Balanced line

As Symmetrical cable is referred to in the electrical cables to be balanced signal transmission. It has one or more of each other and mutually electrically insulated pairs. If the line is provided with a shield, it is not used for signal transmission; Although it forms an additional conductor, which is not counted as core.

To optimize the electromagnetic properties of the wires of a pair are usually twisted against each other ( " stranded "); such a cable is also called twisted- pair cable.

The Balanced signal transmission has its origins in the analog phone and sound equipment, where it still applies today. The twisted -pair cable is a further development and is mainly used in digital data transmission technology.

There are now cables up to 2000 pairs of copper wire.

General

In contrast to the symmetric is asymmetric signal transmission. The signals produce only one wire a potential difference to the reference earth. Interference is picked up directly from the potential difference in the useful signal at the receiver.

Symmetric data transmission lines to avoid this effect by their specific transmission technology and the targeted twisting of the two conductors: In the symmetric transfer the signal from the transmitter is placed on the opposite phase two wires of a pair. This means that a vein runs exactly the opposite voltage of the other wire. In other words, the transmitter feeds the same signal twice, but with opposite polarity, into the cable. The receiver takes the difference of the two voltages. Ideally done outside interference by the symmetry in the same direction on both wires, so that they cancel each other at the receiver.

Balanced cables in practice

In practice, however, said behavior can not be achieved in whole, but the remaining interference from falling very low. The remaining noise voltage varies depending on the transmission technology in a few microvolts; for 10BASE - T is about is the remaining noise voltage maximum of 92 microvolts.

By shielding of the cable to disturbing influences can be further reduced.

Example: In the so-called phantom power is transferred to a shielded twisted pair microphone signals symmetrical, while the supply voltage for the microphone is evenly placed between two wires of the pair of wires on the one hand and shield on the other. Due to the symmetrical transmission "see" neither microphone nor the output for further processing input amplifier, the supply voltage of the microphone: the microphone signal is only slightly influenced by the supply.