## [SI-LIST] Re: Understanding S-Parameters

• From: "QU Perry" <Perry.Qu@xxxxxxxxxxxxxxxxxx>
• To: "WILSON III Ralph" <wilson@xxxxxxxxxxxxxxxxxx>
• Date: Thu, 3 May 2007 09:04:05 -0500

```Ralph:

I stand corrected about the comments below why a uniform TL needs to be
inserted between waveguide port and discontinuity:  Higher order modes
might be generated at the discontinuity when you excite the line with a
TEM/Quasi TEM mode. You don't want the higher order mode to be reflected
back to the port location to affect S parameter calculation for the main
transmission mode. Usually the higher order mode has very large
attenuation constant and they will be attenuated to insignificant amount
after a certain length of TL. Do a solve port only will show you the
attenuation constant (Alpha) for higher order modes and you can estimate
how much energy is left after a certain length e^(-alpha*length).

Regards

Perry

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
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Perry Qu=20

Design & Qualification, Alcatel-Lucent Canada Inc.

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

Email: perry.qu@xxxxxxxxxxxxxxxxxx=20

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> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx=20
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of QU Perry
> Sent: Wednesday, May 02, 2007 9:39 AM
> To: WILSON III Ralph
> Cc: si-list
> Subject: [SI-LIST] Re: Understanding S-Parameters
>=20
> Ralph:
>=20
> There are two ways to setup ports in CST or many other 3D=20
> full wave tool, one is waveguide port and the other discrete=20
> port. Waveguide port is used to excite TEM or quasi-TEM=20
> transmission with fields excitation on a surface area=20
> perpendicular to the direction of wave propagation.
> Usually a certain length of TL lines is  necessary between=20
> port location and discontinuity (e.g. via) that you want to=20
> characterize so that all higher order modes at excitation are=20
> attenuated before reaching the structure. If you have a=20
> natural transition between one uniform TL and another TL with=20
> a via, I think waveguide port is most appropriate. When=20
> setting up waveguide port, you can define the modes of=20
> excitation and mixed mode or  single-ended S parameter can be=20
> calculated.
>=20
> Another type of port is discrete port and they are used very=20
> often when you don't have room to put a leading TL to setup=20
> waveguide port, e.g., these are used very often in IC=20
> packaging characterization. In general, the ports are defined=20
> between two closely spaced conductors to act as an=20
> excitation. As you mentioned, you can either excite the=20
> structure single-ended by define a port between P/N and=20
> ground, or you can excite between P and N differentially. For=20
> the first case, you end up with a 4 port single ended S=20
> parameter set, which  can be used to extract mixed mode S=20
> parameter if needed. For the second case, you end up with=20
> differential S12/S11 only. I always use single-ended discrete=20
> port and get 4 port model as these model can be used to=20
> extract differential S parameter and also can be used=20
> directly in spice simulation for time domain waveform. Also,=20
> 50 ohm port impedance is usually used when you define the=20
> single-ended discrete port and 100 ohm for the differential port.=3D20
>=20
> About interpretation of S11, I'm not sure if you need to=20
> calculate Z0 of via. The ultimate goal is to minimize return=20
> loss for the frequency of interest and there are only certain=20
> vias and minimize stubs etc. You can setup a few parameter=20
> and do some sweep simulation/optimization.
>=20
> Hope this helps.
>=20
> Regards
>=20
> Perry
>=20
>=20
> =
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>=20
> Perry Qu=3D20
>=20
> Design & Qualification, Alcatel-Lucent Canada Inc.
>=20
>=20
> DID: 613-7846720  Fax: 613-5993642=3D20
>=20
> Email: perry.qu@xxxxxxxxxxxxxxxxxx=3D20
>=20
> =
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> D=3D3D=3D3D=3D3D=3D3D=3D
> =
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>=20
> =3D20
>=20
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx=3D20
> > [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Ralph A=20
> Wilson III
> > Sent: Wednesday, May 02, 2007 9:02 AM
> > To: si-list
> > Subject: [SI-LIST] Understanding S-Parameters =3D20  The question =
was=20
> >asked, "which of these differential vias is=3D20  better?" =20
> Ignoring all=20
> >the philosophical issues of the=3D20  meaning "better", I put=20
> together a=20
> >simple 3D-model of each of=3D20  the vias in Microwave Studio=20
> and took a=20
> >look at S11 and S21=3D20  (among other things).  I have=20
> several questions=20
> >regarding the=3D20  setup of the simulation and the=20
> interpretation of the=20
> >results:
> >=3D20
> > 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?
> >=3D20
> > 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?
> >=3D20
> > 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?
> >=3D20
> > Thanks in advance for the help!
> > Ralph Wilson
> > Alcatel-Lucent
> >=3D20
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