Monolithic crystal filter
In the "abroad" ("Radio", 1969, No. 11, p. 58) was placed note "Monolithic quartz filters". Such a filter can be made at home from a low-frequency quartz.
For the manufacture of filter is suitable quartz rectangular, somewhat elongated plates coated with silver on both sides. The sections forming the electrodes of the filter, you should paint any quick-drying varnish, and after drying etching in ferric (it must be clean and not previously used for etching copper) to remove the excess silver from the middle pane. The etching process lasts from several seconds to several minutes depending on the thickness of the layer of silver applied
the quartz plate. After etching, the plate is washed in running water, remove the solvent in the varnish and wipe the cavity with alcohol.
To connect the filter of thin sheet brass are made electrodes (Fig. 1).
Done this way the filter from the quartz frequency 1312,5 kHz (Fig. 2) had a resonant frequency 1325 kHz and a bandwidth of 3.5-4 kHz.
The filter included in the circuit shown in Fig. 3.
Author: B. Karpov (RI8AAD), Tashkent; Publication: N. Bolshakov, rf.atnn.ru