Hi Jose,
I agree on the de-embedding - there are multiple applications and the
material model identification is just one of them.
The material characterization goal in context of SI is to have a broadband
material models that produce acceptable correlation with measurements within
reasonable variation of cross-sections and lengths. The end result is
material models that we can use in a range of EDA tools and expect good
correlation with the measurements at the end. So far, the laminate
manufacturers and corresponding committees simply ignore it and there are
multiple explanations. There is no common set of models in all major tools.
The models are implemented in algorithms in different ways - it shows up as
bad correlation between the tools (especially in the losses). The numbers
published by the manufacturers are usually accurate for un-clad laminate and
out of plane dielectric constant. In most cases those are reliable data
obtained with standardized resonator techniques. By definition, it should
produce good correlation in phase or group delay for wider strips for low
loss dielectrics. This can be used as a validation test for a field solver.
Correlation in losses is totally different story - there are no models from
manufacturers at all.
I like your idea to have a discussion of different material characterization
techniques at DesignCon. It would be great to have a test board with a set
of test structures, such as single-ended and differential t-line segments,
Beatty resonators. We get S-parameters measured for test fixtures and
cross-section data (not just stackup from PCB manufacturer) and come up with
the dielectric and conductor roughness models. We will compare the models at
DesignCon - that should be the fun part. I am in. Would you like to organize
that?
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
2629 Townsgate Rd., Suite #235, Westlake Village, CA 91361, USA
Office +1-702-876-2882; Fax +1-702-482-7903
Cell +1-206-409-2368; Virtual +1-408-627-7706
Skype: shlepnev
www.simberian.com
Simbeor - Accurate, Productive and Cost-Effective Electromagnetic Signal
Integrity Software
2010 and 2011 DesignVision Award Winner, 2015 Best In Design&Test Finalist
-----Original Message-----
From: Moreira, Jose [mailto:jose.moreira@xxxxxxxxxxxxx] ;
Sent: Wednesday, May 31, 2017 7:44 AM
To: shlepnev@xxxxxxxxxxxxx; johnlinc@xxxxxxxxx; 'si-list'
Subject: RE: [SI-LIST] Re: Differential vs. SE for PCB Dk/Df extraction
Hi Yuriy
I understand your comment about de-embedding. Most commercial tools have it
as a black box with confidential IP that makes it hard to gain "trust". This
is a topic being addressed by the IEEE P370 standard. In the context of that
effort at the SIPI 2017 conference, Heidi, Eric and I have a paper where we
show a NIST traceable way to evaluate the accuracy of a de-embedding
algorithm using real PCBs and not simulation data. Apart from this
discussion, de-embedding is really critical for test and measurement and we
need to improve our confidence on it with real metrics which the P370
standard wants to do.
I also understand the comment on the need to use the target structure (e.g.
a tightly coupled differential trace) for the model identification instead
of just a single-ended structure. We did look into differential Beatty
standards but not in a lot of depth. What is important is to understand the
improvement in accuracy. If I use spec sheet values my correlation to
measurement is very bad which we all agree. If I use a single-ended
structure like the Beatty standard how inaccurate would I be compared with
using the target tightly coupled structure. Of course this answer will not
be generic and depends on the details and also how much accuracy I need for
my task.
As you pointed out, there are multiple techniques and I also think the best
technique to use depends a lot on what one wants to achieve and the
conditions/tools/time available. As usual.
What I think we can all agree is that no EM simulation model or tool is
perfect (especially if we discuss plated microstrips which some of us in
test and measurement use a lot), we cannot assume that dielectric spec sheet
values are the values that one needs to use in our simulation models to get
results that correlate with reality.
I think this would be a good follow-up topic for DesignCon 2018. I believe
each approach has advantages and drawbacks that define in which situations
they are the right ones to be used. So maybe a tutorial where an expert in
each "technique" presents and provides a fair judgement on its advantages
and drawbacks and the kind of situations the technique is best suited would
be helpful for the community.
Jose
-----Original Message-----
From: Yuriy Shlepnev [mailto:shlepnev@xxxxxxxxxxxxx]
Sent: Dienstag, 23. Mai 2017 22:04
To: Moreira, Jose <jose.moreira@xxxxxxxxxxxxx>; johnlinc@xxxxxxxxx;
'si-list' <si-list@xxxxxxxxxxxxx>
Subject: RE: [SI-LIST] Re: Differential vs. SE for PCB Dk/Df extraction
Hi Jose,
Technically, any structure on PCB with measurable parameters can be used to
"tune" the material parameters if we can build a model of the structure. I
agree with your statement that "A simple trace is not a good structure".
However, use of Beatty standard with de-embedding may be not the best
approach either. First of all because of the de-embedding (as I can see in
your paper) - that is difficult and error-prone for PCB materials with
inhomogeneous dielectrics and large manufacturing variations. Also, the
number of parameters to match simulations with measurements is also
excessive - reflection and transmission parameters of the de-embedded
structure should be matched simultaneously. This problem is common for all
material identification techniques based on de-embedding - too complicated
and error-prone for PCBs. So, what is better than a simple trace? - it is
two simple traces :-) S-parameters of two trace segments can be used to
extract either reflection-less generalized modal S-parameters - the simplest
form of S-parameters of a t-line segment for any line with any number of
traces (not an approximation). It is just one complex transmission for
single ended case and two transmissions for differential case (reflections
and mode transformations are zero by definition, not by approximation). It
is easy to fit same reflection-less model for a t-line segment, to find the
material properties - the technique is in practical use since 2009 - see
#2010_01 and all papers after that at http://www.simberian.com/AppNotes.php.
The logarithm of the generalized modal transmission parameter divided by the
length difference is the modal complex propagation constant or Gamma - that
can be also used for the material identification in the same way (sometime
called eigenvalue technique). Though, the techniques with Gamma, such as SPP
light with S-parameters (see #2016_02 at
http://www.simberian.com/AppNotes.php), requires one additional step -
taking the logarithms. Usually it is easy step and produces the same result
as the technique with GMS-parameters. And, as Scott mentioned, both GMS or
Gamma techniques allow easy dielectric and conductor loss separation.
Considering the single-ended vs. differential, as Gert already mentioned,
the identification results can be very different. This is because of the
layered type of inhomogeneity of the PCB materials. We always identify some
effective permittivity averaged by the applied electric field. Techniques
based on a wide strip line resonator have preliminary out of plane component
of electric field - the identified value of Dk are good for structures with
primarily out of plane electric field. The other extreme is techniques with
E parallel to dielectric surface - they identify in plane value of Dk. It is
consequence of the layered structure. A strip line with regular width (close
to target impedance) has both out of plane and in plane electric fields - Dk
identified with it will be between the out or plane (min value) and in plane
(max value). See more on that and references in the "Material World..."
tutorial #2016_01 at http://www.simberian.com/TechnicalPresentations.php The
bottom line is that the end result of the identification should be usable
for the modeling of traces within the actual interconnects width range.
Which value would be better for that? - the answer is obvious, the values
identified with the traces used as the actual interconnects. Values
identified with single-ended can be safely used for loosely coupled
differential, simply because of almost the same structure of the electric
field. However, tightly coupled differential traces have more energy in the
out of plane electric field. The space between the tightly coupled traces
with mostly in plane electric field can be filled mostly with the resin and
property of the resin may be different from the rest of the resin-fiber
mixture. That changes the identified Dk. The effect shows up as difference
in the phase or group delay of the differential and common modes or as NEXT
on single-ended S-parameters. Layered dielectric model should be used in
such cases to improve accuracy for both differential and common mode
modeling. Use of differential traces is essential for such cases - see
examples is at the "Lessons learned..." paper #2014_01 at
http://www.simberian.com/AppNotes.php ;
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
2629 Townsgate Rd., Suite #235, Westlake Village, CA 91361, USA Office
+1-702-876-2882; Fax +1-702-482-7903 Cell +1-206-409-2368; Virtual
+1-408-627-7706
Skype: shlepnev
www.simberian.com
Simbeor - Accurate, Productive and Cost-Effective Electromagnetic Signal
Integrity Software
2010 and 2011 DesignVision Award Winner, 2015 Best In Design&Test Finalist
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Moreira, Jose
Sent: Tuesday, May 23, 2017 1:36 AM
To: johnlinc@xxxxxxxxx; si-list
Subject: [SI-LIST] Re: Differential vs. SE for PCB Dk/Df extraction
In my opinion differential or single-ended makes no difference for Dk/Df
extraction. Important is the kind of structure you use. A simple trace is
not a good structure. I suggest a resonant standard like a Beatty Standard.
Check the Designcon 2018 paper "Non-Destructive Analysis and EM Model Tuning
of PCB Signal Traces using the Beatty Standard"
Jose
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of John Lin
Sent: Dienstag, 23. Mai 2017 05:53
To: si-list <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Differential vs. SE for PCB Dk/Df extraction
Hi SI gurus,
While using VNA to measurement traces on PCB for loss and Dk/Df extraction,
my colleagues say the differential handhold probe head can have wider
bandwidth than single ended handhold probe head. Is it true?
Can someone help to explain the reasons?
Also, for Dk/Df extraction, which is better structure, using single ended
or differential traces? Why?
Thank you for helps in advance.
Thanks,
John Lin
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