FreeLists / si-list / [SI-LIST] Re: Signal crossing Split plane

[Charles Harrington <ch_harrington@xxxxxxxxx>]

Chris,
   
  I completely agree with you. We need methods rather than just validated tools.
   
  I disagree with Yuriy that a tool can define the boundaries of discontinuties 
and decompose the channel. If he still believe this is true, then he should 
name the tool.
   
  Channel decomposition is a very old and simple procedure to impement. This I 
agree with Yuriy. But what Yuriy does not understand is that you can do 
decomposition only when you have one dominant wave mode at points or interfaces 
along the channel where you wish to do the decomposition. Let me explain what I 
mean. If you have a stripline, competely surrounded by a homogeous dielectric 
and losses can be neglected, then the TEM approximation can be used. Even if 
this stripline is as long as 1 km, you can decompose it and use quasi-static or 
even static approximations to analyse the different segments. These segments 
can later be integrated together. But when ever you have higher order modes, 
then straight-forward decomposition, which is implemented in most tools, fails. 
As Prof. R. Collins (field theory of guided waves) explains, once there are 
discontinuities, then the waves at the output of the segment containing the 
discontinuity will have a combination of the dominant
 modes (which you can competely characterize) and higher order modes (which you 
may not even know). These waves will then serve as incident waves to the second 
segment and so on. That's why in most tools, it is recommended to do 
decomposition only at points where the higher order modes have decayed. The 
user must define interfaces or points along the channel where the higher order 
modes have decayed and only the main mode (which propagates power) is present. 
This is one of the motivations why boundaries of discontinuties are defined. 
So, don't rely on tools to do channel decomposition whenever you have 
discontinuties at higher frequencies. Remember, at lower frequencies most of 
the effects of the highe order modes can be neglected. 
  I am sure no body in this list (including myself) will be able to propose you 
a solution much better than the one you proposed yourself in your previous 
mail. Below, I will try to outline your methodology. Please correct me, if I 
don't understand something.
   
  1. Characterise the 3D geometies for your PCB technology considering the 
return paths, plane stitches etc. at your frequencies of intertest. You may 
consider the highest frequency. Whenever discontinuties are placed too close to 
each other, then you consider them as one discontinutity and  use a 3D field 
solver to compute the fied solution.
  2. Use a statistical method to make sure you cover possible dimensions of the 
3D geometries (such as via pads, via holes, stitcing vias and so on) and 
what-if scenarios.
  3. Place your design rules in a data base for your post route verification 
analysis.
   
  I think if you proceed as you proposed yourself, then you will not encounter 
any uncalculated discontinuity, because you define your layout yourself. You 
can always localize any discontinuity you encounter. You just have to make sure 
that the return current is kept close to signal current. There is nothing new 
or difficult in this. Dr. Howard Johnson in his book on advanced black magic 
explains this very well.
   
  I hope it helps. If not, I'm sorry, I can not help you further.
   
  Best regards
  Charles
   
  
 
   
   
  
Chris Cheng <Chris.Cheng@xxxxxxxx> wrote:
  JP,
I have no beef in who has a better 3D algorithm. My question is specifically on 
methodology to both Charles and Yuriy in integrating 3D models in a system 
environment and I didn't find Yuriy pimping his own tool in his response to me. 
In fact I think his description is somewhat closer to what Charles has been 
saying, there are non-localize models one has to take into consideration.
I am just looking out from my short bus asking all the experts you mention to 
chime in. Namely, what is the correct methodology to integrate 3D models in a 
complete system interconnect simulation environment.
As far as I know there are not that many brave souls that model the entire 
system interconnect in one big giant 3D full wave model from end to end. That 
means at certain point of time the model is partitioned, most likely between 
pure interconnect (lossy line) and discontinuity (3D models). 
I think Yuriy correctly point out some of the cases are not localizable and in 
those cases, what are you going to do ? What tool can tell me I can safely 
break my trace read from the PC CAD database at what distance before I have to 
extract my 3D model (as in case a) on my original question)? What tool can 
precompute the discontinuity in 3D and then calibrate out the ideal 
interconnect part out of the extended port (probably by some kind of pseudo TRL 
algorithm) so that a user can simply extract the trace length information from 
the PC CAD database and then just reuse the pre-compute 3D models everytime he 
encounter the discontinuity ?
To me, the second option is very attractive because it gives the dumb user like 
me a very straight forward modeling methodology. All those via drills and 
return models, plane switches can be pre-compute and constrain in PCB design 
rules and the post route verification will be a snap by simply reading out the 
trace length and location of the discontinuity and substituting the pre-compute 
model. 
On the other hand, what if we hit one of those giant non-localize discontinuity 
? What tool can fall back and extract the necessary area and model ?
I am not an academic, I ship products. With that in mind, I need a methodology 
and not just a tool that can be validated to a terahertz. 
________________________________

From: Jean-Pierre Maurice [mailto:mauricejeanpierre@xxxxxxxxxxxxxx]
Sent: Sat 12/1/2007 4:28 PM
To: shlepnev@xxxxxxxxxxxxx; Chris Cheng; ch_harrington@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Signal crossing Split plane



Yuriy,

stop confusing yourself and others with lengthy emails and explanations that 
have no proof.

Which of the references you quote in the 40s or any other EM principle warrants 
you to model discontinuities the way you do in your application notes? I went 
through all the examples in your website using the link you provided ( 
http://www.simberian.com/AppNotes.php) and realized that you don't have even 
one example where you correlated your simulations with measurements. Why? Are 
you hiding something? 

At the beginning, Lee advised you to do some measurements. Charles also said 
the very thing, but in a rather harder way.

I may disagree with the way Charles pointed this out to you, but there is a lot 
of truth in everything he said, especially regarding your models. Indeed, the 
models in your application notes are not correct and also misleading, I would 
say. You make terrible claims about your solver's ability to compute complex 
multilayer geometries and yet provide no example to make your case solid. 

In the case of via-hole modeling; If you have now learned and agree that 
via-holes are not just barrels and pads as you represent them in your notes, 
and if you now also agree that you need to be far away with your ports, then 
why do you still have these unrealistic models in your application notes? You 
even go as far as posting them in this list. By doing so, you mislead the young 
and unexperienced. You even mislead the users of your solver. If you claim that 
any of the models (multilayer geometries, slots, via, planes, transmission 
lines, etc) in your application notes is correct, then show us how they match 
with measurement results. 

There are also a lot of weakness in the way you explain some fundamental issues 
which do not reflect the 25 years of experience you claim to have. Unlike 
Charles, I will not talk about that openly in this forum. May be privately, if 
you permit me. You even forge explanations to justify your solver and models. 
This is inappropriate. 

I am now on holidays. When I get back to work, I would like to evaluate your 
solver using some of our multilayer geometries (if you provide me the 3 day 
evaluation license you promise on your website). As long as I don't see any 
good correlation with measurements and your simulation models are also weak, 
then there is no way I can believe you. 

Chris: I don't think Yuriy is the person to ask questions concerning real PCB 
designs when he can not provide realistic examples in the application notes of 
his own solver. If you don't believe me, use the link given above. Charles too 
is a bit impolite, I would say. There are a lot of other well respected experts 
in this forum (Steve, Istvan, Lee, Eric Bogatin etc) from whom I learn a lot. I 
think they will give you real answers, not some theory that does not help. 

Best regards
Jean Pierre




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