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question regarding the boundary behavior of electric current

Discussion in 'Electronic Design' started by laura, Apr 10, 2007.

  1. laura

    laura Guest

    Hi,

    I have a cable and I send electric current through it. I want to know
    the moment when the current has arrived at the other end of the cable.

    Can I visualize this moment by using an oscilloscope ?

    If the answer is yes, I have another question for a more complex
    experiment.

    I have a very simple network with 2 nodes (A and B). There are 2
    cables which are connecting these nodes. The cables have different
    lengths L1 and L2. Assume that L1 is shorter than L2.


    I have draw a small picture here:

    --A*-------L1---------*B--
    | |
    | |
    |______L2_____|


    I apply electric power to this device. Because the cables have lengths
    greater than zero I assume that it will take a while until the current
    traverse the path from A to B.

    More than that, in node A, the current is split in 2, because there
    are 2 cables linking A with B. Because one of the cables is shorter I
    assume that "a part" of the current arrives earlier (denote this by
    moment M1) than the other "part" because it has to traverse a shorter
    path. Am I correct ?

    The other "part" of the current which has traversed cable L2 will
    arrive later in B. Lets denote this by moment M2.

    Can I measure these 2 moments by using an oscilloscope?
    Or what other options for measurement I have ?

    Are there some fluctuations of the electric current at moments M1 and
    M2 ?

    Are there high precision oscilloscopes for this experiment? Or should
    I use very very long cables?

    I'm not interested what happens after those moments.


    Thanks,
    Laura
     
  2. Guest


    You are looking for a TDR.
     
  3. Tom Bruhns

    Tom Bruhns Guest


    When you're dealing with the transmission time along a cable, it's
    good to realize that controlled cables are important. Specifically,
    it's very helpful to use transmission lines with controlled impedance.

    Yes, you can measure such things with oscilloscopes. It doesn't even
    take a particularly fast 'scope. You can use, say, 100 feet of
    coaxial cable for your line L1, and 150 feet for your line L2. The
    line will be much cheaper than a high speed 'scope, or very much
    cheaper than the very high speed 'scope you'd need if you tried to use
    lines, say, 1 inch and 1.5 inches long. With 100 feet of common RG-6
    coaxial cable like is used to wire houses for television, you'll get
    about 125 nanoseconds of delay, and with 150 feet, you'll get 1.5
    times as much. Even a 'scope with 5MHz bandwidth should be able to
    easily resolve that sort of delay. You also need a signal generator
    that can generate a pulse that transitions quickly from one state to
    another, but that's pretty easy with modern digital ICs, if you don't
    already have an appropriate signal or pulse generator available. Use
    shorter cables, maybe 10 feet and 20 feet, if you have a scope that
    can resolve shorter times--maybe one with 50MHz or 100MHz bandwidth.

    But let's make it even easier. Download LTSpice from the Linear
    Technology web site, and you'll have a simulation tool that can easily
    show you what happens. Once you learn to run the program (which isn't
    difficult), you can set up and simulate just what you've proposed in a
    very few minutes. If/when you do, try some different things: Make
    the lines both 100 ohms impedance. At B, try it with no load, and try
    it with a load of 200 ohms across the line, and 100 ohms across the
    line, and 50 ohms across the line.

    Cheers,
    Tom
     
  4. Guest

    Direct current (battery) or alternating current (wall plug)?


    Mark L. Fergerson
     
  5. Robert Baer

    Robert Baer Guest

    Methinks you are going to have a lot of fun making measurements
    related to speed-of-light travel.
     
  6. Robert Baer

    Robert Baer Guest

    Well.. the OP mentioned wires, and the diagram implied that the
    longer one wiggled around some to get from A to B even if the shorter
    one was in a straight lin.
    No specs on the wires (shape,size, insulation, etc) and the rest too
    fuzzy.
    Hence the "fun" comment...
     
  7. laura

    laura Guest

    Dear Tom,

    Thanks for your answer.

    I have downloaded the LTSpice software, but I dont know how create 2
    cables having different lengths. I'm not very sure, but I think that
    the software assumes that the length of cables are not important.

    Can you help me with this example?

    thank you very much,
    Laura
     
  8. Tom Bruhns

    Tom Bruhns Guest


    You need to place two "tline" components on a schematic (F2; type
    tline). You can then place the cursor over the "Td=50n Z0=50" text,
    right-click, and change it to a new time and new impedance. I suggest
    making one of the times 100n (100 nanoseconds) and one of the times
    150n. Connect the bottom left and bottom right terminals to ground,
    for both lines. Place a source (F2; type vo) to the left of the
    lines. Connect the source "-" terminal to ground. Connect the "+"
    terminal through a 50 ohm resistor to the upper left terminal of one
    of the lines. Connect the "+" terminal through another 50 ohm
    resistor to the upper left terminal of the other line. Change the
    source to a pulse; let's say delay 10n, rise time 0.5n, fall time
    0.5n, initial voltage 0, pulse voltage 10, pulse time 1u, period 10u.
    Connect the upper right terminal of each line together, and optionally
    to a load resistor to ground. Label that net with some name like
    "out". Label the upper left terminals of the line something like
    "in1" and "in2". Do a time simulation for, say, 500n or 1u.

    Cheers,
    Tom
     
  9. laura

    laura Guest

    Dear Tom,

    Many, many thanks.

    I did created the design as you have suggested. But I think that I've
    made some mistakes there ...

    Could you help me to correct them? I've sent you the design by email.

    Thanks again,
    Laura
     
  10. Ian

    Ian Guest

    Insert "tline" components (enter tline in the component selection field,
    left click at a couple of positions on your schematic to place them).
    These are transmission lines (cables).

    Note they arrive with default Td (electrical length in nsec) of 50n
    and Z0 (characteristic impedance) of 50 ohms.

    If you right click on the symbols you can edit these parameters and change
    them to Tom's suggestions.

    Add a voltage source and the resistors, label the inputs and outputs you
    want to look at with names you will recognise. Right click on the voltage
    source, select "Advanced", select "PULSE" and set up the parameters
    you want. Right click on the schematic and run the simulation.

    Regards
    Ian
     
  11. laura

    laura Guest

    Hi Ian,

    I have created the example ... but is not working. I'm sure that I've
    made some mistakes there.

    Could you help me to debug it if I'll send it by email?

    thanks,
    Laura
     
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