Improved quality factor for quartz oscillators

For the past 17 years, Hewlett-Packard's Santa Clara crystal facility has produced only SC-cut, 10 MHz, 3rd overtone quartz crystals for precision oven-controlled crystal oscillators. Test data on crystals and oscillators form a data base of almost 200,000 units. All of the units are essentially identical. The resonator design, holder design, and basic oscillator design have not changed during this period. This permits a direct comparison of many properties of a quartz resonator as a function of time, material, and the manufacturing processes used. We have found that the Allan Deviation at a tau of 2700 seconds is an excellent predictor of future oscillator performance and is useful as a new quality control parameter. This is especially important for oscillators required to have good time-domain stability for sampling times of 2,000 to 5,000 seconds. Of interest to crystal and oscillator manufacturers is that the new quality control parameter shows strong correlation with many factors important to oscillator users. In particular, we see a strong relationship between the Allan deviation with a tau of 45 minutes (2700 seconds) and frequency jumps, short-term time domain stability, and several mechanical processes in resonator fabrication (especially surface finish). The use of this single parameter is a more reliable indication of overall oscillator performance than the combination of factors previously used.