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A crystal filter allows some frequencies to 'pass' through an electrical circuit while attenuating undesired frequencies. An electronic filter can use quartz crystals 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 In physics and engineering, the quality factor or ''Q'' factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the initial energy stored in the resonator to the energy lo ...
(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 steep
band-pass A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. Description In electronics and signal processing, a filter is usually a two-por ...
characteristics. Typical crystal filter attenuation in the band-pass is approximately 2-3 dB. Crystal filters are commonly used in communication devices such as radio receivers. Crystal filters are used in the intermediate frequency (IF) stages of high-quality 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 A Ceramic Resonator is an electronic component consisting of a piece of a piezoelectric ceramic material with two or more metal electrodes attached. When connected in an electronic oscillator circuit, resonant mechanical vibrations in the device g ...
s (which also exploit the piezoelectric effect) or tuned
LC circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
s. Very high quality "crystal ladder" filters can be constructed of serial arrays of crystals. The most common use of crystal filters are at frequencies of 9 MHz or 10.7 MHz to provide 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 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, though as quartz has an inherently high temperature stability, its shape does not change much with temperatures found in typical radios. By contrast, less expensive ceramic-based filters are commonly used with a frequency of 10.7 MHz to provide filtering of unwanted frequencies in consumer FM receivers. Additionally, a lower frequency (commonly of 455 kHz) 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 utilizing quartz crystals as a filtering component was first established by Walter Cady in 1922, but it was largely
Warren P. Mason Warren Perry Mason (September 28, 1900 – August 23, 1986) was an American electrical engineer and physicist at Bell Labs. A graduate of Columbia University, he had a prolific output, publishing four books and nearly a hundred papers. He was is ...
's work in the late 1920s and early 1930s that devised methods for incorporating crystals into LC lattice filter networks which set the groundwork for much of the progress in telephone communications. Crystal filter designs from the 1960s allowed for true
Chebyshev Pafnuty Lvovich Chebyshev ( rus, Пафну́тий Льво́вич Чебышёв, p=pɐfˈnutʲɪj ˈlʲvovʲɪtɕ tɕɪbɨˈʂof) ( – ) was a Russian mathematician and considered to be the founding father of Russian mathematics. Chebyshe ...
,
Butterworth Butterworth may refer to: Places * Butterworth (ancient township), a former township centred on Milnrow, in the then Parish of Rochdale, England, United Kingdom * Butterworth, Eastern Cape, now also known as Gcuwa, a town located in South Africa * ...
, and other typical filter characteristics. Crystal filter design continued to improve in the 1970s and 1980s with the development of multi-pole monolithic filters, widely used today to provide IF selectivity in communication receivers. Crystal filters can be found today in
radio communication Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
s, telecommunications,
signal generation In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing ...
, and
GPS The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
devices.Kinsman, R. G. (1998). "A History of Crystal Filters". IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society. Retrieved from


See also

*
Crystal oscillator A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. The oscillator frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable clock ...


References

{{DEFAULTSORT:Crystal Filter Linear filters Wireless tuning and filtering Signal processing filter