Noise in sensor crystals: hermetically sealed and unsealed

Noise limits the accuracy with which the frequency of an oscillator can be determined. Similarly, in a resonant sensor, noise limits the measurement resolution, i.e., it limits the minimum quantity of a measurand that can be measured with a specified uncertainty. In some sensor applications, such as in quartz crystal microbalances (e.g., chemical sensors), the sensor crystals are not hermetically sealed. The noise [/spl sigma//sub y/(/spl tau/) at several /spl tau/] of hermetically sealed 5 MHz fundamental mode AT-cut quartz resonators was measured at laboratory ambient temperature, and in an oven near the turnover temperatures (83/spl deg/C). Then the hermetic seals were broken and the resonators were measured again at ambient and turnover temperatures. As one would expect, breaking the hermetic seal increased the equivalent series resistance and the aging of the resonators. When the resonators were in the oven near their turnover temperatures, the unsealed resonators' (drift-removed) short-term stability was not significantly different from that of the sealed ones. In fact, in some cases, the unsealed resonators exhibited better short term stability. The noise due to temperature fluctuations at laboratory ambient temperatures dominated the effects of breaking the hermetic seal.