Crystal filters are electronic filters that use monolithic crystals. Crystal filters are used in all kinds of electronic and radio communication devices. A monolithic crystal filter uses one large crystal rather than an array of several discrete crystals.
Crystal filters are usually made of synthetically-manufactured quartz. The quartz crystal is cut into blanks and then polished to produce a smooth surface. After the crystal is smoothed, the surface is chemically-etched to further refine the finish and minimize the effects of aging. In a monolithic filter, the single crystal is fitted with two sets of electrodes on either side. The electrodes are deposited on the top and bottom surfaces of the crystal, and are usually made of silver, gold, or aluminum. The electrodes use the resonance qualities of the crystal to pass signals.
Crystal filters use the piezo-electric effect to transmit filtered signals from one set of electrodes to the other set. The signals on the originating electrodes cause the crystal to vibrate at a certain frequency. Only the signals within the bandpass of the filter are allowed across the crystal to the second set of electrodes. This crystal bandpass filter operates efficiently and cost effectively at a frequency at or below 30 MHz.
Monolithic crystal filters may also serve as a high frequency filter for use in wireless applications. A surface mount filter is a crystal filter mounted directly to an integrated circuit. A crystal ladder filter consists of several crystal filters of the same frequency and is often used to build a single sideband transmitter in amateur radio applications.
Crystal filters designed for two way radio communications include front end filters, which are designed with multiple pole discrete crystal filters. A front end filter is often used in radio frequency environments that produce strong signals, such as high power transmitters.