Crystal filter

A crystal filter is a special form of quartz crystal used in electronics systems, in particular communications devices. It provides a very precisely defined centre frequency and very steep bandpass characteristics, that is a very high Q factor—far higher than can be obtained with conventional lumped circuits.

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.

The most common use of crystal filters, is 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.

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.

Very high quality IF filters, called crystal ladder filters, can be constructed by using serial arrays of crystals,