[SI-LIST] Re: Looking for good source of information regarding Via tuning
- From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
- To: "'Denomme, Paul S.'" <Paul.Denomme@xxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sun, 6 Jul 2008 19:28:23 -0700
Paul,
I am afraid that there is no one tuning technique that may cover all
possible via-hole structures and in some cases even 3-D full-wave analysis
may not be helpful. Here are some thoughts on that.
First of all, via-holes are not transmission lines, thus there is no unique
definition of the via impedance. One of the definitions may be as follows:
Via-hole impedance is the normalization impedance for S-parameters that
gives minimal return loss over some frequency band. Note that some
structures like differential vias with large cutouts may be closer to a
transmission line impedance definition than other like single-ended vias for
instance. Another exception is a case when via-hole goes through multiple
planes with equal distance between the planes - definition of impedance for
periodic structure may be used in this case.
Second, direct optimization with a full-wave solver may not always lead to a
desirable result if physics of the problem is not taken into consideration.
An approximate or surrogate model capturing the physics of the problem and
limit cases investigation is a good way to start. Note that vias can be
described as low-pass filters in general and the extension of the bandwidth
of such filter changing some elements of geometry can produce transparent
via-holes up to dozens of gigahertz.
Third, some via-hole structures cannot be analyzed locally with a 3-D
full-wave solver at all. Those are cases with no stitching vias between ALL
reference planes of the transmission lines in connected layers. The
stitching vias must be at a distance much smaller than the wavelength in
dielectric at highest frequency of interest. Otherwise the structure is
either non-localizable or conditionally localizable at lower frequencies.
Electromagnetic analysis of such structures shows that S-parameters are
dependent on either boundary conditions or on size of the simulation area.
If so - the via-hole behavior will depend on termination of parallel planes
on the real board and will become essentially unpredictable.
Finally, correlation of the analysis and optimization results with your
measurements may be also challenging, especially in the cases when the
localizability requirements for via-holes are violated. Accurate
de-embedding technique is required both for measurements and for
electromagnetic analysis to correlate the results. Knowing that, our company
is recently partnered with Teraspeed Consulting Group (www.teraspeed.com)
with the goal to build and investigate a typical FR4 board with different
structures commonly used on a low-cost PCB, including different types of
via-holes, at frequencies up to 20 GHz. The board with the measurements and
advanced calibration and de-embedding methodology together with the
simulation results will be available soon from Teraspeed.
Best regards,
Yuriy Shlepnev
Simberian Inc.
www.simberian.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Denomme, Paul S.
Sent: Wednesday, July 02, 2008 10:56 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Looking for good source of information regarding Via
tuning
Hi All,
I have a customer that is looking for assistance in
providing Via tuning. Basically they are looking to minimize the impact
their via's have on their high speed signals. I am familiar with the
various techniques to minimize the impact a via has on the signal
integrity ( ie minimize pad size, maximize anti pad, remove NFP's,
backdrilling, etc) but is their any good source of information on how to
tune a via to a particular impedance? What about software? I'm familiar
with the 3D field solvers, but is their any reasonably priced software
available that will support this type of work? Any assistance on this
issue would be greatly appreciated.
Thank you
Paul Denomme
Viasystems
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