*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= =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 =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= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20 =20 > -----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 > things you can do, e.g., anti-pad size, adding ground shield=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 > = =3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D= 3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3 > D=3D3D=3D3D=3D3D=3D3D=3D > = =3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D= 20 >=20 > Perry Qu=3D20 >=20 > Design & Qualification, Alcatel-Lucent Canada Inc. >=20 > 600 March Road, Ottawa ON, K2K 2E6, Canada=3D20 >=20 > DID: 613-7846720 Fax: 613-5993642=3D20 >=20 > Email: perry.qu@xxxxxxxxxxxxxxxxxx=3D20 >=20 > = =3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D= 3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3 > D=3D3D=3D3D=3D3D=3D3D=3D > = =3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D3D=3D= 20 >=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 > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the=20 > Subject field=20 > >=3D20 or to administer your membership from a web page, go to: > > http://www.freelists.org/webpage/si-list > >=3D20 > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject=20 > field =3D20 =3D20 =20 > >List technical documents are available at: > > http://www.si-list.net =3D20 > > List archives are viewable at: =3D20 > > http://www.freelists.org/archives/si-list > > or at our remote archives: > > http://groups.yahoo.com/group/si-list/messages > > Old (prior to June 6, 2001) list archives are viewable at: > > http://www.qsl.net/wb6tpu > > =3D20 > >=3D20 > >=3D20 > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >=20 > or to administer your membership from a web page, go to: > http://www.freelists.org/webpage/si-list >=20 > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >=20 >=20 > List technical documents are available at: > http://www.si-list.net >=20 > List archives are viewable at: =20 > http://www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > =20 >=20 >=20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: http://www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

**References**:**[SI-LIST] Understanding S-Parameters***From:*Ralph A Wilson III

**[SI-LIST] Re: Understanding S-Parameters***From:*QU Perry

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