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: [​IMG]

    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:

    [​IMG]

    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.
    ___________

    Then, I have terminated the cable with a **short circuit** and I have obtained this:
    [​IMG]

    Could you explain me what's up at the points indicated by the two arrows and the yellow and red points?
    ________________

    Then, I have terminated the cable with a **resistor of 100 ohm** and I have obtained this:
    [​IMG]
    Could you explain me, why the step pointed by the green arrow is higher than the step pointed by the red arrow?
     
  2. RichardO

    Well-Known Member

    May 4, 2013
    1,228
    382
    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? :D What should I have obtained?

    Many thanks
     
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