Editing Crystal filter

{{More citations needed|date=December 2009}}
[[File:Crystal ladder.svg|thumb|250px|A 9&nbsp;MHz crystal ladder filter with four matched crystals.]]

A '''crystal filter''' allows some frequencies to pass through an electrical circuit while attenuating undesired frequencies. An [[electronic filter]] can use [[quartz]] [[crystal]]s as resonator components of a filter circuit. Quartz crystals are [[piezoelectric]], so their mechanical characteristics can affect electronic circuits (''see'' [[mechanical filter]]). In particular, quartz crystals can exhibit mechanical resonances with a very high [[Q factor|{{mvar|Q}} factor]] (from 10,000 to 100,000 and greater &ndash; 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 steep [[Band-pass filter|band-pass]] characteristics. Typical crystal filter attenuation in the band-pass is approximately 2-3[[Decibel|dB]].<!-- not 6? --> Crystal filters are commonly used in [[Telecommunications|communication]] devices such as radio receivers.

Crystal filters are used in the [[intermediate frequency]] (IF) [[Superheterodyne receiver|stages]] of high-quality [[radio]] [[receiver (radio)|receivers]]. 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 from [[ceramic resonator]]s (which also exploit the [[piezoelectric]] effect) or tuned [[LC circuit]]s. Very high quality "crystal ladder" filters can be constructed of serial arrays of crystals.<ref>{{cite magazine |author1=Stader, Horst |author2=Hardcastle, Jack A. |date=Nov–Dec 2009 |title=Crystal ladder filters for all |magazine=[[QEX|QEX Magazine]] |pages=14–18 |place=Newington, CT |publisher=[[American Radio Relay League]] |url=http://www.arrl.org/files/file/QEX_Next_Issue/Nov-Dec_2009/QEX_Nov-Dec_09_Feature.pdf}}</ref>

The most common use of crystal filters are at frequencies of 9&nbsp;MHz or 10.7&nbsp;MHz to provide [[selectivity (radio)|selectivity]] in communications receivers, or at higher frequencies as a [[roofing filter]] in receivers using up-conversion. The vibrating frequencies of the crystal are determined by its "cut" (physical shape), such as the [[Crystal oscillator#AT cut anchor|common 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.<ref>{{cite web |author=Poole, I. |date=n.d. |title=Quartz crystal filter |website=Radio-Electronics.com |url=https://www.radio-electronics.com/info/data/crystals/crystal_filter.php |access-date=2023-06-04}}</ref>

By contrast, less expensive [[Ceramic resonator#Ceramic filters|ceramic-based filters]] are commonly used with a frequency of 10.7&nbsp;MHz to provide filtering of unwanted frequencies in consumer [[FM radio|FM]] receivers. Additionally, a lower frequency (typically 455&nbsp;kHz or nearby) can be used as the second intermediate frequency and have a piezoelectric-based filter. Ceramic filters at 455&nbsp;kHz can achieve similar narrow bandwidths to crystal filters at 10.7&nbsp;MHz.

The design concept for using quartz crystals as a filtering component was first established by [[Walter Guyton Cady|W.G. Cady]] in 1922,{{citation needed|date=June 2023}} but it was largely [[Warren P. Mason|W.P. Mason]]'s work in the late 1920s and early 1930s{{citation needed|date=June 2023}} that devised methods for incorporating crystals into [[LC filter|LC]] [[Electronic filter topology |lattice filter networks]]{{Clarification needed|reason=Bad formatted text. Is this trying to hyperlink to [[Lattice network]] and include a thumbnail for an LC filter?|date=June 2023}} which set the groundwork for much of the progress in telephone communications. Crystal filter designs from the 1960s allowed for true{{Clarification needed|reason=What is the meaning of "true" here?  Are those two filter types already "true"?  And how are these crystals configured to make these filters?|date=June 2023}} [[Chebyshev filter|Chebyshev]], [[Butterworth filter|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 provide [[intermediate frequency|IF]] [[selectivity (radio)|selectivity]] in [[radio receiver|communication receivers]]. Crystal filters can be found today in [[radio communication]]s, [[telecommunications]], [[signal generator|signal generation]], and [[GPS]] devices.<ref>{{cite journal |author=Kinsman, R.G. |year=1998 |title=A history of crystal filters |journal=IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society |url=http://www.ieee-uffc.org/main/history.asp?file=crystal |department=UFFC History |access-date=2011-12-17 |url-status=dead |archive-url=https://web.archive.org/web/20110909013316/http://ieee-uffc.org/main/history.asp?file=crystal |archive-date=2011-09-09}}</ref>

==See also==
* [[Bandpass filter]]
* [[Crystal oscillator]]

==References==
{{reflist|25em}}

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[[Category:Linear filters]]
[[Category:Wireless tuning and filtering]]
[[Category:Signal processing filter]]
[[Category:Radio technology]]