Concept of Characteristic Impedance

Characteristic Impedance refers to the impedance of the cable when the cable is infinitely long. It is a resistance name that prevents the current from passing through the conductor. It is not a conventional DC resistance. The Characteristic Impedance of a cable is a combination of electrical conductivity, capacitance, and resistance. Assume that an even cable is infinitely stretched. The Impedance at the transmitting end at a certain frequency is called Characteristic Impedance ). These parameters are determined by physical parameters such as the conductor size, the distance between conductors, and the characteristics of cable insulation materials.

When measuring the characteristic impedance, the equivalent resistance of the characteristic impedance can be used at the other end of the cable. The measurement result is related to the frequency of the input signal. The measurement unit of characteristic impedance is ohm. When the frequency in the high-frequency band is continuously increased, the characteristic impedance will be gradually fixed. For example, the coaxial line will be 50 or 75 ohm, while the Characteristic Impedance of common unshielded twisted pair is 100 ohm, and the characteristic impedance of shielded twisted pair is 150 ohm.

Normal physical operation relies on the constant characteristic impedance of the entire system cable and connector. The characteristic impedance can usually be slightly changed by the connection and termination of the cable. The hard turns or knots of the cable also change the characteristic impedance of the cable. In the case of nonconsecutive light, because the reflected signal is weak and attenuation of the cable, it can still run on the network. Large impedance disconnections will interfere with data transmission. This kind of discontinuity is caused by poor electrical connections, incorrect end-to-end knots, use of mismatched cables and mismatched connectors, and mismounting of twisted pair cables..

In the integrated wiring system, there are many taboos, such as: If the twisted pair wires are disconnected, they cannot be directly connected together, and the tension cannot be too large, do not mix cables with Different Characteristic Impedance for the outer diameter of a non-shielded cable whose bending radius is at least 4 times. These will lead to changes in the characteristic impedance. During the acceptance test, it is shown that the return loss test parameter is low.

1. Network Cabling Testing Instrument
2. Integrated Wiring Technology