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 IF stages of high-quality radio receivers. Cheaper sets may use ceramic filters (which also exploit the piezoelectric effect), or tuned circuits. The use of a fixed IF frequency allows a crystal filter to be used because it has a very precise fixed frequency.