Crystal Filter

CRYSTAL FILTER

Crystal filter allows certain frequencies to 'pass' through an electrical circuit and cut the undesired frequencies. The crystal filters are used in radio communications, telecommunications, signal generation, GPS devices and many electronic devices.

Working

Crystal filters are electronic filters that use monolithic crystals. A monolithic crystal filter uses one large crystal rather than an array of several discrete crystals. (Fig. 1.31)

 Crystal filters are usually usually made of synthetically- manufactured quartz. The quartz crystal is cut into planks and then polished to produce a smooth surface.

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 made of silver, gold, or aluminium are deposited on the top and bottom surface of the crystal. The electrodes use the resonance qualities of the crystal to pass signals.

The 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 band-pass of the filter are allowed across the crystal. This crystal band-pass filter operates efficiently at a frequency below 30 MHz. 

Applications

1.Crystal filters are used in the intermediate frequency (IF) stages of high-quality radio receivers. They are very stable mechanically. Thus, only a small change in resonant frequency with changes in operating temperature.

2. The most common use of crystal filters are at frequencies of 9 MHz or 10.7 MHz to provide selectivity in communications receivers.