Crystal filter
Encyclopedia
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 circuit
s. 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,
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
used in electronics
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
systems, in particular communications
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
devices. It provides a very precisely defined centre frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
and very steep bandpass characteristics, that is a very high Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
—far higher than can be obtained with conventional lumped circuits.
A crystal filter is very often found in the intermediate frequency (IF) stages
Superheterodyne receiver
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
of high-quality radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
receivers
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
. Cheaper sets may use ceramic filters (which also exploit the piezoelectric effect), or tuned LC circuit
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
s. 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
Roofing filter
A roofing filter is a type of filter used in a HF receiver . It is usually found after the first receiver mixer. The goal of a roofing filter is to reduce the passband of the 1st intermediate frequency to about 6-20 kHz, so that overloading and distortions by the following amplifier stages and...
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,