The principle of shielded twisted-pair cables suppressing common-mode interference

Twisted-pair cable consists of two insulated wires twisted together, making it particularly suitable for differential signal transmission. Compared to parallel wires, it can more effectively suppress interference. The characteristics of twisted-pair cable are reflected in the following two aspects:

1. Elimination of capacitive coupling

Compared to parallel wires, the coupling capacitance values ​​of each wire in a twisted-pair cable to the interference source or ground are closer, resulting in a more balanced impedance, as shown in Figure 1.

Figure 1

Because the twisted-pair cables are tightly wound together, the coupling capacitance between the two wires and the noise source, as well as their impedance to ground, are essentially the same. The interference current flowing from the noise source into the two signal lines is basically the same, and the difference between the two signal lines remains unchanged. The current in the coupling capacitor is converted into common-mode interference. As shown in Figure 2, the coupling capacitors C1=C2 and Z1=Z2. The current flowing from the interference source into C1 and C2 is equal, meaning that the voltages generated by wires 1 and 2 are equal, Vn=0. Since differential signal transmission has good common-mode rejection capability, the effect of capacitive coupling can be eliminated.

2. Eliminating Inductive Coupling

If parallel lines are used, the two signal lines will form a very narrow loop, which will pick up magnetic field interference from the environment. The structure of a twisted-pair cable involves twisting the two conductors of the transmission line at a fixed interval, causing the direction of the electromotive force induced by the magnetic field to reverse at each adjacent "small loop," thus sequentially canceling it out. From a circuit perspective, the mutual inductance at each adjacent "small loop" is opposite to the noise source, and the overall mutual inductance of the conductors becomes zero. As shown in Figure 3, when parallel lines are subjected to external magnetic field interference, the induced currents in the two conductors cannot cancel each other out, resulting in a large induced voltage that affects signal transmission. The structure of a twisted-pair cable, however, causes the induced currents in the conductors to cancel each other out, preventing the generation of an induced voltage.

In differential transmission applications, twisted-pair cables can eliminate capacitive and inductive coupling with external interference sources. Therefore, twisted-pair cables are widely used in differential signal transmission applications such as CAN and RS-485.

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