[SI-LIST] Re: Doug Brooks's Question
- From: steve weir <weirsp@xxxxxxxxxx>
- To: Sainath Nimmagadda <intrinsi@xxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Sat, 15 Nov 2003 14:21:25 -0800
Sainath, no, that is not the way that I see it. Unless I misread Doug,
like you he noted that the LC at the end of the line, like all the
increments along the line has Zline. The problem is that a transmission
line does not behave like a lumped LC. I don't personally see a way to try
to replace the Tx line with a lumped LC, nor do I see a useful point.
If the idea is to think about the behavior intuitively, I personally like
the parallels to conservation of momentum.
Jim Knighten's discussion of Kirchoff's Laws fits that nicely as well.
What doesn't seem to fit is a lumped LCR. The response doesn't
match. With a lumped LCR the ringing time constant is determined by LC,
and L/R. With an underdamped transmission line, it is determined by the
length of the line. All we get at the end of the line is L/C for Zline. L
and C get scaled for convenience to solve the equations. The higher the
frequency range we want to evaluate, the smaller both L and C become. So,
the LCR time constant moves towards zero.
Regards,
Steve.
At 01:08 PM 11/15/2003 -0800, Sainath Nimmagadda wrote:
>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 <weirsp@xxxxxxxxxx> 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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