The cut also determines some temperature characteristics, which affect the stability of the resonant frequency, though as quartz has an inherently high temperature stability, its shape does not change much with temperatures found in typical radios. The vibrating frequencies of the crystal are determined by its "cut" (physical shape), such as the common AT cut used for crystal filters designed for radio communications. The most common use of crystal filters are 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.
#Crystal filter design serial#
Very high quality "crystal ladder" filters can be constructed of serial arrays of crystals. For the highest available stability applications, crystals are placed in ovens with controlled temperature making operating temperature independent of ambient temperature.Ĭheaper sets may use ceramic filters built from ceramic resonators (which also exploit the piezoelectric effect) or tuned LC circuits. They are preferred because they are very stable mechanically and thus have little change in resonant frequency with changes in operating temperature. Crystal filters are commonly used in communication devices such as radio receivers.Ĭrystal filters are used in the intermediate frequency (IF) stages of high-quality radio receivers. Typical crystal filter attenuation in the band-pass is approximately 2-3 dB. The crystal's stability and its high Q factor allow crystal filters to have precise center frequencies and steep band-pass characteristics. In particular, quartz crystals can exhibit mechanical resonances with a very high Q factor (from 10,000 to 100,000 and greater - far higher than conventional resonators built from inductors and capacitors). Quartz crystals are piezoelectric, so their mechanical characteristics can affect electronic circuits ( see mechanical filter). An electronic filter can use quartz crystals as resonator components of a filter circuit. value in Hz 30dB, 60dB, etc.A 9 MHz crystal ladder filter with four matched crystals.Ī crystal filter allows some frequencies to 'pass' through an electrical circuit while attenuating undesired frequencies.
Every filter parameter can be moderately to severely distorted by them. The maximum attenuation guaranteed at the specified frequency range.Īre produced by unwanted vibrational modes in the crystals. The difference in attenuation between the highest peak and the lowest valley within the passband. May be measured at center frequency or within 1db passband limits. The amount of attenuation within the passband of the filter compared to the input signal level. The number if hertz expressing the difference between the upper 3db frequency and lower 3db frequency of a bandpass filter. In a bandpass filter, these limits, or passband edges, are generally the frequencies at which 3db of attenuation is measured. Also used as the design midpoint of the passband. Typical Construction of a Discrete Crystal and a Monolithic Crystalįor bandpass or band reject filters, it is the arithmetic mean of the 3db point frequencies. If the acoustic coupling is correct a filter response will be achieved. This forms 2 resonators with acoustic (mechanical) coupling between them. What is a filter? Filters are electronic circuits that emphasize certain signals and reject other signals.ĭefinition of a Monolithic Crystal Filter: A monolithic (Greek: one stone) crystal filter has 2 sets of electrodes deposited on the same quartz disc.