electronic device has two types: radiated noise and conducted noise generated from the power cable and signal cable. These two categories are divided into two types: common mode noise and differential mode noise [1].
Differential mode conducted noise is the noise current generated by the internal noise voltage of the electronic device and the same path as the signal current or the power supply current, as shown in Fig. 7. The way to reduce this noise is to combine a differential mode choke, a shunt capacitor, or a low-pass filter with a capacitor and inductor on the signal and power lines to reduce high-frequency noise, as shown in Figure 8.
The differential mode radiated noise is the radiation produced by the signal current loop in the cable of Figure 7. The intensity of the electric field generated by this noise is inversely proportional to the distance from the cable to the observation point, proportional to the square of the frequency, and proportional to the area of the current and current loop. Therefore, the method of reducing this radiation is to add an LC low-pass filter at the signal input to prevent noise current from flowing into the cable; use a shielded cable or a flat cable to transmit return current and signal current in adjacent wires to make the loop The area is reduced.
Common mode conducted noise is generated by the noise current flowing between the earth and the cable, driven by the noise voltage inside the device, as shown in Figure 9. The method of reducing the common mode conduction noise is to connect a common mode choke coil in a signal line or a power line, connect a capacitor between the ground and the wire, form an LC filter for filtering, and filter common mode conducted noise. Its circuit is shown in Figure 10. The common mode choke coil is formed by winding the neutral line of the power line and the live line (or the return line and the signal line) in the same direction on the ferrite core, and the differential mode signal current and the source current impedance flowing between the lines. It is very small, and the common mode current impedance flowing between the two wires and the ground is very large. FIG 7
differential mode noise suppression
8
FIG differential mode noise of the common mode noise
9
Figure 10 Common mode noise suppression
Common mode radiated noise is due to the common mode voltage on the cable port, driven by the common mode current flowing from the ground to the cable. The intensity of the radiated electric field is inversely proportional to the distance from the cable to the observation point (when the cable length is shorter than the wavelength of the current) is proportional to the frequency and the length of the cable. Ways to reduce this radiation are by using ground planes on the board to reduce ground impedance and using LC low-pass filters or common mode chokes at the ports of the cable. In addition, minimizing the length of the cable and using shielded cables also reduces radiation.
In some circuits, the anti-interference transformer shown in Figure 11 can also be connected to prevent differential mode and common mode noise.