Re: [SI-LIST] : Basic Transmission Line question

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From: Steve Corey (steve@tdasystems.com)
Date: Thu May 24 2001 - 12:31:38 PDT


Tim -- Lumped and distributed have to do with how the line is modeled for
computations (hand calculations or simulator). Lossless vs. lossy are
intrinsic characteristics of the transmission line itself. Both lumped and
distributed models are capable or modeling both lossless and lossy
transmission lines.

Both lumped and distributed models are only valid within a certain
frequency range in which they accurately represent the characteristics of
the device.

Practically speaking, much of the transmission line modeling work done to
date in the time-domain, SPICE-like simulators has focused on so-called
distributed models. This has particularly been true for those models made
to handle loss mechanisms such as skin effect and dielectric loss. This
has given the impression to many that a distributed model is necessary to
model skin and dielectric loss, which is not necessarily true in the
technical sense, but to date has been true for all practical purposes.

Finally, just for the sake of interest, I would mention that from a
time-domain simulator's perspective, the concept of a distributed model vs.
a lumped model is a very blurry one, since as the simulator chooses its
time steps, every model gets effectively discretized into lumps of some
sort.

  -- Steve

-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Modeling Company."
http://www.tdasystems.com

email: steve@tdasystems.com
phone: (503) 246-2272
fax: (503) 246-2282
-------------------------------------------

tcoyle wrote:

> Hello,
> I have a basic transmission line question. I am reading through Howard
> Johnson's book and he's talking about lumped and distributed
> transmission lines.
> It basically states that if the length of the system is small enough,
> then the lumped model can be used. If it is not, then a distributed
> model can be used.
> It is also stated that the lumped model has fixed component values good
> over a low frequency range. The distributed model is good for high freq
> range because it takes into account skin effect and so forth. My
> confusion comes from comparing this to a lossless and lossy lines. Is
> the lumped model a lossless line, because it is only accuarate at low
> frequencies? Or can it be both lossless and lossy?
>
> Thanks for any insight provided
>
> Tim
>
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