This crystal ladder filter calculator is a simple tool for dimensioning
crystal bandpass filters composed by 2 to 8 identical quartz crystals and some
capacitors. Quartzes are used in their fundamental mode (even if they are
overtone) in their series resonance (even if they were designed for parallel
mode operation).
The series and parallel resonance of the crystals must be
precisely known, as well as the crystal's series capacitance Cs. Those values
cannot be found on datasheets with the necessary accuracy and need to be
manually measured with the method described before. It's also important to
select crystals with very similar characteristics (the resonance frequencies
should be within 100 Hz).
If no bandpass ripple is allowed the filter
will be of Butterworth type and the transitions between pass-band and stop-band
will not be very fast and the ultimate attenuation (horizontal asymptote) will
be poor. If some bandpass ripple is allowed the filter will be of Chebytheff
type with much better stop-band characteristics.
The number of poles of the
filter equals the number of crystals and depends on how many identical crystals
one happens to have.
The target bandwidth is the bandwidth the filter should
pass: the maximum possible bandwidth depends on all the data entered and is
usually only a few kHz.
Hitting the "Calculate" button will start the
simulation.
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