talk about characteristic impedance vs electrical length

[kent616]

talk about characteristic

could u talk about characteristic impedance vs electrical length??

How they related to each other??

 

[IanP]

impedance vs length

The CharacteristicImpedance of a Transmission line is determined by taking the limit of the Impedance given by the unit length capacitance and unit length inductance of a transmission line as unit length tends towards zero. The CharacteristicImpedance is irrespective of the SeriesResistance of the cable and is only valid when the cable length is more than 4-8 times longer than the electrical length of the smallest electrical feature (usually the rising edge). This is because a reflection on a short cable will return before the edge is complete, avoiding the (maximum) effect of the TransmissionLine's CharacteristicImpedance.

 

[biff44]

electrical length - degree

Strictly speaking, they are not related at all.

Characteristic impedance, on something like a 2 conductor cable, is simply a measure of the voltage divided by current at a physcial point. More specifically, it is the amplitude of the voltage wave travelling in one direction on the cable divided by the amplitude of the current wave travelling in that same direction.

Electrical length implies time delay or phase shift.

impedance and electrical length are independent.

You can have a 50 ohm cable that is 1 degree long, or 100000000 degrees long. In either case it is exactly the same cable, only with 2 different lengths.

You can have a 20 degree long cable that is 10 ohms, and another 20 degree long cable that is 200 ohms. IF the two cables are made of the same material (same dielectric material mostly), both would have the same physical length, but quite different cross sectional geometries.

Note, a cable with an electrical length of "20 degree long" has a thru phase shift of 20 degrees at the ONE frequency that you make the measurement at. At a different frequency, the electrical length will be a different phase.