[SI-LIST] Re: Signal crossing Split plane
- From: steve weir <weirsi@xxxxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx
- Date: Tue, 20 Nov 2007 17:42:33 -0800
Lee I don't think the original problem would have warranted
measurements. But the thread has moved significantly since then. It
might be a fun test of Simbeor to model your test vehicle and compare
results. Rather than just drive with a TDR, it might be fun to drive
with a pulse generator that can be set to the simulated resonant
frequency and see if doing so impacts measured results in real life the
way simulation predicts.
Best Regards,
Steve.
Lee Ritchey wrote:
> 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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