# Coaxial cable, resistor and oscilloscope

Discussion in 'General Electronics Chat' started by sunrise, Jul 2, 2013.

1. ### sunrise Thread Starter New Member

Jul 2, 2013
2
0
I'm studying the trasmission of signals though coaxial cable; I'm using a coaxial cable (of course!), an oscilloscope, a generator an some different endings. This is the scheme:

I was told that if I use a very long cable that has impedence \\$Z_0= 50 \Omega\\$ and put, at the end of it, a **resistor of \\$47\Omega \\$**, the signal will be reflected.

I have obtained this:

If I have correctly understood, the height of the step pointed by the green arrow is the half of the V given by the generator.

I can't understand why the step pointed by the red arrow is shorter than the one that is on the other side. And I can't understand what's the physical meaning of the step pointed by the red arrow.
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Then, I have terminated the cable with a **short circuit** and I have obtained this:

Could you explain me what's up at the points indicated by the two arrows and the yellow and red points?
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Then, I have terminated the cable with a **resistor of 100 ohm** and I have obtained this:

Could you explain me, why the step pointed by the green arrow is higher than the step pointed by the red arrow?

2. ### RichardO Senior Member

May 4, 2013
2,268
877
Good question.
I think that you are seeing the effect of multiple reflections. Try increasing the pulse width of your signal and see if that shows more reflections with decreasing amplitudes.

3. ### sunrise Thread Starter New Member

Jul 2, 2013
2
0
Thanks for your answer but I can't repeat the experience because now I haven't the instrumentation... Can you help me to have some ideas about these horrible results? What should I have obtained?

Many thanks