A crystal filter is a special form of quartz crystal used in electronics systems, most commonly in communication devices such as radio receivers. It provides a very precisely defined center frequency and very steep band-pass characteristics, giving it a very high Q factor factor, from 10,000 to 100,000 and greater - far higher than can be obtained with conventional LC components. Typical crystal filter attenuation in the band-pass is approximately 2-3dB.
A crystal filter is very often found in the intermediate frequency (IF) stages of high-quality radio receivers. Cheaper sets may use ceramic filters (which also exploit the piezoelectric effect), or tuned LC circuits. The use of a fixed IF stage frequency allows a crystal filter to be used because it has a very precise fixed frequency. Very high quality IF filters, called crystal ladder filters, can be constructed by using serial arrays of crystals.[1]
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 cut of the quartz crystal determines the crystal's vibrating frequencies, such as the common AT cut used for crystal filters designed for radio communications. The cut of the quartz also determines certain temperature characteristics of the component, of which quartz has a very high temperature stability.[2]
Ceramic filters tend to be used at 10.7 MHz to provide selectivity in broadcast FM receivers, or at a lower frequency (455 kHz) as the second intermediate frequency filters in a communication receiver. Ceramic filters at 455 kHz can achieve similar 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 Walter Mason's work in the late 1920's and early 1930's that devised methods for incorporating crystals into LC lattice filter networks which set the groundwork for much of the progress in telephone communications. It was in the 1960's where crystal filter designs allowed for filters with true Chebyshev, Butterworth, and other typical filter characteristics. The 1970's and 1980's continued to improve crystal filter design with the development of multi-pole monolithic filters, which allowed communication technologies to advance past frequency limitations. The monolithic crystal filter is, today, a widely used component in communication receiver IF selectivity. Today, crystal filters are widely used in radio communications, telecommunications, signal generation, GPS devices, and many more.[3]