page_title: Filter (signal processing)
section_title: Quartz filters and piezoelectrics
hierarchical_section_title: Filter (signal processing) / Technologies / Quartz filters and piezoelectrics
caption_attribution_description: English: Crystal filter for fcenter=45 Mhz and with B3dB=12 KHz, Z=300 Ohm and Cp=8pF.
context_page_description: In signal processing, a filter is a device or process that removes some unwanted components or features from a signal. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal. Most often, this means removing some frequencies or frequency bands. However, filters do not exclusively act in the frequency domain; especially in the field of image processing many other targets for filtering exist. Correlations can be removed for certain frequency components and not for others without having to act in the frequency domain. Filters are widely used in electronics and telecommunication, in radio, television, audio recording, radar, control systems, music synthesis, image processing, and computer graphics.
  There are many different bases of classifying filters and these overlap in many different ways; there is no simple hierarchical classification. Filters may be:
  non-linear or linear
  time-variant or time-invariant, also known as shift invariance. If the filter operates in a spatial domain then the characterization is space invariance.
context_section_description: In the late 1930s, engineers realized that small mechanical systems made of rigid materials such as quartz would acoustically resonate at radio frequencies, i.e. from audible frequencies (sound) up to several hundred megahertz. Some early resonators were made of steel, but quartz quickly became favored. The biggest advantage of quartz is that it is piezoelectric.  This means that quartz resonators can directly convert their own mechanical motion into electrical signals.  Quartz also has a very low coefficient of thermal expansion which means that quartz resonators can produce stable frequencies over a wide temperature range. Quartz crystal filters have much higher quality factors than LCR filters.  When higher stabilities are required, the crystals and their driving circuits may be mounted in a "crystal oven" to control the temperature.  For very narrow band filters, sometimes several crystals are operated in series.
  A large number of crystals can be collapsed into a single component, by mounting comb-shaped evaporations of metal on a quartz crystal.  In this scheme, a "tapped delay line" reinforces the desired frequencies as the sound waves flow across the surface of the quartz crystal. The tapped delay line has become a general scheme of making high-Q filters in many different ways.
caption: Crystal filter with a center frequency of 45 MHz and a bandwidth B3dB of 12 kHz.
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