RESEARCH INTO NEW APPROACHES FOR VHF FILTER CRYSTALS.
Rept. no. 12 (Final), 1 May 63-31 Oct 66,
CLEVITE CORP CLEVELAND OH ELECTRONIC RESEARCH DIV
Pagination or Media Count:
An understanding of the behavior of thickness shear mode quartz resonators can be obtained by considering the problem of wave propagation in the quartz medium. The unelectroded portion of the wafer surrounding the electrodes of a resonator has a cut-off frequency for wave propagation that is higher than the resonant frequency of the electroded portion. The vibratory energy is therefore essentially confined to the electroded region with an energy distribution that decreases exponentially with distance from the electrode edge. This energy trapping phenomenon is the basis for the high Q sub m observed for crystals mounted on low Q supports and for the degree of acoustical isolation observed between individual resonators on a multi-resonator wafer. Consideration of the boundary conditions at the electrode edge yields a relationship between the system of standing waves that can exist in the electroded region, the lateral electrode dimensions, and the frequency-lowering or ratio of cut-off frequencies of the electroded and unelectroded regions of the wafer. This relationship shows that unwanted vibrational modes i.e., inharmonic overtone responses can be eliminated by varying either electrode diameter andor electrode thickness. As a result, the range of motional parameters of HF and VHF filter crystals has been extended. Several new resonator structures, namely, the multi-electrode resonator, the large area single electrode resonator, and the mesa and inverted mesa resonators, have resulted from a consideration of energy trapping theory. Useful fabrication techniques have also evolved, most significant of which is the insulating film tuning of resonators.
- Electrical and Electronic Equipment