DESIGN CONSIDERATIONS FOR CRYSTAL RESONATORS WITH PRESCRIBED MODE SUPPRESSION.
ARMY ELECTRONICS COMMAND FORT MONMOUTH N J
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In virtually all filter crystal applications and in certain specific oscillator crystal applications, such as fixed tuned oscillators, the reduction or elimination of unwanted modes is of paramount importance. It has only recently become apparent, however, that a need also exists for crystal resonator types possessing mode suppression figures intermediate to those usually associated with conventional oscillator crystals, on one hand, with little or no suppression, and filter crystals, on the other, where a maximum suppression is desired. Such intermediate levels are particularly useful inasmuch as a partial relaxation of the mode suppression requirement allows the gamut of achievable values to be extended for the other modal parameters. This report shows how, in order to control the mode spectrum in quartz crystal resonators, the techniques of energy trapping are employed, and how a compromise is effected in such a fashion that unwanted mode suppression can be traded off against crystal resistance and motional capacitance. A procedure is formulated which is general enough to permit the design of crystal resonator types to be made in a straightforward, logical manner with a minimum of assumptions. This procedure is applied to the design of third overtone oscillator units in the range 35 to 60 MHz because the unwanted mode problem appears to be most pronounced with these types. Author
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