What is the cable Characteristic Impedance ?

Characteristic impedance corresponds to the input impedance of a uniform transmission line of infinite length, i.e.,

It also corresponds to the input impedance of a transmission line of finite length that is terminated in its own characteristic impedance.

In general, the characteristic impedance is a complex number with a resistive and reactive component. It is a function of the frequency of the applied signal, and is unrelated to length. At very high frequencies, the characteristic impedance asymptotes to a fixed value which is resistive. For example, coaxial cables have an impedance of 50 or 75 Ohms at high frequencies. Typically, twisted-pair telephone cables have an impedance of 100 Ohms above 1 MHz.

Characteristic impedance is of prime importance for good transmission. Maximum power transfer occurs when the source has the same impedance as the load. Thus for sending signals over a line, the transmitting

equipment must have the same characteristic impedance as the line to get the maximum signal into the line. At the other end of the line, the receiving equipment must also have the same impedance as the line to be able to get the maximum signal out of the line.

Where impedances do not match, some of the signal is reflected back towards the source. In many cases this reflected signal causes problems and is therefore undesirable.

The formulae for characteristic impedance are:

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