For accurate DK/DF extraction, we should measure differential traces
and match all IL, RL, NEXT, FEXT and TDR/TDT after causal
de-embedding. Because the glass/resin composite makes PCB stripline
structures inhomogeneous, the extracted DK/DF will depend on the
cross-sectional model being used. The extracted DK/DF can be
considered "effective" values and they are self consistent with the
model being used when all IL, RL, NEXT, FEXT and TDR/TDT are matched.
We showed that FEXT and its polarity, among others, can have profound
implication in DK/DF extraction in a DesignCon paper:
http://www.ataitec.com/PDF/Paper_AfullyautomatedSIPlatform.pdf and
http://www.ataitec.com/PDF/MPX.pdf
Regards,
Ching-Chao Huang
www.ataitec.com
On Wed, May 24, 2017 at 8:28 AM, Bert Simonovich
<dmarc-noreply@xxxxxxxxxxxxx> wrote:
John,------------------------------------------------------------------
To add to the discussion, the roughness of the copper used in the
fabrication of the core laminate and etching before final lamination will
affect total phase delay which translates into and effective Dk (Dkeff). So
the Dkeff you extract is only good for the particular geometry measured. It
is not the intrinsic Dk of the dielectric material.
See my DesignCon2017 paper, "A Practical Method to Model Effective
Permittivity and Phase Delay Due to Conductor Surface Roughness".
http://bit.ly/2qWcHPm
Furthermore, since the dielectric is non-homogeneous, the glass style, resin
content, number of dielectric layers used and where the traces are
positioned relative to the glass weave pattern will affect results.
Best regards,
Bert Simonovich
Signal/Power Integrity Practitioner | Backplane Specialist | Founder
LAMSIM Enterprises Inc.
Email:Lsimonovich @lamsimenterprises.com
Web Site: http://lamsimenterprises.com
Blog: http://blog.lamsimenterprises.com/
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Yuriy Shlepnev
Sent: 24-May-17 10:37 AM
To: johnlinc@xxxxxxxxx
Cc: jose.moreira@xxxxxxxxxxxxx; 'si-list'
Subject: [SI-LIST] Re: Differential vs. SE for PCB Dk/Df extraction
Hi John,
Let's take a practical case and analyze possible outcomes - such as FR408HR
in the "Lessons Learned" project - paper #2014_01 at
http://www.simberian.com/AppNotes.php
The dielectric around the strips is mostly resin.
If we use single ended strip and identify one effective model for
dielectric, the result is usable for either single-ended strips or for
loosely coupled differential. Nothing else is needed to cover those cases.
If we use tightly coupled differential traces, the complex propagation
constants for differential and common modes will be different due to the
resin around the strips (result of the spatial "averaging" of E-field). To
identify one dielectric model, we can use either differential or common mode
for the model identification. The model identified with the common model
will be close to the single-ended case, but not accurate for the
differential mode analysis - does not matter what solver is used. The
dielectric model identified with the differential propagation will be
different, but usable only for the analysis of differential modes in the
line with similar geometry. It will give wrong result for the common mode
propagation. Simply put, isotropic one dielectric model will not be
sufficient for such case. Two dielectric models are needed - one for layer
around the strips (resin) and one for the rest of the cross-section
(three-layer model for strips, anisotropic dielectric is an alternative).
Such model will be more accurately for all cases - single-ended, loosely and
tightly coupled differential for both differential and common modes.
Dielectric models for such layered model of the cross-section can be
identified with the two modes in the tightly coupled differential traces.
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 John Lin
Sent: Tuesday, May 23, 2017 9:27 PM
To: shlepnev@xxxxxxxxxxxxx
Cc: jose.moreira@xxxxxxxxxxxxx; si-list
Subject: [SI-LIST] Re: Differential vs. SE for PCB Dk/Df extraction
Thank all to shed light on my questions. I appreciate.
Hi Dr. Yuriy,
Thank you for your insightful explanation.
Based on it, can I conclude that the Dk/Df extraction for Single
Ended shall be close to the that of loosely coupling differential but
different from tightly coupling due to the E field in between the tightly
coupling traces and common mode convention?
If the extracted dk/df are used for simulator to model any structures, ie
SE, tightly/ loosely coupling differential pair ...etc., does Dk/Df
extracted from SE makes more sense and more accurate assuming the tool
automatically calculate the E field in between two tightly coupling
differential traces?
Thanks again for helps.
John Lin
2017Е││5Ф°T24Ф?г 04:04О+-"Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>Е│LИ│?О+
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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