[SI-LIST] Re: reflection coeficient
- From: "Lynne D. Green" <lgreen22@xxxxxxxxxxxxxx>
- To: <kgrhoads@xxxxxxxxxxxx>
- Date: Mon, 20 Aug 2007 13:41:34 -0700
Kevin-
I agree there is no replacement for visual demos (100 foot clothesline is
under $5). My Physics prof showed the tied vs slip ring termination effect
at roughly 30 feet (10 m).
Another fun one is to use a motor with "stick" to drive one end of the
"clothesline" to demo standing waves. Back then, I was most impressed by
the fact that there was a fixed point of zero amplitude (voltage), and yet
energy was clearly moving past that point. Today, I look back at that and
visualize PCB resonances.
After that, they taught us all the math....
Cheers,
Lynne
"IBIS training when you need it, where you need it."
Dr. Lynne Green
Green Streak Programs
http://www.greenstreakprograms.com
425-788-0412
lgreen22@xxxxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Kevin G. Rhoads
Sent: Monday, August 20, 2007 10:49 AM
To: kpsubramani@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: reflection coeficient
>I was reading about signal reflections.
It is too bad that most people use clothing dryers and not clotheslines
anymore. If you've ever "pulsed" a clothesline and watched the pulse
propagate and reflect off the far end as a kid, then when you get to
transmission lines 101 you can visualize just exactly what is going on
phenomenologically. (So all that time when I was "playing" as a child was
not totally wasted ...)
Absent a clothesline, take a good string or twine and run a taut line over
at least 10 feet, longer if you can. Ping the line by plucking it near one
end and watch the propagating pulse. You can literally see the pulse
reflect from the far end. If one uses the natural analogy of displacement
<-> voltage and transverse velocity <-> current, then a tied end is
analogous to a shorted line. An opne line would be simulated by having the
twine tied to a slip ring that can freely move up and down, which is much
harder to get working.
There is no substitute for understanding the math -- but good visualization
can be very helpful as well.
HTH
Kevin
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