[SI-LIST] Re: S parameter translation to different characteristics impedance
- From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
- To: <piyush.bhatt@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 12 Apr 2010 09:55:42 -0700
Piyush,
In case if all ports of a multiport are normalized to the same impedance,
the re-normalization you are trying to do is particularly simple. One
possible way is outlined here:
1) Convert S-matrix into a normalized Z-matrix with the Cayley transform (or
use formulas for 2-port structure):
Z1n=(U+S)*(U-S)^-1, where U is the unit matrix.
2) Multiply all elements of Zn by ratio of the old normalization impedance
to new one Z1o/Z2o, getting the re-normalized Z-matrix Z2n.
3) Convert the re-normalized impedance matrix Z2n into new S-matrix S2:
S2=(Z2n-U)*(U+Z2n)^-1
Do 1-3 for every frequency in your sweep.
The re-normalization is a little more complicated in case of different
normalization impedances for different ports or in case of complex
normalization. All kinds of conversion and renormalization operations can be
done with our free software Simbeor L0 - it can be downloaded from
www.simberian.com/Downloads.php (requires registration only).
As Istvan noted, measured and re-normalized S-parameters may not have
expected zero reflection parameters due to frequency-dependency of the line
characteristic impedance and due to measurement noise. Transitions from
coaxial cable to your t-line may be also reflective and may require
de-embedding to get the parameters of pure line segment.
Best regards,
Yuriy
www.simberian.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of piyush.bhatt@xxxxxxxxx
Sent: Sunday, April 11, 2010 9:07 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] S parameter translation to different characteristics
impedance
For getting S parameter from VNA we should set impedance of VNA to nearly
equal to transmission line. But I already have data with wrong input
impedance. I want to change that to a different impedance value. "High-speed
Digital system Design, Hand book of interconnects theory and design practice
by Stephen hall, Garrett hall and James McCall" gives a method in appendix
D.2. To convert S parameter to a impedance matrix and then subtract this
value from each number. We should also verify that S11,S22 should nearly
equal to zero.
But how accurate will be this method when done in practice as actual
impedance of trace will change with freq and S parameter file will have
value for different freq but which impedance to subtract from that. Is there
any tool with can help me with this?
regards
Piyush Bhatt
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