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Electronic filters are Electronic Circuit s which perform Signal Processing functions. Electronic filters can be:
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. Most filters are a type of Resonant System . HISTORY The oldest forms of electronic filters are passive analog linear filters, constructed using only Resistors and Capacitor s or resistors and Inductor s. 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 Line s 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 Resistor s, Inductor s and Capacitor s. 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, therefore 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 are called stubs. Multipole types Second order filters are measured by their quality or "''Q''" Factor . A filter is said to have a high ''Q'' if it selects or rejects a narrow range of frequencies compared with its centre frequency. ''Q'' is defined as center frequency/3 dB bandwidth. Active filters Active Filter s are implemented using a combination of passive and active (amplifying) components. Operational Amplifier s are frequently used in active filter designs. These can have high ''Q'', and achieve resonance without the use of inductors. However, their upper frequency limit is limited by the bandwidth of the amplifiers used. Digital filters . Other filter technologies Quartz filters and piezoelectrics In the late 1930s , engineers realized that small mechanical systems made of rigid materials such as Quartz would acoustically resonate at radio frequencies, i.e. from audible frequencies ( Sound ) up to several hundred megahertz. Some early resonators were made of Steel , but quartz quickly became favored. The biggest advantage of quartz is that it is Piezoelectric . This means that quartz resonators can directly convert their own mechanical motion into electrical signals. Quartz also has a very low coefficient of thermal expansion which means that quartz resonators can produce stable frequencies over a wide temperature range. Quartz Crystal filters have much higher quality factors than LCR filters. When higher stabilities are required, the crystals and their driving circuits may be mounted in a "crystal oven" to control the temperature. For very narrow band filters, sometimes several crystals are operated in series. Engineers realized that a large number of crystals could be collapsed into a single component, by mounting comb-shaped evaporations of metal on a quartz crystal. In this scheme, a "tapped delay line" reinforces the desired frequencies as the sound waves flow across the surface of the quartz crystal. The tapped delay line has become a general scheme of making high-''Q'' filters in many different ways. SAW filters SAW ( Surface Acoustic Wave ) filters are Electromechanical devices commonly used in Radio Frequency applications. Electrical signals are converted to a mechanical wave in a Piezoelectric crystal; this wave is delayed as it propagates across the crystal, before being converted back to an electrical signal by further Electrode s. The delayed outputs are recombined to produce a direct analog implementation of a Finite Impulse Response filter. This hybrid filtering technique is also found in an Analog Sampled Filter . Garnet filters See Also: Yttrium iron garnet filter Another method of filtering, at Microwave frequencies from 800MHz to about 5 GHz, is to use a synthetic Single Crystal Yttrium Iron Garnet sphere made of a chemical combination of Yttrium and Iron (YIGF, or '''yttrium iron garnet filter'''). The garnet sits on a strip of metal driven by a Transistor , and a small loop Antenna touches the top of the sphere. An Electromagnet changes the frequency that the garnet will pass. The advantage of this method is that the garnet can be tuned over a very wide frequency by varying the strength of the Magnetic Field . Atomic filters For even higher frequencies and greater precision, the vibrations of atoms must be used. Atomic Clock s use Caesium Maser s as ultra-high ''Q'' filters to stabilize their primary oscillators. Another method, used at high, fixed frequencies with very weak radio signals, is to use a Ruby maser tapped delay line. FILTERS BY TRANSFER FUNCTION GENERAL CLASSES SPECIFIC DESIGNS
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