Crystal filter

A quartz filter is an electro-mechanical filter based on the piezoelectricity, consisting of a quartz crystal and by means of specific resonance frequency components of a signal, for example in the form of a bandpass filter can be suppressed or can pass.

Construction

Parameters of the crystal filter, such as its cutoff frequency and attenuation values ​​are determined by the mechanical dimensions of the crystal and the crystal interface. Quartz filters have, in contrast to passive electronic filters, a very high quality factor Q on - typical values ​​for Q are in the range of 10,000 to 100,000 which is also the relatively high production cost for quartz filters justifies. The primary scope of quartz filters are wireless communication systems, where they are used in the intermediate frequency stages with common values ​​such as 9 MHz or 10.7 MHz. In order to further increase the quality factor, several single quartz filter or multiple identical oscillating crystals can be connected in series, as well as English Crystal Ladder designated filter.

Closely related and similar in construction are ceramic filters, which use a ceramic resonator instead of the quartz plate. Since the ceramic resonator has a lower quality factor, not as high as grades are achieved with quartz filters with ceramic filters.

History

For the first time quartz filters were described in 1922 by Walter Guyton Cady, with the scope of the newly created telephone network. Dama Lige quartz filters were due to the not yet so far advanced manufacturing technologies, pure Einzelpolfilter. That is, the transfer function of the crystal filter has only a single pole and thus the possibility of design of the filter was limited. In the 1960s and 1970s succeeded in the simulation of complex transfer functions, such as the Butterworth filters or Chebyshev filters, and the development of multi-pole filters. Crystal filters are used in various wireless communication devices and better quality as filters in intermediate frequency stages.