Electronic Filter

What is an electronic filter?
An electronic device that performs signal processing functions.
An electronic filter is an electronic circuit, component, or device that performs signal processing functions, specifically to remove unwanted components or enhance desired components in a signal.

Basic information on electronic filters
Electronic filters include audio filters (Wave Filter) and noise filters (Noise Filter) and other application devices, which can be:
     passive or active filter
     Analog or digital filters
     Discrete-time (sample) or continuous-time filters
     Linear or nonlinear filters
     Infinite Impulse Response (IIR) or Finite Impulse Response (FIR)
Regardless of their differences in design, the most common type of electronic filter is the linear filter. See the article on linear filters for details on their design and analysis.

History of Electronic Filters
As early as the 1880s, resistor and capacitor filter circuits had appeared. An inductor and capacitor resonant circuit with frequency selection function can be used as the simplest filter.
In 1915, K.W. Wagner of Germany and G.A. Campbell of Bell Laboratories in the United States respectively proposed papers on filters, which have been recognized as the independent inventors of filters in the world.
After 1923, O.J. Chabell of Bell Labs proposed a design method for K-type and m-inducible image parameter filters.
In 1939, W. Kaul of Germany and S. Darlington of the United States proposed the theory of filter design with working parameters respectively. Since many circuits and systems have to distinguish signals of different frequencies, filters are widely used in communications, broadcasting, radar, and many instruments and equipment.
Many complex multipole LC filters have also been around for many years, and there are many books on the operation of such filters.
The oldest forms of electronic filters are passive analog linear filters built using resistors and capacitors or resistors and inductors, called RC and RL single-pole filters, respectively.
Hybrid filters have also been developed, typically combining analog amplifiers with mechanical resonators or delay lines. Devices such as CCD delay lines are also used as discrete-time filters. Active digital filters have become common due to the widespread use of digital signal processing.

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According to filtering technology classification:
Passive Filters (Passive Filtering)
single pole filter
The simplest electronic implementation of a linear filter is a combination of resistors, inductors, and capacitors. These filters are available in various forms such as RC, RL, LC, and RLC. All of these types of filters are collectively referred to as passive filters because they do not require external power supplies.
Inductors block high-frequency signals and allow low-frequency signals to pass, while capacitors have the opposite characteristics. A filter that can pass a signal through an inductor, or a filter that is connected to ground through a capacitor, has less attenuation for low-frequency signals than high-frequency signals, and is called a low-pass filter. If the signal is connected to the ground through a capacitor or through an inductor, the filter will attenuate the high-frequency signal less than the low-frequency signal, which is called a high-pass filter. The resistance itself has no frequency-selective characteristics, but it is added to the inductance and capacitance to determine the time constant of the circuit and therefore the corresponding frequency.
At very high frequencies, probably over 100 MHz, sometimes the inductor consists of a single ring or piece of metal, and the capacitor consists of adjacent pieces of metal, called stubs.

multipole filter
Second-order filters are measured in quality or Q-factor. A filter is said to have a high Q factor (Q value) if the bandwidth of frequencies it passes or blocks is very narrow compared to the center frequency. Q is defined as center frequency/3dB bandwidth.

Active Filter (Active Filtering)
Active filters are constructed using passive and active amplifying components, and operational amplifiers are often used in active filter designs. Active filters can achieve high Q factors and resonance without the use of inductors. However, their upper frequency limit is limited by the bandwidth of the amplifier used.
Switched Capacitor Filters
Compared with active filtering, a filter with a switched capacitor (Switched Capacitor) circuit instead of a resistor.
digital filter
Digital signal processing can be used to cheaply construct different types of filters. The signal is sampled and converted into a digital stream by analog-to-digital conversion, and the output digital stream is often calculated using a computer program running on a central processing unit or a special digital signal processor rather than a hardware algorithm. Its output is then converted into a signal by digital-to-analog conversion. Noise can be introduced during the conversion, but they can be controlled and limited with many useful filters. Since sampling is involved, the input signal must be limited to a certain frequency, otherwise aliasing will occur.
power filter
The power filter refers to the noise filter (Noise Filter) installed on the side of the AC power supply. The device used to eliminate interference clutter, the input or output is filtered to obtain pure alternating current, which can control the internal noise inside and prevent the noise from being transmitted to other electrical appliances through the cable.