Practical Consequences of Modal Parameter Control in Crystal Resonators

摘要

The extension of the design technique to permit the fabrication of units to 150 MHz is presented along with a discussion of the latitude which exists between the conflicting constraints imposed by the nature of the crystal element. Included are charts and nomograms to facilitate the design of these units, together with attainable motional parameter values consistent with an adequately suppressed unwanted mode spectrum. This new technology is also applicable to oscillator crystals. One area explored concerns the specific problem of achieving moderate mode suppression in VHF oscillator crystals which must work in fixed-tuned circuits. Here the operating characteristics of the oscillator are such that the use of conventional designs in the crystal fabrication results in poor equipment performance. The difficulties are resolved by using energy-trapped elliptical electrodes. Not only are the original motional resistance and capacitance values preserved, but all troublesome modes are suppressed by more than 8 dB; these units perform well in the equipment. The report concludes with a discussion of certain inherent boundaries on resonator design and performance arising from intrinsic properties of quartz and vibrator configuration. Based on the Bommel, Mason, and Warner curves these boundaries are related to practical frequency ranges, orders of overtone, and values of attainable Q, including Q enhancement by mode trapping and control. (Author)