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what is 75 ohm impedence?

Discussion in 'Electronic Basics' started by nanofuc, Jan 2, 2005.

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  1. nanofuc

    nanofuc Guest

    here what I know (or what I beleave to know): impedence is the
    difficult electric current encounter to pass trough a wire / device.
    It is possible to measure that keeping 2 terminals. Now, the coaxial
    cable is 75 ohm impedence wire, but where can I measure this
    impedence?between what point and what point of wire I can measure it?
    More, the impedence is variable in depending on the frequency
    alimentation: 75 ohm is refered at what range of frequency?
     
  2. Here is an easy way to think of cable impedance.
    Suppose you live in a world where 1 nanoSecond seems like a
    long time, and you can get several things done in such a time.
    You pick up a meter long piece of 75 Ohm twinax and hook
    your Ohm-meter up to it, at one end. It applies a voltage and
    measures the current, or applies a current and measures the
    voltage, computes the ratio, and then displays "impedance".
    You observe "75.000 Ohms", after a few picoSeconds of
    measurement. You start the next phase of whatever project
    induced you to make this measurement, leaving the meter
    connected and running. Then, about 10 nS later, a strange
    thing happens: The reading changes! The current or voltage
    applied by the meter when you hooked it up has traveled
    down the cable, bounced off the far end because there was
    no 75.000 Ohm terminator there, and come back so as to
    modify the observed terminal conditions.

    If you think through this story, and imagine that a circuit
    can indeed exist in that fantasy world, provided it delays
    its displays enough for us slower folk to perceive them,
    you should find your questions answered.
     
  3. Rich Webb

    Rich Webb Guest

    An easy way (for my simple mind ;-) to understand cable impendence is to
    picture the resistance to motion of my hand by a rope that's attached
    to, say, a block of concrete sitting on the floor.

    If the rope is short, say a foot or so, and the back-and-forth motion of
    my hand is also slow then the resistance is mostly a function of the
    load: how heavy the block and how much friction with the floor.

    But if the rope is longer, say twenty feet, and I move my hand very
    quickly then the resistance to motion that I feel is almost all due to
    moving the rope and not the load.

    If the "wavelength" of my hand is very short with respect to the rope, I
    see the characteristic impedance of the rope. If the wavelength is very
    long (say "DC" or a constant pull) then I see the resistance of the
    load.

    When you measure the resistance with a multimeter, you're seeing the
    results of a constant pull on the rope
     
  4. Miles Harris

    Miles Harris Guest

    IOW, if the cable is of an infinite length, it will show 75 ohms
    impedance; if the cable is of a short length *and* terminated with a
    75 ohm resistance, it will show 75 ohm impedance. Also, the
    chatracteristic impedance of coaxial line is *not*
    frequency-dependent. But the cable loss *will* vary with length.
     
  5. Impedance is a two dimensional version of DC resistance (volts per
    ampere). When you deal with AC signals instead of DC, the current is
    not necessarily in phase with the voltage, so one way to have the math
    take care of all possibilities is to have one dimension take account
    of current that is either in phase or exactly out of phase with the
    voltage (+- real current) and the other dimension take account of the
    component of the current that is +- 90 degrees out of phase with the
    voltage.

    Transmission lines carry AC energy as traveling waves, and the
    impedance of the transmission line tells you the ratio of voltage to
    current for those waves. Since those waves are a form of AC, the two
    dimensional measure of that ratio is needed, so it is called
    impedance, not resistance.

    If you wish to launch 7.5 volt waves into a transmission line of 75
    ohms impedance, the source will have to supply .1 ampere of current to
    the line. When the waves reach the other end, if there is not a 75
    ohm load absorbing that ratio of voltage to current, some of the
    energy will be launched back into the line from that end (reflected
    off the mismatched load). If the line is much less than a wavelength
    long, the reflections bounce back and forth so often and overlap so
    many times that it gets hard to measure the line impedance.
     
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