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Voltages along on a transmission line...

Discussion in 'Electronic Design' started by billcalley, Jun 5, 2007.

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  1. billcalley

    billcalley Guest

    I know this should be obvious, but I just can't visual the power
    or voltage distribution on a short, matched transmission line that is,
    lets say, just one wavelength long: For such a short line, wouldn't
    the voltage or power vary in amplitude along the matched 360 degree
    long line, considering that this signal will sinusoidally start out at
    zero volts at the beginning of the transmission line (0 degrees), then
    90 degrees later reach a peak, then 90 degrees later drop to 0V, etc?
    Yet I never here mention of any amplitude variations on a lossless
    transmission line except when I read of standing waves on a mismatched
    line -- but why don't we get measurable amplitude variations due to
    the rise and fall of the RF signal itself (which would affect RF
    circuits vs. distance at high microwave frequencies)??

    Confused,

    -Bill
     
  2. Phil Allison

    Phil Allison Guest

    ** Groper in the dark alert

    ** You are indeed.

    Try to figure out the difference between "amplitude" and instantaneous
    value.

    All continuous waves have both.


    ....... Phil
     
  3. John Larkin

    John Larkin Guest

    Yes, at any one instant it might look like that. A little later, it
    will look different. The generator is extruding a sine wave that
    travels along the line, towards the load, at a goodly fraction of the
    speed of light. At any intermediate point along the line, there's the
    same sine wave voltage, just shifted in phase.
    If you measure any point along the line with an oscilloscope, you'll
    see the sine wave rise and fall. If you measure it with a voltmeter,
    it will look the same everywhere. It's a matter of the time scale over
    which you take the measurements.

    If the line is unmatched, there will be waves traveling in both
    directions, in which case the voltmeter will see different voltages in
    different places, as the waves add in some locations and subtract in
    others.

    John
     
  4. billcalley

    billcalley Guest


    Fantastic info John -- that really cleared up my confusion on this
    subject!

    Thanks again for the great help!

    -Bill
     
  5. PeteS

    PeteS Guest

    Although you seem to have cleared this up, here's a thought exercise:

    You have a driver output that has an effective rise time of 300
    picoseconds (not uncommon nowadays). It is driving a 6 inch track.

    On FR4, the transmission velocity is approximately 160 picoseconds per
    inch. So at the initiation of the transmission, the line is 0V everywhere.

    At 50% amplitude output (at the driver, and assuming 3.3V systems and a
    linear output) the voltage 1 inch away **is still zero**; i.e. the
    energy has not yet reached this point.

    Now continue this until the entire system has stabilised.

    Cheers

    PeteS
     
  6. A matched line will have uniform RF voltage along its length, since
    there are no standing waves. A (theoretical) time domain voltage probe
    would show the same signal, delayed in phase by (vf * distance from
    source)/wavelength in free space.

    Frank Raffaeli
     
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