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Acrystal filter allows some frequencies to pass through an electrical circuit while attenuating undesired frequencies. Anelectronic filter can usequartzcrystals as resonator components of a filter circuit. Quartz crystals arepiezoelectric, so their mechanical characteristics can affect electronic circuits (seemechanical filter). In particular, quartz crystals can exhibit mechanical resonances with a very highQ factor (from 10,000 to 100,000 and greater – far higher than conventional resonators built from inductors and capacitors). The crystal's stability and its high Q factor allow crystal filters to have precise center frequencies and steepband-pass characteristics. Typical crystal filter attenuation in the band-pass is approximately 2-3dB. Crystal filters are commonly used incommunication devices such as radio receivers.
Crystal filters are used in theintermediate frequency (IF)stages of high-qualityradioreceivers. They are preferred because they are very stable mechanically and thus have little change in resonant frequency with changes in operating temperature. For the highest available stability applications, crystals are placed in ovens with controlled temperature making operating temperature independent of ambient temperature.
Cheaper sets may use ceramic filters built fromceramic resonators (which also exploit thepiezoelectric effect) or tunedLC circuits. Very high quality "crystal ladder" filters can be constructed of serial arrays of crystals.[1]
The most common use of crystal filters are at frequencies of 9 MHz or 10.7 MHz to provideselectivity in communications receivers, or at higher frequencies as aroofing filter in receivers using up-conversion. The vibrating frequencies of the crystal are determined by its "cut" (physical shape), such as thecommon AT cut used for crystal filters designed for radio communications. The cut also determines some temperature characteristics, which affect the stability of the resonant frequency. However,quartz has an inherently high temperature stability, its shape does not change much with temperatures found in typical radios.[2]
By contrast, less expensiveceramic-based filters are commonly used with a frequency of 10.7 MHz to provide filtering of unwanted frequencies in consumerFM receivers. Additionally, a lower frequency (typically 455 kHz or nearby) can be used as the second intermediate frequency and have a piezoelectric-based filter. Ceramic filters at 455 kHz can achieve similar narrow bandwidths to crystal filters at 10.7 MHz.
The design concept for using quartz crystals as a filtering component was first established byW.G. Cady in 1922,[citation needed] but it was largelyW.P. Mason's work in the late 1920s and early 1930s[citation needed] that devised methods for incorporating crystals intoLClattice filter networks[clarification needed] which set the groundwork for much of the progress in telephone communications. Crystal filter designs from the 1960s allowed for true[clarification needed]Chebyshev,Butterworth, and other typical filter types. Crystal filter design continued to improve in the 1970s and 1980s with the development of multi-pole monolithic filters, widely used today to provideIFselectivity incommunication receivers. Crystal filters can be found today inradio communications,telecommunications,signal generation, andGPS devices.[3]
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