Filters
Electronic filters are electronic circuits which perform signal processing functions, specifically intended to remove unwanted signal components and/or enhance wanted ones. Electronic filters can be: more...
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passive or active;
analog or digital;
discrete-time (sampled) or continuous-time;
linear or non-linear;
infinite impulse response (IIR type) or finite impulse response (FIR type);
The most common types of electronic filters are linear filters, regardless of other aspects of their design. See the article on linear filters for details on their design and analysis.
History
The oldest forms of electronic filters are passive analog linear filters, constructed using only resistors and capacitors or resistors and inductors. These are known as RC and RL single pole filters respectively. More complex multipole LC filters have also existed for many years and the operation of such filters is well understood with many books having been written about them.
Hybrid filters have also been made, typically involving combinations of analog amplifiers with mechanical resonators or delay lines. Other devices such as CCD delay lines have also been used as discrete-time filters. With the availability of digital signal processing, active digital filters have become common.
Classification by technology
Passive filters
Single pole types
The simplest electronic implementations of linear filters are based on combinations of resistors, inductors and capacitors. These filters exist in so-called RC, RL, LC and RLC varieties. All these types are collectively known as passive filters, because they do not depend upon an external power supply. Inductors block high-frequency signals and conduct low-frequency signals, while capacitors do the reverse. A filter in which the signal passes through an inductor, or in which a capacitor provides a path to earth, presents less attenuation to low-frequency signals than high-frequency signals and is a low-pass filter. If the signal passes through a capacitor, or has a path to ground through an inductor, then the filter presents less attenuation to high-frequency signals than low-frequency signals and is a high-pass filter. Resistors on their own have no frequency-selective properties, but are added to inductors and capacitors to determine the time-constants of the circuit, and therefore the frequencies to which it responds.
At very high frequencies (above about 100 megahertz), sometimes the inductors consist of single loops or strips of sheet metal, and the capacitors consist of adjacent strips of metal. These inductive or capacitive pieces of metal are called stubs..
Read more at Wikipedia.org
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