Twisted pair principle

- Jun 06, 2018-

The twisted pair is formed by twisting a pair of mutually insulated metal wires. In this way, not only can some electromagnetic wave interference from the outside world be resisted, but also mutual interference between multiple pairs of twisted wires can be reduced. Twist the two insulated wires together, the interference signal is consistent on the two twisted wires (this interference signal is called common mode signal), and the common mode can be used in the differential circuit receiving the signal. The signal is cancelled, thereby extracting a useful signal (differential mode signal).


The function of the twisted pair is to make the noise generated by the external interference on the two wires (in the professional field, the useless signal is called noise) the same, so that the subsequent differential circuit extracts the useful signal, the differential circuit is a subtraction circuit, two The in-phase signals (common mode signals) of the inputs are canceled each other (mn), and the inverted signals are equivalent to x-(-y), which is enhanced. In theory, in the twisted pair and differential circuits, m=n, x=y, which is equivalent to the interference signal being completely eliminated, and the useful signal is doubled, but there is a certain difference in actual operation.


In a cable sleeve, different pairs have different twist lengths. Generally speaking, the twist length is between 38.1mm and 140mm, twisted counterclockwise, and the twist length of the adjacent pair is 12.7mm. Within. The length of a twisted pair twisted loop is called the pitch. The smaller the pitch (the denser the twisted wire), the stronger the anti-interference ability.