# What is characteristic impedance?

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

1. ### twomilimeter Thread Starter New Member

Dec 23, 2009
4
0
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 AAC Fanatic!

Nov 6, 2005
1,015
69
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).