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lenght cable tester?

Discussion in 'Electronic Basics' started by Impeller, Feb 15, 2005.

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  1. Impeller

    Impeller Guest

    I heard about a tester which measure the lenght of the cables, but I
    don't know how it work. Does it measure only the resistence cable or
    does it measure even inductance and/or capacitance? When I do the
    measure do I have to connect end cables or have I to leave ends
    opened? Thanks.
     
  2. Some ethernet PHY chips have this built in. I believe they measure the
    delay of a pulse and it's reflection off of the unterminated end of the
    cable. They can use this to report opens, and how far they are away from
    the PHY.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  3. Rich Grise

    Rich Grise Guest

    This is called a "Time-Domain Reflectometer", TDR. In general, what it
    does is put a calibrated step voltage into one end of a cable, and
    monitors the voltage by displaying it on, essentially, an oscilloscope. So
    you see a trace that starts at 0, and at time t0, steps to V. The voltage
    reading at that point stays constant as long as the pulse propagates down
    the line. When it meets a discontinuity of some kind, some or all of the
    signal is reflected back to the source. This reflection shows up on the
    display as another edge, which can tell you a remarkable amount of stuff
    about the condition of the cable. People with more education than I have
    can tell what things like overshoot, undershoot, rise/fall times, and so
    on have to do with it - what I was interested in the time I used one was
    finding the break in a buried cable. A break (open) shows up as a step to
    2 * V, and a short shows up as a step to 0V. Other values mean an
    impedance mismatch somewhere between a short (0 ohms) and an open
    ("infinite" ohms). And the time from t0 to the step gives the length of
    cable from the tester to the target. My selective memory wants to tell me
    that the unit I used was actually calibrated in feet; I know it got me to
    the exact connector where the break was. Unfortunately, where it was
    "buried" was almost dead center inside the tail section of an F-4 Phantom
    jet, directly above the exhaust nozzles. And the inside surface was lined
    with the points of sheet-metal screws. I had to resolder a coax to two
    pins of a 144-pin Cannon plug. By flashlight light. =:-O

    I think that kind of work was paying about $187.00/week in those days. But
    the food was provided, all you can eat, and was surprisingly good! Maybe
    not surprising - the cooks have to eat their own cooking, after all. ;-)

    Hope This Helps!
    Rich
     
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