![]() ![]() 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. #8 khz bandwidth crystal filter design serialVery 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. Pages 20-23.A crystal filter allows some frequencies to 'pass' through an electrical circuit while attenuating undesired frequencies. Change the coupling capacitors to C2, and set the input and output impedance to R2.Ĩ. Calculate C for the desired bandwidth (BW2), using C2 = C1*(BW1/BW2)^2.Ħ. If it isn't you've either got the calculation wrong, or set the pots incorrectly.ĥ. The ripple should be about 1 dB with two peaks. #8 khz bandwidth crystal filter design generatorSweep the signal generator across the filter pass-band. Set the filter input and output impedance on the test rig to this value, measuring between A-B and C-D using a DVM.ģ. Separate ten-turn pots might make the settings easier if you want to get the resistance settings 'spot-on'. Since the input and output impedance should be equal I used a dual pot. I just wired it up on a scrap of PCB material. The following test rig needs to be constructed. I've extracted the relevant information from John's article and presented it, somewhat simplified, in 'cookbook' form.īasically, G3JIR's technique involves measuring the bandwidth of a simple two-pole filter, modifying it for the desired bandwidth, and then calculating the shunt capacitors required for a filter of the same bandwidth, but with more crystals to improve the shape factor. John Hardcastle, G3JIR, described an interesting ladder filter design technique in 1980 that appears to have been forgotten. ![]()
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