Differential mode and common mode signal

Understanding the difference between common mode and differential mode signals is critical to understanding the relationship between pulsed magnetic circuits and working modules. important. Termination of transformers, common mode chokes, and autotransformers plays a key role in reducing common mode interference in local area network (LAN) and communication interface circuits. Common mode noise is a major cause of RF interference in communication systems using unshielded twisted pair cables, so understanding common mode noise will help to better understand the electromagnetic compatibility arguments of our concerned magnetic interfaces. The main purpose of this paper is to illustrate the key features of differential and common mode signals and the main uses of common mode chokes, autotransformer terminations, and why common mode signals can cause noise emissions on unshielded twisted pair cables. While introducing the characteristics of these signals, the methods commonly used to suppress general noise are also introduced.

Figure 1 Differential Mode Signal

Figure 2 Waveform of Differential Mode Signal
2 Differential Mode and Common Mode Signal
We studied a simple two-wire cable with a load impedance at its termination. The voltage of each line to ground is indicated by the symbols V1 and V2. The differential mode signal component is VDIFF, the common mode signal component is VCOM, and the parasitic capacitance existing between the cable and ground is Cp. Its circuit is shown in Figure 1, and its waveform is shown in Figure 2.
2.1 Differential Mode Signal
The pure differential mode signal is: V1=-V2(1)
The size is equal, the phase difference is 180°
VDIFF=V1-V2(2)
Since V1 and V2 are symmetrical to ground, no current flows through the ground. All differential mode currents (IDIFF) flow through the load.
When transmitting a signal by cable, the differential mode signal is a signal that carries the information "want". Differential mode devices are installed in the structure of a wireless transceiver used in a local area network (LAN) and communication. The sum of the instantaneous values ​​of the two voltages (V1+V2) is always equal to zero.
2.2 Common Mode Signal
The pure common mode signal is:
V1=V2=VCOM(3)
The size is equal, the phase difference is 0 deg;
V3=0(4)< Br> The circuit of the common mode signal is shown in Figure 3, and its waveform is shown in Figure 4.
Since there is no potential difference across the load, no current flows through the load. All common mode current flows to the ground through the parasitic capacitance between the cable and ground. When transmitting a signal by cable, since the common mode signal does not carry information, it is a signal that does not want to be "

Fig. 3 Common mode signal

Fig. 4 Waveform diagram of common mode signal

Fig.5 Differential mode signal in unshielded twisted pair system

Figure 6. Common mode signal in an unshielded twisted pair system
The sum of the two voltage instantaneous values ​​(V1 + V2) is not equal to zero. There is a varying potential difference on each cable relative to ground. This varying potential difference will emit electromagnetic waves from the cable.