There are three main measures to prevent electromagnetic interference, namely shielding, filtering and grounding. Shielding alone often does not provide complete EMI protection because the cable on the device or system is the most effective interference receiving and transmitting antenna. Many devices have no problem when they perform electromagnetic compatibility experiments. However, when the two devices are connected, the electromagnetic compatibility requirements are not met. This is the function of the cable to receive and radiate the antenna. The only measure is to add a filter to cut off the path of electromagnetic interference along the signal line or power line. Together with the shielding, it forms a perfect electromagnetic interference protection, whether it is to suppress the interference source, eliminate the coupling or improve the resistance of the receiving circuit. Filtering techniques can be used.
First, the type of online interference
Interference currents on the line can be divided into two categories according to their flow paths: one is differential mode interference current and the other is common mode interference current. The differential mode interference current is the interference current flowing between the live line and the neutral line. The common mode interference current is the interference current flowing between the live line, the neutral line and the earth (or other reference object), due to the suppression of these two types of interference. Different, so correctly identifying the type of interference is a prerequisite for implementing a correct filtering method.
Common mode interference is generally caused by interfering electromagnetic waves from the outside or other parts of the circuit in the loop of the cable and the "ground". Sometimes, due to the different "ground" potentials at both ends of the cable, common mode interference is also generated. It is very harmful to electromagnetic compatibility. On the one hand, common mode interference will cause the cable to emit strong electromagnetic radiation, interfere with other parts of the circuit or peripheral electronic equipment; on the other hand, if the circuit is unbalanced, in the cable When the amplitude and phase of the common mode interference current on different wires are different, the common mode interference will be converted into differential mode interference, which will seriously affect the quality of the normal signal, so people are trying to suppress common mode interference.
Differential mode interference is mainly caused by electromagnetic interference generated by other parts of the circuit entering the signal line loop through conduction or coupling, such as higher harmonics, self-oscillation, and grid interference. Since the differential mode interference current flows in the loop simultaneously with the normal signal current in the same direction, its interference with the signal is severe and must be suppressed.
In summary, in order to meet the requirements of electromagnetic compatibility, both common mode interference and differential mode interference should be suppressed.
Second, the classification of the filter
A filter is a network of resistors, inductors, and capacitors that concentrate parameters, or resistors, inductors, and capacitors that distribute parameters. This network allows some frequencies to pass while suppressing other frequency components. The interference filter has a low-pass filter, a high-pass filter, a band-pass filter, a band-stop filter, and the like according to the relative relationship between the frequency of the interference signal to be filtered and the operating frequency.
The low-pass filter is the most commonly used one, and is mainly used when the interference signal frequency is higher than the operating signal frequency. In digital devices, pulse signals are rich in higher harmonics. These higher harmonics are not necessary for circuit operation, but they are a strong source of interference. Therefore, in the digital circuit, the commonly used low-pass filter filters out unnecessary higher harmonics in the pulse signal, and only retains the lowest frequency that can maintain the normal operation of the circuit. The power line filter is also a low-pass filter that allows only 50 Hz of current to pass, which greatly attenuates other high-frequency interference signals.
● A commonly used low-pass filter is a combination of an inductor and a capacitor. The capacitor is connected in parallel between the signal line to be filtered and the signal ground (filtering differential mode interference current) or between the signal line and the chassis ground or ground. (Filter out common mode interference current) The inductor is connected in series on the signal line to be filtered. According to the circuit structure, there are single capacitor type (C type), single inductor type, L type and reverse type, T type, π type.
● High-pass filter is used when the interference frequency is lower than the signal frequency, such as the interference caused by power supply harmonics on some sensitive signal lines close to the power line.
● Bandpass filter is used when the signal frequency only occupies a narrow bandwidth. For example, a bandpass filter is installed on the antenna port of the communication receiver, and only the communication signal is allowed to pass.
● The band-stop filter is used for the interference frequency bandwidth is narrow, and the signal frequency is wide. For example, a band-stop filter with a band-stop frequency equal to the transmission frequency of the station should be installed at a cable port close to the high-power station.
There are two main differences between the filter circuits of different structures:
1. The more filter components in the circuit, the greater the attenuation of the filter stopband and the shorter the transition between the filter passband and the stopband.
2. Filter circuits of different structures are suitable for different source impedances and load impedances, and their relationship should follow the principle of impedance mismatch. However, it should be noted that the impedance of the actual circuit is difficult to estimate, especially at high frequencies (electromagnetic interference problems often occur at high frequencies). Due to the influence of circuit parasitic parameters, the impedance of the circuit varies greatly, and the impedance of the circuit is often It is also related to the operating state of the circuit, plus the circuit impedance is different at different frequencies. Therefore, in practice, which filter is effective depends mainly on the results of the test.
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