What is characteristic impedance?

Discussion in 'General Electronics Chat' started by twomilimeter, Mar 21, 2010.

  1. twomilimeter

    twomilimeter Thread Starter New Member

    Joined:
    Dec 23, 2009
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    4
    Hi,

    Based on my understanding, characteristic impedance is the constant and instantaneous impedance a signal sees as it travels along a transmission line. If the impedance along the line is different, then the voltage will get reflected. But
    [1] what is causing the voltage to be reflected?
    [2] has characteristic impedance related to the resonant in the LC circuit?
    [3] is it necessary that the transmission line be purely resistive or could it be a combination of RLC network that form a 50ohm impedance line?

    Kindly advise and thank you for the help.

    Twomilimeter
  2. rjenkins

    rjenkins AAC Fanatic!

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    Sheffield, England
    Have a look here for some basic info:
    http://en.wikipedia.org/wiki/Characteristic_impedance

    Think of a single pulse of power, and what voltage that equates to on a single resistor.
    For the same power, a higher resistance needs higher voltage or a lower resistance needs lower voltage.

    Now think of that pulse travelling along a coaxial cable. If the impedance suddenly changes the voltage must change to suit. If the impedance gets higher, more voltage is needed and a negative reflection will go back down the cable. If the impedance gets lower, a positive reflection occurs. The amplitude of reflection depends on the amount of impedance change.

    To visualise this, think of a single wave or ripple travelling down a canal with perfectly even sides. If the canal suddenly gets wider or narrower, a wave is reflected back, either a dip or bump (negative or positive) in the surface.


    A transmission line can be modelled as repeated descrete sections with capacitance & inductance (and resistance, though that is not needed to set the impedance).
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