Vladimir: Thanks for your reply. If the issue comes from the improper normalized impedance, can I fit Y parameters instead of S parameters to fix this problem? Also, what's the meaning of "deviate only slightly or stick flat to the lines S=3D1 or S=3D-1."? What are these two lines? Could you explain a little more? Regards, Neoflash "Dmitriev-Zdorov, Vladimir" <vladimir_dmitriev-zdorov@xxxxxxxxxx> wrote: Hi Neoflash, The cause of your problem can also be an improper choice of the normalizing impedance when creating S-parameters. The "average" impedance of the two ports you measured first is perhaps very different from the port you added later. An interesting thing about S-parameters is that even if you don't change anything about ports 1 and 2 and only add the port 3, the components of the S-matrix that corresponds to your ports 1 and 2 (such as s11, s22 and s12) may change drastically, compared to your case 1. For example, in case 1 the parameters 1 and 2 will be as if nothing is connected to the port 3 (current is identically zero). In case 2, with e.g. normalizing impedance 50 Ohm (or whatever you used for port 3) the S11/S22/S12 will appear exactly as if you terminate the port 3 with 50 Ohm resistor and still measure only the ports 1 and 2. Of course, the result might be quite different, and S12 is what probably contains most of you mutual inductance. The rule of thumb is that the S-parameter dependences - real and imaginary parts - when you observe them with some sort of viewer must use the entire range (between -1 to +1) but should not deviate only slightly or stick flat to the lines S=3D1 or S=3D-1. If you see the = latter, the normalizing impedance must be changed for certain ports before you start fitting. It is possible to re-normalize S-parameters without re-creating them, provided that your frequency points are taken dense enough to cover all important resonances. If you could send it, I can investigate and transform your touchstone data. Vladimir >Msg: #1 in digest >Date: Tue, 18 Mar 2008 07:37:23 -0700 (PDT) >From: H B >Subject: [SI-LIST] Curve fitting of center-tapped inductor >Hi, folks:=20 =20 >I come up with a strange thing when doing curve fitting for a differential >center-tapped inductor.=20 =20 >The original version of the inductor is a two-terminal differential >inductor. I did curve fitting to find the equivalent circuit model. And the >fitting seems good.=20 =20 >The next step I added a center-tap and got a 3-port s-parameter from field >solver. The problem is when I'm doing the curve fitting again. the mutual >inductance in the circuit model was forced to zero by the fitting tool. The >fitting seems OK but not as good as two-port one. The inductance and Q is >ok but the s-parameter related with the third port are not well fitted.=20 =20 >Any one ever experience same thing before? Any suggestions?=20 =20 >Regards,=20 >Neoflash=20 =20 > I've attached the link where the equivalent circuit model could be seen. > http://www.designers-guide.org/Forum/YaBB.pl?num=3D1205764700 ------------------------------------------------------------------ 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: //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: //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 --------------------------------- Looking for last minute shopping deals? Find them fast with Yahoo! Search. ------------------------------------------------------------------ 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: //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: //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