[SI-LIST] Re: Discontinuities on PCB
- From: Charles Harrington <ch_harrington@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sun, 15 May 2005 02:32:52 -0700 (PDT)
Thanks to everyone who sent me guiding information and advice, both off and
online on this subject. They were all of great help to me.
Charles
Ivan Ndip <ndip@xxxxxxxxxxxxxxxxx> wrote:
Hello Charles,
I have been working on RF/microwave modeling and analysis of
discontinuities in chip packages and boards for quite some time now, so
I think I can share my experience with you.
Most of the contributions made so far on microstrip discontinuities,
especially by Silvester, Benedek, Itoh, Gopinath, Thompson etc. in the
1970s, centered around the calculation of discontinuity capacitance and
inductance using quasi-static methods. So, concentrating only on these
publications won't help you know how far away from the discontinuity the
excited higher-order modes can go. However, Menzel, Mehran, Kompa and
Wolf published quite a lot also in the 70's on the calculation of
frequency-dependent properties of microstrip discontinuities.
I suggest that you first of all go through these publications in order
to fully well understand what happens in a discontinuity. (Dr. Ronald De
Smedt already gave a very good explanation of the phenomenon).
Secondly, you must also note that the number and characteristics of
higher-order modes excited don't depend only on frequency (as you think)
but also on the geometry of the discontinuity under consideration (don't
expect a via and bend to excite same higher-order modes). However, since
these modes are evanescent (this is mostly the case for discontinuities
in chip packages and PCBs used for RF/high-speed applications), they
attenuate rapidly from the discontinuity. (Note: use either Hoffmann's
Handbook on MIC or Microstrip Lines and Slotlines by Gupta et al. to
calculate the cut-off frequency in your case). To define the precise
distance away from the discontinuity that these modes can go, you need a
3D full-wave solver (e.g., HFSS) to compute the field behavior in the
vicinity of the discontinuity. I would suggest you excite only one mode
(certainly the fundamental mode) and from the extracted S-parameters you
can study the mode conversion that occurred. Repeat the experiment for
different lengths of line interconnecting the wave port and the
discontinuity. You'll realize that in the immediate vicinity of the
discontinuity, the magnitude of the considered S-parameter (at a
particular frequency), computed from the fundamental mode changes very
rapidly. This can be attributed to the presence of the fields of
higher-order modes which are still very strong. But with increasing
distance away from the discontinuity, the magnitude of the S-parameter
slowly converges to a particular value, because these fields vanish,
leaving the fundamental mode unperturbed. The point away from the
discontinuity where the magnitude of your S-parameter becomes constant
or changes by less than a self-defined value (e.g., 5% or so) defines
the end or insignificance of the higher-order modes. I call it the end
of the "discontinuity effect".
Certainly you have to take into consideration the S-parameters of the
line used. But from my experience, the variation of the magnitude of
S-parameters in the vicinity of the discontinuity is essentially due to
the excited higher-order modes, especially when you consider S11.
I'll be presenting a paper at the 35th European Microwave Conference
(European Microwave Week - Oct. 3-7, 2005) in Paris entitled "Efficient
RF/Microwave Modeling of Discontinuities in Chip Packages and Boards". I
strongly recommend you also go through this paper, when available.
Regards,
Ivan Ndip
Charles Harrington wrote:
>Hello Mark,
>
>because Eric's book is so beautifully written, I've
>gone through it almost twice already. However, it has
>very little or nothing to do with my question. I also
>went through Johnson's book (Advanced Black Magic),
>Silvester and Benedek?s publications on discontinuties
>and quite a lot of other electromagnetic texts. So,
>it's not a question for beginners.
>Jose recommended me Gupta's book (thanks Jose), which
>I already read. Here, Wolf and Mehran's waveguide
>model are used to analyze T-bends, Gap and other
>discontinuities, but the questions on how far the
>excited modes go away from the discontinuity and how
>this depend on the frequency are not dealt with.
>As far as I know, this subject has not being properly
>dealt with yet.
>So may the gurus recommend any other publications on
>this subject or some advice.
>
>Thanks,
>Charles
>
>
>
>
>
>
>
>
--
***************************************************************************
Dipl.-Ing. Ivan Ndip
Research Engineer
Fraunhofer Institut für Zuverlässigkeit und Mikrointegration (FhG-IZM)
Dept.: Advanced System Engineering (ASE)
Gustav-Meyer-Allee 25
D-13355 Berlin
Germany
Phone: +49 (0) 30 46403 679
Fax: +49 (0) 30 46403 158
E-mail: ndip@xxxxxxxxxxxxxxxxx | Web: http://www.izm.fraunhofer.de
ndip@xxxxxxxx
***************************************************************************
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