[SI-LIST] Re: S-parameter Bisection
- From: "Alfred P. Neves" <al@xxxxxxxxxxxxx>
- To: "'Luciano Boglione'" <l.boglione@xxxxxxxx>, "'Grossman, Brett'" <brett.grossman@xxxxxxxxx>, "'Moeller, Merrick'" <mmoeller@xxxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 11 Nov 2009 07:52:19 -0800
Guys,
I would first create a specification and requirement list detailing THRU
(transmission sans DUT) accuracy, insertion and return loss numbers versus
bandwidth, etc., your calibration has to be significantly better than
your DUT, but not a LOT better to make most practical measurements. If a
simple THRU on your test board has -15dB return loss (using simple SOLT cal)
at 5GHz and your DUT has similar numbers your in trouble no matter what you
do, simply put your fixture may need a redesign. We have found poor
fixture design, and inconsistency wreaks havoc on ALL the de-embedding and
calibration approaches (TRL/LRM, T-matrix, using TDNA measure-modeling
methods, etc.,).
So as to not fall into a rat hole and succumb to matrix and calibration
madness we typically specify our calibration objectives first, based on our
overall measurement objectives (again, related to the DUT performance), then
select test board materials (FR4 versus low loss dielectrics), calibration
approach, calibration kit (for TRL), launch (simple SMA versus 2.92mm).
This approach becomes much more important when there is a team of folks
addressing the problem. By establishing a specification and having the
team (and customer) first buy into it you make life easier for yourself
also.
Alfred P. Neves <*)))))><{
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Luciano Boglione
Sent: Tuesday, November 10, 2009 5:06 PM
To: 'Grossman, Brett'; 'Moeller, Merrick'; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: S-parameter Bisection
Hi,
Just a few comments:
- mathematically, any transmission type of matrix could do (ABCD or
transmission S par) as long as the single matrices MA and MB (MA preceding
MB) can be cascaded into M = MA*MB
- MA and MB may not represent the same physical object (e.g. given a TX line
L unit of lengths long, MA is L/3 and MB is 2*L/3), hence the major
indetermination in the phases of the 12 and 21 terms (what counts is the
product 12*21)
- if MA and MB correspond to the same physical object (e.g. each tx line is
L/2), then the square root approach may make sense, assuming symmetricity of
the physical object (e.g. if the M object is SMA connector+tx line L+ SMA
connector and it is to be split into MA=SMA+L/2 tx line and MB=L/2 tx
line+SMA connector, MA and MB are reciprocal but not symmetrical)
- the best approach in my opinion would be to make a second tier TRL
calibration of your fixture if possible (I used this approach to
characterize two Cascade GSG probes at Q band and worked very well);
otherwise, the mathematical deembedding may be possible, but pay a great
deal of attention to the physical implication of your calculations (and of
the assumptions that may be made to get to the final result)
- even if obvious, once MA and MB are determined, you must get M=MA*MB back:
MA and MB may still be "wrong", but if M<>MA*MB then something is certainly
wrong
- Agilent ADS, AWR Microwave Office and most likely other programs have S
parameter blocks that provide the deembedding feature (based on cascading
blocks with matrix algebra) once MA and MB are known
Looking forward to any comments.
Luciano
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Grossman, Brett
Sent: Tuesday, November 10, 2009 6:08 PM
To: Moeller, Merrick; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: S-parameter Bisection
Merrick,
I've run into this situation bunches of times, and I will say that your
success may depend on how many assumptions you are able to make.
Consider that if the data you have is a 2-port s-parameter matrix, you have
essentially 4 knowns (S11, S12, S21, S22).
If you wish to split it into two 2-port matrices (a & b), with no
assumptions, this is 8 unknowns (S11a, S12a, ..., S11b, S12b,...).
So with no assumptions, you can see the problem will be difficult to solve
(8 unknowns vs. 4 knowns). However, it may not be unrealistic to say that
the two s-parameter matrices are equivalent (i.e. you split your passive
device in half and each half is the same). In which case one matrix becomes
the transpose of the other, such that:
S11a = S22b
S12a = S21b
...
By using these assumptions and simplifying the problem in this way, it is
possible you could arrive at a solution in theory. In practice I've found a
few of the assumptions I'd need to make, I simply can't accept.
Also keep in mind that strictly speaking you can't cascade s-parameters, and
would need to convert them into t-parameters to solve the problem (and
likely back to s-parameters for your application). In the case you are
dealing with > 2-ports, the translation to t-parameters can be ambiguous.
Check out the following:
J. Frei, X.-D. Cai, and S. Muller, Multiport S-parameter and T-parameter
Conversion with Symmetry Extension, IEEE Trans. On Microwave Theory and
Tech., Nov. 2008, pp2493
for a reference.
There is also a reference (though I do not have a citation), which address
this problem with a software tool that I believe you can purchase. You may
look up a company called Ultimetrix or google the name Vahe Adamian for the
approach they have developed for splitting a fixture into 2-halves.
Best regards,
-Brett
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Moeller, Merrick
Sent: Tuesday, November 10, 2009 10:04 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] S-parameter Bisection
Is it possible to split a passive S-parameter matrix into two equal
sections?
For example; given s-parameters for a transmission line of a 10" length
can the matrix be split into two 5" lengths. Assume that the passive
s-parameter obeys reciprocity.
The reason I'm asking is that I have a test fixture on each end of my
measurement that I'm trying to de-embed. The insertion loss can be
subtracted very easily, but the return loss and other parameters have a
great deal of error dealing with de-embedding from one side of the DUT
only. I'm hoping to break the fixture into two equal parts without
having a direct point of measurement to do so otherwise.
Merrick M. Moeller
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