at excitation are attenuated before reaching the
structure. If you have a natural transition between one uniform TL and
another TL with a via, I think waveguide port is most appropriate. When
setting up waveguide port, you can define the modes of excitation and
mixed mode or  single-ended S parameter can be calculated.

Another type of port is discrete port and they are used very often when
you don't have room to put a leading TL to setup waveguide port, e.g.,
these are used very often in IC packaging characterization. In general,
the ports are defined between two closely spaced conductors to act as an
excitation. As you mentioned, you can either excite the structure
single-ended by define a port between P/N and ground, or you can excite
between P and N differentially. For the first case, you end up with a 4
port single ended S parameter set, which  can be used to extract mixed
mode S parameter if needed. For the second case, you end up with
differential S12/S11 only. I always use single-ended discrete port and
get 4 port model as these model can be used to extract differential S
parameter and also can be used directly in spice simulation for time
domain waveform. Also, 50 ohm port impedance is usually used when you
define the single-ended discrete port and 100 ohm for the differential
port.=20

About interpretation of S11, I'm not sure if you need to calculate Z0 of
via. The ultimate goal is to minimize return loss for the frequency of
interest and there are only certain things you can do, e.g., anti-pad
size, adding ground shield vias and minimize stubs etc. You can setup a
few parameter and do some sweep simulation/optimization.

Hope this helps.

Regards

Perry


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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20

Perry Qu=20

Design & Qualification, Alcatel-Lucent Canada Inc.

600 March Road, Ottawa ON, K2K 2E6, Canada=20

DID: 613-7846720  Fax: 613-5993642=20

Email: perry.qu@xxxxxxxxxxxxxxxxxx=20

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=20

> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx=20
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Ralph A Wilson III
> Sent: Wednesday, May 02, 2007 9:02 AM
> To: si-list
> Subject: [SI-LIST] Understanding S-Parameters
>=20
> The question was asked, "which of these differential vias is=20
> better?"  Ignoring all the philosophical issues of the=20
> meaning "better", I put together a simple 3D-model of each of=20
> the vias in Microwave Studio and took a look at S11 and S21=20
> (among other things).  I have several questions regarding the=20
> setup of the simulation and the interpretation of the results:
>=20
> 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?
>=20
> 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?
>=20
> 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?
>=20
> Thanks in advance for the help!
> Ralph Wilson
> Alcatel-Lucent
>=20
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