A selective damping technique for the suppression of unwanted resonances in high-frequency filter crystals

A new technique for suppressing unwanted resonances in thickness-shear-mode quartz crystals having spherically-bevelled edges has been developed. The technique is based on the fact that the strain associated with inharmonic overtones of vibration extends closer to the perimeter of the crystal along certain axes than along others, while the fundamental thickness-shear mode may, with correct geometry, have negligible amplitude near the crystal perimeter. In view of this it has been found possible, by placing small amounts of an adhesive such as an epoxy resin at certain points on the edge of the crystal, to damp the inharmonic modes selectively without greatly influencing the main response. The new technique permits relatively large variations in electrode diameter to be made without having to alter the crystal geometry to maintain suppression of the unwanted resonances. This facility greatly reduces the problem of crystal-filter design by extending the range of crystal inductances which are available. The paper describes the basic theory underlying the technique, called `selective damping¿, and illustrates its application to obtain single-response crystals in the frequency range 2¿8 Mc/s.