When the interference frequency is lower than the signal frequency, use a high-pass filter, such as filtering out the interference caused by power harmonics on some sensitive signal lines near the power line.
● When the signal frequency only occupies a narrow bandwidth, use a band-pass filter. For example, a band-pass filter should be installed on the antenna port of the communication receiver to allow only communication signals to pass.
● Band-stop filter is used when the bandwidth of interference frequency is narrow and the signal frequency is wide. For example, a band-stop filter with a band-stop frequency equal to the transmitting frequency of the radio station is installed at the cable port near the high-power radio station.
There are two main differences between different filter circuits:
The more filter devices in 1. circuit, the greater the attenuation of filter stopband and the shorter the transition band between filter passband and stopband.
2. Filter circuits with 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 it is difficult to estimate the impedance of the actual circuit, especially at high frequencies (electromagnetic interference problems often occur at high frequencies). Due to the influence of parasitic parameters of the circuit, the impedance of the circuit changes greatly, and the impedance of the circuit is often related to the working state of the circuit, and the impedance of the circuit is different at different frequencies. Therefore, in actual operation, which filter is effective mainly depends on the test results.
4. Basic principle of filter
The filter is a low-pass filter circuit composed of inductance and capacitance, which allows the current of useful signal to pass and greatly attenuates the interference signal with high frequency. Because there are two kinds of interference signals: differential mode and * * * mode, the filter should attenuate these two kinds of interference. There are three basic principles:
A) Using the characteristics of capacitor passing high frequency and isolating low frequency, the high frequency interference current of live wire and neutral wire is introduced into the ground wire (* * * mode), or the high frequency interference current of live wire is introduced into the neutral wire (differential mode);
B) reflecting the high-frequency interference current back to the interference source by using the impedance characteristics of the inductance coil;
C) Using the characteristic that the interference suppression ferrite can absorb the interference signal in a certain frequency band and convert it into heat, the appropriate interference suppression ferrite magnetic ring and magnetic bead are selected for the interference signal in a certain frequency band and directly covered on the cable to be filtered.
5 Importance of high frequency insertion loss of power filter
Although the restriction on conducted emission in various electromagnetic compatibility standards is only 30MHz (50MHz in the old military standard and 10MHz in the new army standard), the influence of high frequency can never be ignored in the suppression of conducted emission. Because the high-frequency conduction current on the power line will cause radiation, the radiation emission of the equipment will exceed the standard. In addition, the test waveform in transient pulse sensitivity test often contains high-frequency components. If these high-frequency interferences are not filtered, the sensitivity test of equipment will also fail.
There are two main reasons for the poor high frequency characteristics of power line filters. One is the spatial coupling caused by internal parasitic parameters, and the other is the imperfection of filter elements. Therefore, the method to improve the high-frequency characteristics also starts from these two aspects.
Internal structure: the connecting wires of the filter should be arranged in one direction according to the circuit structure. If space permits, a certain distance should be kept between inductance and capacitance. If necessary, some isolation plates can be set to reduce spatial coupling.
Inductor: Control the parasitic capacitance of the inductor as described above. If necessary, use multiple inductors in series.
Differential mode filter capacitor: the lead of the capacitor should be as short as possible. Understand the meaning of this requirement: the connection between the capacitor and the wires to be filtered (live wire and neutral wire) should be as short as possible. If the filter is mounted on the circuit board, the traces on the circuit board are also equivalent to the leads of the capacitor. At this time, we should pay attention to the shortest capacitor lead to ensure the timing.
* * * Mode capacitor: The lead of the capacitor should be as short as possible. The understanding and precautions of this requirement are the same as those of differential mode capacitors. However, the * * * mode high-frequency filtering characteristics of the filter are mainly guaranteed by the * * * mode capacitor, and the frequency of * * * mode interference is generally high, so the high-frequency characteristics of the * * * mode filtering capacitor are more important. Using three-terminal capacitor can obviously improve the high frequency filtering effect. But pay attention to the correct use of three-terminal capacitor. That is, the grounding wire should be as short as possible, and the length of the other two wires has little effect on the effect. If necessary, through capacitors can be used. At this time, the performance of the filter itself can be maintained above 1GHz.
Special note: When the radiation emission of the equipment at a certain frequency does not meet the standard requirements, don't forget to check the * * * mode conducted emission of the power cord at that frequency. Radiation emission may be caused by * * * mode emission current.
6 filter selection
According to the characteristics of interference sources, frequency range, voltage and impedance, and the requirements of load characteristics, it is generally considered that the electromagnetic interference filter is required to meet the attenuation characteristics required by the load in the corresponding working frequency band. If the attenuation of a filter can not meet the requirements, multi-stage cascade can be used to obtain higher attenuation than single stage, and different filters can be cascaded to obtain good attenuation characteristics in a wide frequency band. Secondly, in order to meet the requirements of the working frequency and the frequency to be suppressed of the load circuit, if the frequency to be suppressed is very close to the frequency of the useful signal, a filter with very steep frequency characteristics is needed to filter out the suppressed interference frequency, and only the signal with the useful frequency is allowed to pass. Thirdly, the impedance of the filter must not match the impedance of the interference source and the load at the required frequency. If the load is high impedance, the output impedance of the filter should be low impedance. If the impedance of power supply or interference source is low, the input impedance of the filter should be high; If the impedance of power supply or interference source is unknown or varies in a large range, it is difficult to obtain stable filtering characteristics. In order to obtain good and stable filtering characteristics of the filter, a fixed resistor can be connected in parallel with the input and output terminals of the filter. Fourthly, the filter must have a certain withstand voltage, and the filter should be selected according to the rated voltage of power supply and interference source, so that it has a sufficiently high rated voltage to ensure reliable operation under all expected working conditions and withstand the impact of input instantaneous high voltage. Fifth, the allowable pass rate of the filter should be consistent with the rated current of continuous operation in the circuit. Higher rated current will increase the volume and weight of the filter; Low rated current will reduce the reliability of the filter. Sixth, the filter should have sufficient mechanical strength, simple structure, light weight, small volume, convenient installation, safety and reliability.
7 the use of filters
In order to improve the quality of power supply, improve the linearity of the circuit and reduce all kinds of clutter, nonlinear distortion interference and harmonic interference, filters are used. As far as the weapon system is concerned, the places where filters are used are as follows: First, in addition to the power filters installed in the general distribution system and distribution system, filters should also be installed in the power supply entering the equipment. It is best to use line-to-line filters instead of line-to-ground filters. Secondly, when the equipment sensitive to impulse interference and transient interference is powered by isolation transformer, a filter should be installed at the negative terminal. Third, when supplying power to a weapon system with an electric explosive device, a filter should be added. When necessary, the lead wire of the electric explosion device should also be equipped with a filter. Fourthly, at the interface of subsystem or equipment, there should be filters to suppress interference and ensure compatibility. Fifth, filters or bypass capacitors should be added at the input and output ends of control signals of equipment and subsystems.