What is characteristic impedance?

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

  1. twomilimeter

    twomilimeter Thread Starter New Member

    Dec 23, 2009

    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.

  2. rjenkins

    rjenkins AAC Fanatic!

    Nov 6, 2005
    Have a look here for some basic info:

    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).