The bandwidth and response curve of a crystal filter can be adjusted to some extent. In building your
filter you may have picked the 4 crystals which were closest in frequency and used these for the filter.
This will give you the steepest sideband skirts. The capacitors to ground inside the filter diagram
(C54, C55, and C56) may be changed to alter the overall filter bandwidth. While it may seem
counterintuitive, increasing capacitance decreases the filter bandwidth. The value of C55 should
equal approximately twice the value of C54 or C56, but there is some room to experiment to see how
changes affect the ripple factor across the passband and steepness of sideband skirts.
There are several mechanized ways to monitor your crystal filter passband, including use of a Spectrum
Analyzer, a swept frequency source with RF detector and oscilloscope display, or using a PC sound
card with one of the AF Spectrum Analysis software packages. These have the advantage over manual
measurement and manual production of graphs because they allow you to see a realtime result from
any changes you might make.
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