[SI-LIST] Fwd: RE: Re: Doug Brooks's Question

  • From: Sainath Nimmagadda <intrinsi@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Sun, 16 Nov 2003 19:24:18 -0800 (PST)

For some reason, Tom Dagostino's post did not make it to the List. Here is a 
copy.
 
Sainath
Tom Dagostino <tom@xxxxxxxxxxxxx> wrote:
From: "Tom Dagostino" 
To: ,
,

Subject: RE: [SI-LIST] Re: Doug Brooks's Question
Date: Sat, 15 Nov 2003 14:14:52 -0800

There are many different combinations of L and C that when applied to the
formula R=Z=(L/C)^.5 will yield for example, 50 Ohms. Intuitively for a
given dielectric, a length of cable 1 inch, 1 foot, 1 mile or 1 light-year,
can all have an Zo of 50 Ohms. Likewise, if the dielectrics are different
you can still get 50 Ohms but the time delays per unit of the cables will be
different.

Thus if the L and C equivalents from these different lengths are applied to
the equation, you will get 50 Ohms. Now, what insight do you get by using
these L's or C's in the equation RC=L/R?

Tom Dagostino
Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC
2926 SE Yamhill St. Device Modeling Division
Portland, OR 97214 13610 SW Harness Lane
Beaverton, OR 97008
http://www.teraspeed.com 503-430-1065
tom@xxxxxxxxxxxxx


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Sainath Nimmagadda
Sent: Saturday, November 15, 2003 1:09 PM
To: weirsp@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Cc: intrinsi@xxxxxxxxx
Subject: [SI-LIST] Re: Doug Brooks's Question


Steve,

I suppose Doug's track assumes that the line is matched with upstream
networks and his concern is about the termination- specifically, about
interpreting time constants when it is not matched to the line. The end LC
of the line also represents characeristic impedance of the line.

Do you see it differently?

Sainath


steve weir wrote:
Sainath, trying to combine just the end LC with the termination R ignores
the effects of the upstream networks and yields the wrong behavior.

Regards,


Steve.
At 09:54 AM 11/15/2003 -0800, Sainath Nimmagadda wrote:
>I understand the responses(using different tracks) to Doug's
>question- but don't know why his track is wrong altogether. Someone,
>please explain why and where his approach doesn't work.
>
>Thanks,
>Sainath
>
>[ from Archives]
>==========================================================
>From: Doug Brooks [mailto:doug@xxxxxxxxxx]
>Sent: Wednesday, November 12, 2003 4:43 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Transmission line match
>
>I am trying to develop a *qualitative* (NOT quantitative) explanation of
>what happens at the terminating end of a transmission line. I think I am on
>the track with this model and discussion:
>1. The transmission line model is a lumped model of a string of Ls and Cs.
>2. The termination value forms a time constant with these. That is, if
>R=Z=(L/C)^.5, then RC=L/R. That is, when correctly terminated, the time
>constants are matched.
>3. If the terminating resistance is high, then the RC time constant is
>relatively longer. If it is low, then the L/R time constant is relatively
>longer.
>
>At this point I am having trouble getting over the "so what?" and
>completing the argument. What is the next step in this argument..... or am
>I on the wrong track altogether?
>
>Thanks
>Doug Brooks
>=======================================================
>
>
>
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