[SI-LIST] Re: TEM approximation
- From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
- To: "'steven.corey@xxxxxxxxxxxxxx'" <steven.corey@xxxxxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
- Date: Fri, 19 Sep 2003 13:52:55 -0700
Noop, BTDT. Any TEM approximation will be way off. Especially when you
calculate crosstalk, impedance and prop delay (if the mesh is uniformly
defined). I am talking about 50%-200% error. If you don't believe me, try it
yourself.
BTW, this is a well known problem. IBM engineers realise this way back when
they design the TCM modules for their mainframe. Albert Ruehli's PEEC method
works great on these problems.
-----Original Message-----
From: Steve Corey [mailto:steven.corey@xxxxxxxxxxxxxx]
Sent: Thursday, September 18, 2003 11:32 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: TEM approximation
This is a good example of one of the basic assumptions underlying TEM
transmission line theory -- that the lines have to be uniform in the
direction of propagation. In other words, a 2D problem.
A more interesting question from my perspective as a tool developer is
what is the "right answer" to which you refer. From an analytical
perspective, no tool will give the exact answer due to imprecision and
inherent model inaccuracies. From an engineering perspective, there are
many "right answers", since any answer that falls within an acceptable
error range of the exact answer is a right answer. So I can see where
in some applications a TEM approximation for the example below will be
fine and give a right answer, whereas in others it will not. It depends
how much accuracy you need to get a reliable design.
Since our tools have both TEM and non-TEM capabilities, we recommend
that our customers use measurement validation to determine which
approach is necessary to get them the accuracy they need.
-- Steve
-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Analysis Company."
http://www.tdasystems.com
email: steven.corey@xxxxxxxxxxxxxx
phone: (503) 246-2272
fax: (503) 246-2282
-------------------------------------------
Chris Cheng wrote:
> I can easily give you an example where TEM assumption will break down even
> with relatively slow edge rate.
>
> Take a simple uniform stripline. Start meshing the reference planes
directly
> below and above the trace. The size of the hole is dimensionally large
> compared with the trace itself but electrically small compared with the
edge
> rate so that quasi-static mode still holds. e.g. drill 50-100 mil holes
> uniformly spaced along a 5 mil stripline trace. No TEM mode tools or
> approximation will give you the right answer. The imagine current on the
> reference plane is forced to run with orthogonal components to the signal
> current, thus breaking the TEM mode. This is a very real life problem as
it
> exist in many signal traces in organic substrate packages.
>
> -----Original Message-----
> From: Dima Smolyansky [mailto:Dima.Smolyansky@xxxxxxxxxxxxxx]
> Sent: Wednesday, September 17, 2003 4:01 PM
> To: swamydp@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: TEM approximation
>
>
> Hello, Swami,
>
> Interestingly, we at TDA Systems determined that the TEM assumptions can
be
> used to surprisingly high risetimes if the parameters are accurately
> extracted and verified. Our lossy line examples verify this.
>
> I hope this helps.
>
> Thanks,
>
> -Dima
>
> ----- Original Message -----
> From: "Swamy Dhoss" <swamydp@xxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Sunday, September 07, 2003 4:28 PM
> Subject: [SI-LIST] TEM approximation
>
>
>
>>Hi
>>
>>Could anyone tell me about the concept of TEM
>>approximation for the propagation of an electrical
>>signal through a transmission line ?
>>Over what range of frequencies TEM approximation is
>>valid ? and what model can be used above this
>>frequency limit ?
>>
>>Regards
>>swamy
>>
>>
>>__________________________________
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