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Passive filter, also known as LC filter, is a filter circuit composed of inductance, capacitance and resistance, which can filter out one or more harmonics. The most common and easy to use passive filter structure is to connect the inductance and capacitance in series, which can form a low impedance bypass for the main harmonics (3, 5 and 7); Single tuned filter, double tuned filter and high pass filter are all passive filters.
advantage
Passive filter has the advantages of simple structure, low cost, high operation reliability and low operation cost. It is still widely used as a harmonic control method.
classification
The characteristics of LC filter shall meet the specified technical index requirements. These technical requirements are usually working attenuation in frequency domain, or phase shift, or both; Sometimes, time response requirements in time domain are proposed. Passive filters can be divided into two categories: tuned filters and high pass filters. At the same time, according to different design methods, it can be divided into image parameter filter and working parameter filter.
Tuning filter
The tuning filter includes a single tuning filter and a double tuning filter, which can filter out one (single tuning) or two (double tuning) harmonics. The frequency of the harmonics is called the resonant frequency of the tuning filter.
High pass filter
High pass filter, also known as amplitude reduction filter, mainly includes first-order high pass filter, second-order high pass filter, third-order high pass filter and C-type filter, which are used to significantly attenuate harmonics lower than a certain frequency, which is called the cut-off frequency of high pass filter.
Image parameter filter
The filter is designed and implemented based on the theory of image parameters. This filter is composed of several basic sections (or half sections) cascaded according to the principle of equal image impedance at the connection. The basic section can be divided into fixed K-type and m-derived type according to the circuit structure. Taking LC low-pass filter as an example, the stopband attenuation of the fixed K-type low-pass basic section increases monotonically with the increase of frequency; The m-derived low-pass basic node has an attenuation peak at a certain frequency in the stopband, and the position of the attenuation peak is controlled by the m value in the m-derived node. For a low-pass filter composed of Cascaded Low-Pass basic sections, the inherent attenuation is equal to the sum of the inherent attenuation of each basic section. When the internal impedance and load impedance of the power supply terminated at both ends of the filter are equal to the image impedance at both ends, the working attenuation and phase shift of the filter are equal to their inherent attenuation and phase shift respectively. (a) The filter shown is composed of a fixed K section and two m derived sections in cascade. Z π and Z π m are the image impedance. (b) Is its attenuation frequency characteristic. The positions of the two attenuation peaks /f ∞ 1 and f ∞ 2 in the stopband are respectively determined by the m values of the two m derived nodes.
Similarly, high pass, band-pass and band stop filters can also be composed of corresponding basic sections.
The image impedance of the filter cannot be equal to the pure resistive internal resistance of the power supply and load impedance in the whole frequency band (the difference is greater in the stopband), and the inherent attenuation and working attenuation are greatly different in the passband. In order to ensure the realization of technical indicators, it is usually necessary to reserve sufficient inherent attenuation margin and increase the passband width in the design.
Operating parameter filter
This filter is not composed of cascaded basic sections, but uses network functions that can be physically realized by R, l, C and mutual inductance elements to accurately approximate the technical specifications of the filter, and then realizes the corresponding filter circuit by the obtained network functions. According to different approximation criteria, different network functions can be obtained, and different types of filters can be realized. (a) It is the characteristic of the low-pass filter realized by the flattest amplitude approximation (bertowitz approximation); The passband is the most flat near zero frequency, and the attenuation increases monotonically when it approaches stopband. (c) Is the characteristic of low-pass filter realized by equal ripple approximation (Chebyshev approximation); The attenuation in the passband fluctuates between zero and the upper limit, and increases monotonically in the stopband. (e) It uses elliptic function approximation to realize the characteristics of low-pass filter, and the attenuation presents constant voltage change in both pass band and stop band. (g) Is the characteristic of the low-pass filter realized by; The attenuation in the passband fluctuates in equal amplitude, and the attenuation in the stopband fluctuates according to the rise and fall required by the index. (b) , (d), (f) and (H) are the corresponding circuits of these low-pass filters respectively.
High pass, band-pass and band stop filters are usually derived from low-pass filters by means of frequency transformation.
The working parameter filter is designed by the synthesis method accurately according to the requirements of technical indicators, and can obtain a filter circuit with excellent performance and economy,
LC filter is easy to make, low in price, wide in frequency band, and widely used in communication, instrumentation and other fields; At the same time, it is often used as the design prototype of many other types of filters.

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