[SI-LIST] Re: si-list Digest V8 #92
- From: "Dmitriev-Zdorov, Vladimir" <vladimir_dmitriev-zdorov@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 21 Mar 2008 10:09:31 -0600
Hi Neoflash,
If the purpose is to preserve the appearance of the X11/X22/X12/X21
parameters compared to your previous 2-port case, I would recommend
Z-parameters, not Y. However, the fit quality for Y or Z-parameters is
generally worse than for properly normalized S-parameters simply because
the former are un-bounded. The core of any fitting method is an SVD-like
procedure that minimizes the norm of the difference between the fitted
and target dependences. Hence, this procedure considers the pieces with
high absolute values as "more important". But, this does not agree with
our needs. In most practical cases we shouldn't care if the impedance at
certain point is 1e5 or 1e6 Ohm because this will not affect the
solution (i.e. power consumption is very low). Same way, with
Y-parameters, we don't care if the conductance between certain nodes is
1e6 or 1e7 because there will be no perceptible voltage drop and again,
in the generally non-singular structure, no considerable power
consumption. S-parameters are more suitable in this sense because they
show best the range where the impedance is comparable to the normalizing
value. But there comes the problem of properly choosing these
normalizing values.
Although this could be not exactly your case, but with improperly chosen
normalized port impedance the S-parameter curves may either be barely
deviate from zero, or have 'flat' regions where their modules reach 1 -
the maximum value - hence their real/imaginary parts be close to
real(s)/imag(s) =3D=3D1 or -1.
Vladimir
Msg: #1 in digest
Date: Thu, 20 Mar 2008 06:05:23 -0700 (PDT)
From: Neo <neoflash2008@xxxxxxxxx>
Subject: [SI-LIST] Re: FW: Curve fitting of center-tapped inductor
Vladimir:
=20
Thanks for your reply.=20
=20
If the issue comes from the improper normalized impedance, can I fit Y
parameters instead of S parameters to fix this problem?
=20
Also, what's the meaning of "deviate only slightly or stick flat to
the lines S=3D3D1 or S=3D3D-1."? What are these two lines? Could you =
explain
a little more?
=20
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=3D3D1 or S=3D3D-1. If you see the =
=3D
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=20
>Subject: [SI-LIST] Curve fitting of center-tapped inductor
>Hi, folks:=3D20
=3D20
>I come up with a strange thing when doing curve fitting for a
differential >center-tapped inductor.=3D20
=3D20
>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.=3D20
=3D20
>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.=3D20
=3D20
>Any one ever experience same thing before? Any suggestions?=3D20
=3D20
>Regards,=3D20
>Neoflash=3D20
=3D20
> I've attached the link where the equivalent circuit model could be
seen.
> http://www.designers-guide.org/Forum/YaBB.pl?num=3D3D1205764700
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