[SI-LIST] Re: Signal crossing Split plane
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Charles Harrington" <ch_harrington@xxxxxxxxx>, shlepnev@xxxxxxxxxxxxx, scott@xxxxxxxxxxxxx
- Date: Tue, 20 Nov 2007 16:45:15 -0800
I've watched this thread for a while now and haven't seen anyone suggest
making measurements. I've done lots of that and have several test boards
in my lab that represent the kind of plane splitting that is done to
accommodate two Vdds in the same plane. I've run traces over these splits
and measured them with a TDR with a rise time of 40 pSEC. There is no
detectable disturbance of the signal when this is done. (Of course, the
Vdd PDS designs need to be done such that the frequencies in the signals
crossing the planes can be properly supported. If that has not been done,
worrries about crossing plane splits will be the minor problem.)
I've also measured hundreds of vias used to change layers from top to
bottom of a PCB and from adjacent layer to adjacent layer. Again,
measuring these vias with the same TDR they behave as though someone has
attached a very small parasitic capacitor, on the order of .5 pF for a 12
mil drill in a 100 mil thick PCB. There has been no detectable coupling of
energy into the space between planes.
Same thing for right angle bends. Not a detectable source of signal
degradation or EMI.
Sometimes a little lab time saves a lot of agony and speculation.
If there are those out there who have tests that prove otherwise, perhaps
they should publish the results. I've published mine several times as have
others.
Hope this helps those who are confused by all of the complex explanations
that have been offered without any supporting measurements.
Lee Ritchey
> [Original Message]
> From: Charles Harrington <ch_harrington@xxxxxxxxx>
> To: <shlepnev@xxxxxxxxxxxxx>; <scott@xxxxxxxxxxxxx>
> Cc: <sunil_bharadwaz@xxxxxxxxx>; SI LIST <si-list@xxxxxxxxxxxxx>
> Date: 11/20/2007 2:45:54 PM
> Subject: [SI-LIST] Re: Signal crossing Split plane
>
> Yuriy,
> I agree with some of your views. However, they contradict your via
models.
> I couldn?t reply yesterday, because I was trying search for the
reference I mentioned, since you needed it. Many other people replied
off-line and so needed the reference. Got it from IEEE Xplore.
>
>
> A Novel Methodology for Defining the Boundaries of Geometrical
Discontinuities in Electronic Packages
> Ndip, I.; Reichl, H.; Guttowski, S.;
> Research in Microelectronics and Electronics 2006, Ph. D.
> 12- 15 June 2006 Page(s):193 - 196
>
>
> You mentioned in your mail that the near field zone as a result of the
higher-order modes excited at the via expands with frequency and is very
small. I agree with you.
> But the question is this. How small is it? How small or big is at 1
GHz, 10 GHz, 20 GHz? Have you ever studied it? You have to take this zone
into consideration when studying vias or any other structures that excite
higher order modes.
> The method proposed in this paper is quite illustrative and useful. I
understand it this way (Please correct me if I understand it wrongly):
> These higher-order modes (e.g., TE, TM...) are characteristics of the
trace or transmission line and they die exponentially away from the point
of excitation, i.e., the via-trace interface. S-parameters, like other
network parameters, give us the relation between input and output signals.
Now, to obtain S11, for example, you need to get the ratio of the reflected
and input signals. Both signals must be of the same "type". We can not
directly compare cars and aeroplanes, though both are used for
transportation. You know your input signal (e.g., a transverse
electromagnetic wave), because you excited it at the port. At
discontinuities, an infinite order of given higher-order modes can be
excited. The orders or strength of the excited modes differ from one
discontinuity to another, although the modes can be the same. So, there is
no way you can know all the orders of the higher-order modes excited and
how they interact. Now if you place your ports quite close to the point
> of excitation of these modes, then your S-parameters must be wrong. Why?
In this case, to obtain S11, you need to obtain the ratio of the unknown
higher-order modes and your known excited transverse electromagnetic wave
at the port. That?s why in most 3D full-wave solvers, it is recommended
that ports should be placed far away from the discontinuities, so as to
enable these higher-order modes to die. When they die, then you can easily
define your S-parameters which will then be the ratio of the input signal
you know (transverse electromagnetic wave) and the reflected signal you
know (transverse electromagnetic wave). To define the points where these
modes die or have attenuated substantially, these authors argued that near
the discontinuity, the imaginary part of the Poynting vector describes the
reactive energy associated with these higher-order modes. So they studied
this imaginary part and used it to define the point where the modes die. I
think they mentioned that only
> at a distance of about 1mm away from the via-trace interface, at 20 GHz
(or may be 30 GHz) may you place your ports, to get correct results.
Certainly, this depends on the via geometry and trace type. But I find the
results very helpful and can be used as a base for further experiments. You
can get the details from the paper.
> Unfortunately in your case, you compare what you don?t know (reflected
signal) and what you know (excited input signal). In your via models,
neither did you define the required distance away from the via-trace
interface needed for these modes to die nor did you follow the advice given
in full-wave solvers to be far way from the via-trace interface. You
considered the via just as the barrel and the pads at 20 GHz and beyond.
That?s why I mentioned yesterday that your via models are not correct and
your S-parameter results are misleading. If you wish to study only the
behaivor of the barrel alone at lower frequencies (for what ever reason -
but not for realistic designs), then you don't even need a field solver.
You can get formulas from good SI texts like that of Horward Johnson or
from papers.
>
> At first I was also making the same mistakes as you are making right
now. I had a lot of difficulties to correlate my simulation and measurement
results. So I learnt a lot from this paper, from Professor C. Balanis
(Advanced engineering electromagnetics) and from Professor R. Collins
(Field theory of guided waves). I think these references will be good for
you. You need all three of them.
> There are also a lot of points that you need to modify in your models.
> It?s ridiculous when you talk of -30 dB attenuation of higher-order
modes. Which higher-order mode? Which order of this mode? Basic
electromagnetic theory teaches us that an infinite order of a given
higher-order mode can be excited at any discontinuity. An interaction
between makes matters worst. So how do you separate the different orders of
the modes and tell which one attenuates by -30 dB? Are the modes
propagating or evanescent? Never use rule of thumbs that have no base. I
supposed you meant attenuation of the fundamental mode which is
propagating.
>
> I don?t know anything about the lumped ports you use. All I know is
that some lumped ports in some field solvers assume perfect H boundary
conditions on the sides. Consequently, depending you may not even capture
stray fields. So you can even get the worst results with lumped ports.
>
> You can only shift your reference S-parameters plane and get accurate
results if your model captured all the necessary field behavior. But you
can not simulate the via and traces differently and then do some
post-processing or circuit modeling afterwards and expect to get correct
results at higher frequencies. The traces too are part of the ?via effect?
at least, at the frequencies you are interested in (20 GHz and beyond),
because the stored higher-order modes give rise to additional inductances
and capacitances. These inductances and capacitances can not be captured if
you analyze the vias separately from their traces.
> Finally, the theory of multi-modal decomposition means different things
to different electrical engineers. So I don?t know what you mean. If you
mean that different parts of a system can be analyzed separately and then
put together, then it?s true that it has been done for decades now. But the
question is this. How do you bring the different parts together in the case
where there are discontinuities like vias? How do you define the via? How
small or big is your near field zone? I bet you, we have not yet understood
this type of decomposition and it has not been done, or at least published
for decades. Whenever we have to deal with vias and other discontinuities
at higher frequencies, straight-forward modeling can not be used.
> Please Yuryi, don?t get me wrong. I?m not trying to highlight on your
errors. I have mine too, like any body else. No one is perfect. I?m just
trying to raise the point that we need to be careful when modeling vias at
your frequencies. I agree with most of the points you made, but disagree on
the ones stated above. We learn from each other when we exchange ideas
about such fundamental issues that affect our modeling results. I think
that is one of the reasons why Ray and his team set up this forum.
>
> Best regards.
> Charles
>
>
>
>
>
>
>
>
> Yuriy Shlepnev <shlepnev@xxxxxxxxxxxxx> wrote: 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 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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