In electrical engineering, a symmetrical cable is a cable for symmetrical signal transmission . It has one or more wire pairs that are electrically isolated from one another and from one another. If the cable is provided with a screen , this is not used for signal transmission; although it forms an additional conductor, it is not counted as a wire.
To optimize the electromagnetic properties, the wires of a wire pair are usually twisted against each other ("stranded"); such a cable is also called a twisted pair cable .
Symmetrical signal transmission has its origins in analog telephone and sound technology, where it is still used today. The twisted pair cable is a further development and is mainly used in digital data transmission technology.
There are now cables with up to 2000 copper wire pairs.
In contrast to the symmetrical is the asymmetrical signal transmission . The signals generate a potential difference to reference earth on just one wire . Interference has a direct effect as a potential difference in the useful signal at the receiver.
Symmetrical data transmission lines avoid this effect through their special transmission technology and the targeted twisting of the two individual conductors: With symmetrical transmission, the signal from the transmitter is applied to the two wires of a pair in antiphase . This means that one wire carries exactly the opposite voltage of the other wire. In other words: the transmitter feeds the same signal into the cable twice, but with opposite polarity . The receiver forms the difference between the two voltages. Ideally, external interference occurs in the same direction on both wires due to the symmetry, so that they cancel each other out at the receiver.
Symmetrical cables in practice
In practice, however, this behavior cannot quite be achieved, but the remaining interference fields are very low. The remaining interference voltage varies depending on the transmission technology at a few microvolts; with 10BASE-T, for example, the remaining interference voltage is a maximum of 92 µV.
Interfering influences can be further reduced by shielding the cable.
Example: With what is known as phantom power , a microphone signal is transmitted symmetrically on a shielded pair of wires , while the supply voltage for the microphone is applied equally between the two wires of the pair of wires on the one hand and the shield on the other. Due to the symmetrical transmission, neither the microphone output nor the processing input amplifier “see” the supply voltage of the microphone: the microphone signal is only slightly influenced by the supply.
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