1. Basic principle The filter consists of a low-pass filter circuit consisting of an inductor and a capacitor, which allows the current of the useful signal to pass and attenuates the interference signal with higher 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: (1) 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) by using the high-frequency and low-frequency isolation characteristics of capacitors; (2) Reflect the high-frequency interference current back to the interference source by using the impedance characteristics of the inductor; (3) According to the characteristic that interference suppression ferrite can absorb interference signals in a certain frequency band and convert them into heat, suitable interference suppression ferrite magnetic rings and magnetic beads can be directly sleeved on the cable to be filtered according to the frequency band of interference signals. Filter shape 2. The concept of filter is an important concept in signal processing. The function of the filter circuit is to reduce the AC component of the pulsating DC voltage as much as possible, keep its DC component, reduce the ripple coefficient of the output voltage, and make the waveform smoother. Waveforms before and after filtering Generally speaking, filtering can be divided into classical filtering and modern filtering. Classical filtering is an engineering concept based on Fourier analysis and transformation. According to the theory of advanced mathematics, any signal that meets certain conditions can be regarded as the superposition of infinite sine waves. In other words, the engineering signal is a linear superposition of sine waves with different frequencies, and the sine waves with different frequencies that make up the signal are called frequency components or harmonic components of the signal. A circuit that only allows signal components within a certain frequency range to pass normally and prevents other frequency components from passing is called a classical filter or filtering circuit. Filtering Time Domain and Frequency Domain Diagram In classical filtering and modern filtering, the filter model is actually the same (the hardware filter has not made much progress), but modern filtering has also added many concepts of digital filtering. 3. Main parameter bandwidth: refers to the spectrum width to be passed, BW=(f2-f 1). F 1 and f2 are based on the insertion loss at the center frequency f0. Passband ripple: the change of insertion loss in passband with frequency. The in-band fluctuation in the bandwidth of 1dB is 1dB. Ripple: refers to the peak value of insertion loss fluctuating with frequency based on the average loss curve in the bandwidth (cutoff frequency) of 1dB or 3dB. Delay (Td): refers to the time required for the signal to pass through the filter, which is numerically the derivative of the transmission phase function to the angular frequency, that is, Td=df/dv. In-band phase linearity: this index indicates the phase distortion amplitude of the transmission signal introduced by the filter in the passband. The filter designed according to the linear phase response function has good phase linearity. Insertion loss: Due to the introduction of the filter, the attenuation of the original signal in the circuit shows the loss at the center or cut-off frequency. If full-band insertion loss is required, it should be emphasized. Return loss: The decibel number (dB) of the ratio of input power to incident power of port signal is also equal to 20Log 10ρ, where ρ is the voltage reflection coefficient. When the input power is completely absorbed by the port, the return loss is infinite. Center frequency: generally, the frequency f0 of the filter passband is taken as f0=(f 1+f2)/2, and f 1 and f2 are the edge frequency points of the bandpass or bandstop filter, which is relatively reduced by 1dB or 3dB.. Narrow band filters usually calculate the passband bandwidth with the minimum insertion loss as the center frequency. Cut-off frequency: refers to the right frequency point of low-pass filter passband and the left frequency point of high-pass filter passband. Generally, it is defined by the relative loss point 1dB or 3dB. The reference standard of relative loss is low-pass insertion loss based on DC, and Qualcomm insertion loss based on high-pass band without parasitic stopband. In-band standing wave ratio (VSWR): an important index to measure whether the signal in the filter passband matches well with the transmission. The ideal VSWR= 1: 1, and VSWR >: 1. For a practical filter, VSWR 1), KxdB=BWxdB/BW3dB, (x can be 40dB, 30dB, 20dB, etc. ) all satisfied. The more filter orders, the higher the rectangle-that is, the closer k is to the ideal value 1, the more difficult it is to make. 4. The function (1) separates the useful signal from the noise to improve the anti-interference and signal-to-noise ratio of the signal; (2) filtering out uninteresting frequency components to improve the analysis accuracy; (3) Separate a single frequency component from the complex frequency component. Filter shape Figure 5 The filter shape filter is used to improve the power quality, improve the linearity of the circuit, and reduce all kinds of clutter, nonlinear distortion interference and harmonic interference. For weapon systems, the places where filters are used are: (1) Besides the power filters installed on the main power distribution system and the power distribution system, filters should also be installed on the power supply that enters the equipment. It is better to use a line-to-line filter than a line-to-ground filter. (2) For the equipment sensitive to impulse interference and transient interference, a filter should be installed at the negative terminal when the isolation transformer is used for power supply. (3) When supplying power to a weapon system containing an electric explosive device, a filter should be installed. When necessary, the lead wire of the electric explosion device should also be equipped with a filter. (4) At the interface between each subsystem or device, there should be a filter to suppress interference and ensure compatibility. (5) For the control signals of equipment and subsystems, filters or bypass capacitors should be added at the input and output ends. This paper summarizes the principle, filtering concept, parameters, functions and matters needing attention of the filter. There are many kinds of filters, and each filter has different performance characteristics. Therefore, when selecting the filter screen, it is usually necessary to comprehensively consider the actual use environment and performance requirements of customers in order to make a correct, effective and reliable choice.