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Principle of twisted pair

Updated:26-11-2021
Summary:

Twisted pair (TP) is one of the most commonly used tran […]

Twisted pair (TP) is one of the most commonly used transmission media in integrated wiring projects. It is composed of two copper wires with an insulating protective layer. Two insulated copper wires are twisted together at a certain density, and the radio waves radiated by each wire during transmission will be offset by the radio waves emitted by the other wire, effectively reducing the degree of signal interference.
Twisted pair wire is generally made of two 22-26 gauge insulated copper wires twisted with each other, and the name "twisted pair" is also derived from this. In actual use, the twisted-pair wire is wrapped in an insulated cable sleeve by multiple pairs of twisted-pair wires. If one or more pairs of twisted pairs are placed in an insulating sleeve, it becomes a twisted pair cable, but in daily life, the "twisted pair cable" is generally directly referred to as the "twisted pair".
Compared with other transmission media, twisted pair cables are subject to certain restrictions in terms of transmission distance, channel width and data transmission speed, but the price is relatively low.

Principle

Twisted pair wire is formed by twisting a pair of mutually insulated metal wires. In this way, it can not only resist a part of electromagnetic wave interference from the outside, but also reduce the mutual interference between multiple pairs of twisted wires. The two insulated wires are twisted together, and the interference signal acts on the two wires that are twisted together. The interference signal is called the common mode signal. The common mode can be used in the differential circuit of the received signal. The signal is eliminated to extract the useful signal (differential mode signal).
The function of the twisted pair cable is to make the noise generated by external interference on the two wires (in the professional field, the useless signal is called noise) the same, so that the subsequent differential circuit can extract the useful signal. The differential circuit is a subtraction circuit. The in-phase signals (common-mode signals) at the two input ends cancel each other out (mn), and the inverted signal is equivalent to x-(-y) and is enhanced. Theoretically, m=n and x=y in twisted pair and differential circuit are equivalent to the interference signal is completely eliminated and the useful signal is doubled, but there are certain differences in actual operation.
In a cable sleeve, different wire pairs have different twisting lengths. Generally speaking, the twisting length is within 38.1mm~140mm, twisted in a counterclockwise direction, and the twisting length of adjacent wire pairs is 12.7mm. Within. The length of a twisting cycle of a twisted pair is called pitch. The smaller the pitch (the denser the twisted wire), the stronger the anti-interference ability

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