FreeLists / si-list / [SI-LIST] FW: Curve fitting of center-tapped inductor

["Dmitriev-Zdorov, Vladimir" <vladimir_dmitriev-zdorov@xxxxxxxxxx>]

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 <neoflash2008@xxxxxxxxx>
>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

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