[SI-LIST] Re: Transmsision lines
- From: "Luciano Boglione" <l.boglione@xxxxxxxx>
- To: <lalexman@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sat, 15 Oct 2005 17:31:37 -0400
Leonard,
My 2 cents: the solution to your question can be found by considering
incident v+ and reflected v- voltage waves. At the load x=3DL meters =
away,
GL=3D(zL-1)/(zL+1)=3D1. v-=3DGL*v+ or in this case, v-=3Dv+. Therefore, =
the total
voltage vtot=3D(v+)+(v-) is twice the incident wave (or as you say, "the
voltage that arrives at the load is doubled"). At x=3D0 (where the =
source is),
the incident and reflected waves are such that the total voltage is
vtot=3Dv(source)*Zin/(Zin+Zs) where Zin is what you see when looking =
into the
line at x=3D0 and Zs is the source impedance. If you consider the =
telegraph
equation and solve it under sinusoidal excitation, you should be able to =
see
it yourself.
When I read your question, however, I thought of a step u(t)=3D1V (t>0, =
Zs=3D0)
applied to a lossless tx line at x=3D0: when I get to the load at x=3DL, =
the
reflection is in phase because GL=3Dv-/v+=3D1. So, one more volt is =
reflected
back towards the source. The total voltage along the line increases by =
1V to
2V as the reflected wave travels back along the line. This is also true =
when
the point x=3D0 is reached again by the reflected wave. So the total =
voltage
at x=3D0 after waiting for enough seconds is 2V! Something seems to be
incorrect, unless v+ at x=3D0, t=3D0+ is u(t)/2 (which does not make =
sense
because implies v-=3Du(0+)/2 and the reflected wave has yet to arrive at
t=3D0+)... I'm getting confused, need my book of Laplace transforms!
Regards,
Luciano
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On
Behalf Of Leonard Alexman
Sent: Saturday, October 15, 2005 2:18 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Transmsision lines
Hi,
=20
I am trying to figure out transmission lines and reflections and trying =
to
understand why if the load is open or a high resistance the voltage that
arrives at the load is doubled and the signal is them reflected back to =
the
source. I understand there is an impedance mismatch but in all the =
articles
I have found not explains in basic terms wht the voltage doubles and
reflects back down the line. Can anyone point me to an article that =
might
explain this in basic terms ?
=20
TIA
=20
Leonard Alexman
=20
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=20
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