[Transmission Line] Constant Impedance -> how?

[ivlsi]

Hi All,

As far as I know, an impedance over the whole transmission line should be constant and usually should match 50Ohm.

How could it be achieved? An impedance depends on the conductor length. So, as far as the signal propagates over the transmission line, it should "see" a different impedance since it "see" a different length of the transmission line ahead.

Could anyone explain how the propagated signal "see" the same impedance over the whole length of the transmission line?

Thank you!

 

[KlausST]

Hi,

It seems you are mixing "wave impedance" with the common impedance.
--> Read about "wave impedance".

and usually should match 50Ohm

Not always 50 Ohms. (Wave impedance)
There are different systems: some are 50 Ohms, some are 75 Ohms, some are 120 Ohms...
To avoid echoes: source impedance (driver side) = wave impedance of cable = terminating resistance (receiver side).

Wave impedance depends on the construction and the material of a transmission line.
It does not depend on length.

Klaus

 

[crutschow]

The "characteristic impedance" of a line is determine by the relative resistance, inductance, and capacitance of the line per unit length so is independent of the line length. The equation for that is:

The reason for this is that the impedance is seen by the wave-front as it propagates down the line.
It never "sees" the whole line at any one instant due to the propagation speed of the wave-front (which is some large fraction of the speed of light).

Note that this characteristic impedance value is only true for cables that are electrically much longer than the wavelength or risetime of the signal.