Homemade Tuneable Crystal Filter
It takes only 4 crystals to build a tuneable crystal filter.
Rolf Heine, DL6ZB

This quartz filter is tuned by a variable voltage. The impedance is between approx. 40 and 150 ohms with bandwidths from 3 kHz to 300 Hz. Better roll-off rate and higher stopband attenuation may be achieved by cascading these crystal filters at the expense of higher passband attenuation.

Due to parasitic effects, in particular parasitic capacitances in the structure of the crystals or the RF transformer, sharp secondary resonances at - 30 dB may occur a few hundred kilohertz below or above of the fundamental frequency. Cascading two or more filters almost eliminate this effect.

I have found Zener diodes BZX85C15 to be a good substitute for more expensive varactor diodes. Although this filter works on any fundamental quarz crystal frequency, I would recommend crystal center frequencies around 8 to 12 MHz for best performance.