[SI-LIST] Re: Signal crossing Split plane
- From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
- To: <ch_harrington@xxxxxxxxx>, <scott@xxxxxxxxxxxxx>
- Date: Mon, 19 Nov 2007 10:20:01 -0800
Charles,
I am sorry that the simulation examples were not helpful to you. I will
appreciate if you send me the reference you mentioned - I am preparing to be
shocked:)
You are absolutely right, the via-holes are not just pads and barrels and
there is no one solution that covers all possible cases. Analysis of
different vias has to be done in different ways. Transition to the traces
have to be almost always included in the final model for analysis of
multi-gigabit channels. Moreover sometime the via-hole problem cannot be
solved locally and require analysis of parallel plane structures with all
decoupling structures attached (see technical presentation #1 at
http://www.simberian.com/Presentations.php for more details on different
structures).
Considering the ports and excitation. Analysis of via-holes with lumped
ports provides just rough idea about the via-hole behavior. It is similar to
what you would see from a differential probe attached to the pads of the
via-holes. Transition to traces and transmission line or wave-ports have to
be used for the final extraction of S-parameters for the system-level
analysis (I am sorry that you missed this part in app notes). Note that it
is possible only for the localizable via-holes or via-holes not coupled to
parallel planes in general. Such t-line ports have to be positioned at a
distance from the via-hole that guaranties that the high-order modes are
attenuated substantially (for practical applications we usually use -30 dB
threshold at the highest frequency of interest). After such analysis, the
phase reference planes of S-parameters can be safely shifted closer to the
via-hole at the position where t-lines are still continuous to preserve
causality (to the edges of anti-pads for instance). Such transformation does
not affect the near field or high order modes around the via-holes and the
final model can be safely connected with the transmission line segments in a
system-level solver. Though, the model have to be used with transmission
line segments with length not less than in the electromagnetic analysis (to
avoid the near-field interaction between the vias and possible
discontinuities). This technique called the multi-modal de-compositional
analysis and used in microwave engineering for decades at frequencies even
higher than 20 GHz.
Note, that in typical PCB trace the cut-off frequencies for high-order modes
are extremely high. 10 mil trace on 10 mil dielectric with dielectric
constant 4.2 have cut-off frequency about 120 GHz, and the cross-over with
the surface TM mode may happen only at 200 GHz. Before 120 GHz the
high-order modes are evanescent and essentially form the via-hole near
field. This near-field zone is expanding with the frequency, but at 20 GHz
the area is still relatively small. Thus S-parameters only for the dominant
modes can be safely extracted and used as the via-hole model.
Cases when via-hole excite the non-evanescent parallel-plane modes and
planes are not stitched close to the via-hole cannot be solved locally (non
localizable) and may require the system-level analysis with all decoupling
structures attached.
Best regards,
Yuriy
Yuriy Shlepnev
Simberian Inc.
www.simberian.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Charles Harrington
Sent: Monday, November 19, 2007 8:33 AM
To: shlepnev@xxxxxxxxxxxxx; scott@xxxxxxxxxxxxx
Cc: sunil_bharadwaz@xxxxxxxxx; 'SI LIST'
Subject: [SI-LIST] Re: Signal crossing Split plane
Yuriy,
not only are your slot simulations on your page not so helpful, but your
via simulations are very misleading. I think you'll run into trouble when
you try to compare your simulation and measurement results, because your
simulation models are unrealistic.
At such frequencies (20 GHz and beyond), the via can no longer be considered
to be just the barrel and the pads, as you did. The modes excited at the
via-trace interface don't die abruptly, but extend along the traces to the
ports. So either you seperate these modes from the originally excited modes
at the port (in order to obtain "clean" S-parameters') or you allow the
modes to die before they reach the ports (as recommended in most 3D
full-wave solvers).
I just read a very interesting research paper the other day on defining the
boundaries of discontinuties, in which these issues are properly examined. I
can't really remember the exact title nor its authors at the moment, but the
paper was presented at a Ph.D. research conference on microelectronics and
electronics somewhere in Europe (Italy, I presume). You'll be shocked at the
error you are making when you read this work.
You also connected the models of the via and transmission lines after the
simulations, correct? Here you go wrong again, because how do you know where
the vias "actually" begin and end? And at what freqency? These are very
complicated issues and I suggest you spend a little more time studying them
well.
Thanks.
Charles
Yuriy Shlepnev <shlepnev@xxxxxxxxxxxxx> wrote: Scott,
I agree with you. It was just an illustration of a slot-type discontinuity
in general for some stackup configurations. It shows how a slot-type
discontinuity in a reference plane may reflect the signal even in the case
if slot does not cut across the board or around a patch (though, it might be
obvious for you). As soon as the coupling to a slot is strong, it has to be
simulated at the system level with a complete geometry of the slot or split,
with all relevant traces crossing the slot and all de-caps (if any). I
prefer to do it with the hybrid de-compositional approach on the base of
localized models built with an electromagnetic solver. The localized strip
to slot coupling effect can be captured with a 4-port S-parameter model for
strip crossing the slot for instance (two ports for the strip and two for
the slot). Combined with the strip and slot line models, it produces a
simple and computationally efficient system-level model that captures
practically all coupling and resonance effects.
Best regards,
Yuriy
Yuriy Shlepnev
Simberian Inc.
www.simberian.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Scott McMorrow
Sent: Sunday, November 18, 2007 12:29 PM
To: shlepnev@xxxxxxxxxxxxx
Cc: sunil_bharadwaz@xxxxxxxxx; 'SI LIST'
Subject: [SI-LIST] Re: Signal crossing Split plane
Yuriy
Actually, these sorts of slot simulations are pretty meaningless. Slots
normally occur due to plane splits. As a result, the either extend from
one edge of a board to another edge, or when the plane is a square patch
the slot is a closed loop around the periphery of the plane. When this
happens, it is quite interesting to simulate multiple signals crossing
the slot. There is a very nice slot resonance mode that occurs that is
generally in the signal bandwidth (or at least 3rd harmonic) because of
the length of the slot. This induces a signficant amount of ringing and
crosstalk into neighboring traces.
scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
TeraspeedR is the registered service mark of
Teraspeed Consulting Group LLC
Yuriy Shlepnev wrote:
> Sunil,
>
> A simple example of how an electromagnetic solver can be used to
investigate
> the effect of a slot or split in a reference plane is provided at
> http://www.simberian.com/AppNotes.php - see the topmost app note.
>
> Best regards,
> Yuriy
>
> Yuriy Shlepnev
> Simberian Inc.
> www.simberian.com
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
> Behalf Of sunil bharadwaz
> Sent: Sunday, November 18, 2007 1:26 AM
> To: SI LIST
> Subject: [SI-LIST] Signal crossing Split plane
>
> Hi ,
> I have few signals (@ 80 Mhz & 20 Mhz) crossing the split Power
> plane in the adjacent layer.
>
> The 20 Mhz signal is diffrerential signal.The 80 Mhz is a single
> ended signal.
>
> I want to analyse the affect on Signal Integrity of these two
> signals due to split plane.
>
> I believe one need to define his stack up (Including the
> split) & then extract the layout to simulate.
>
> I'am not too sure if the prevalent SI tools have an option
> of creating split planes .
>
> Pls suggest me a right tool to carry out this.Also , i'am
> looking for a free tool to start with (even if the accuracy
> is slightly limited).
>
> Thanks in Advance!!
>
> Regards
> Sunil.Bh
>
>
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