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The resonant frequency of every quartz crystal is affected by acceleration forces. The nature of the effect depends on the type of force that is being applied. Changes in the static gravitational force being experienced such as tilting or rotation will cause a step offset in frequency. Time dependent acceleration or vibration will frequency modulate the output of an oscillator. This will generate discrete sidebands in the case of sinusoidal vibration or an increase in the noise floor with random vibration. A shock pulse will cause a sharp temporary perturbation in the output frequency.

The magnitude of these frequency shifts is a function of the quartz crystal's acceleration or "g-sensitivity" vector and the magnitude and direction of the applied acceleration force. The acceleration sensitivity of quartz crystals is caused by stresses resulting from the mass of the resonator blank reacting against its mounting structure. This sensitivity is determined by many factors such as the type of cut (AT vs. SC, etc.), the design and processing of the blank, the package type, mounting structure and orientation in the holder. The range of typical g-sensitivities for bulk-mode quartz crystals can span several orders of magnitude, from less than 1x10 ^-10 per g for a carefully made precision SC cut to greater than 1x10 ^-7 per g for a low cost AT.

Since the magnitude of these effects is relatively small, they go undetected in many applications with standard oscillators such as VCXOs and clocks. However with precision ovenized oscillators or those that undergo severe environmental conditions, the inherent acceleration sensitivity can be very significant, even limiting the performance in some instances. If the oscillator is deployed in a high vibration environment such as an airborne platform, increased phase noise or discrete spurious components will appear as modulation on the output signal, degrading the performance more than all other sources of noise combined. But even in a benign environment, a high stability OCXO may experience significant frequency shifts due to static g-forces by moving, tilting or rotating motions. With knowledge of the operating environment that an oscillator will experience and an understanding of the acceleration sensitivity of the quartz crystal, it is possible to predict and plan for the expected frequency errors.

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This article was first published in the October 2004 issue of Microwaves & RF.