Designing smaller SAW oscillators for low vibration sensitivity

All quartz package (AQP) surface acoustic wave (SAW) hybrid circuit oscillators with low vibration sensitivity, typically 3 to 4/spl times/10/sup -10//g, have been routinely achieved, provided that the oscillator package is sufficiently rigid to minimize bending stresses caused by vibration loading. This has been accomplished by bonding a suitably thick ceramic stiffener to the base of the oscillator housing. However, because of size constraints, some applications cannot accommodate a sufficiently thick stiffener. One approach toward overcoming this potential limitation on performance is to use finite element analysis to determine the stresses in the base of an oscillator package that would otherwise have an insufficiently thick stiffener, and then arrange to mount the AQP SAW device in a region of minimal stress. This approach was verified experimentally by comparing the vibration sensitivity of the same oscillator with the AQP SAW device mounted in both high- and low-stress regions. The result of the experiment was a factor of two reduction in the oscillator's vibration sensitivity. An alternative approach is to apply mass loading directly to the AQP SAW device. This is accomplished by bonding two small weights, approximately 60 mg each, to the backside of the AQP SAW substrate. Under vibration loading, these weights alter the stress gradients in the active acoustic area of the SAW substrate which reduces /spl Gamma/~, the magnitude of the vibration sensitivity vector. The same procedure was unsuccessfully applied to AQP SAW oscillators that had sufficiently thick stiffeners. Thus, mass loading is only effective in decreasing /spl Gamma/~ for an oscillator with an insufficiently thick stiffener.<
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