: The fundamentals of quartz frequency standards are reviewed. The subjects discussed include: crystal resonators and oscillators, oscillator types, and the characteristics arid limitations of temperature-compensated crystal oscillators (TCXO) and oven-controlled crystal oscillators (OCXO). The oscillator instabilities discussed include: aging, noise, frequency vs. temperature, warmup, acceleration effects, magnetic field effects, atmospheric pressure effects, radiation effects, and interactions among the various effects. Guidelines are provided for oscillator comparison and selection. Discussions of specifications are also included, as are references and suggestions for further reading. Oscillator, clock, frequency standard, frequency control, frequency stability, time, timing devices, quartz, quartz crystal, quartz oscillator, atomic clock, atomic frequency standard, stability, aging, noise, phase noise.

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Introduction to Quartz Frequency Standards

The stability of resonant frequency for single wafer, thin-film encapsulated silicon MEMS resonators was investigated for both long-term operation and temperature cycling. The resonant frequencies of encapsulated resonators were periodically measured at 25 ± 0.1 °C for >9000 h, and the resonant frequency variation remained within the measurement uncertainty of 3.1 ppm and 3.8 ppm for the two designs of resonators measured. Also, the resonators were temperature cycled for 680 cycles between −50 °C and 80 °C, measuring the resonant frequency each time the temperature reached 30 °C. Again, the change in resonant frequency was seen to remain within the measurement uncertainty. This demonstrates stability of resonant frequency for both long-term operation of more than a year and large number of temperature cycles, emphasizing the stability of both the resonator and the package.

/pdf/frequency-stability-of-wafer-scale-film-encapsulated-silicon-35k34u79py.pdf

Frequency stability of wafer-scale film encapsulated silicon based MEMS resonators

The frequency instabilities of precision bulk acoustic wave (BAW) quartz crystal oscillators are reviewed. The fundamental limits on the achievable frequency stabilities, and the degree to which the fundamental limits have been approached to date are examined. Included are the instabilities as a function of time, temperature, acceleration, ionizing radiation, electromagnetic fields, humidity, atmospheric pressure, power supply, and load impedance. Most of the fundamental limits are zero or negligibly small, a few are finite. We speculate about the progress which may be achievable in the future with respect to approaching the fundamental limits. Suggestions are provided about the paths that may lead to significant stability improvements. >

Fundamental limits on the frequency stabilities of crystal oscillators

This paper presents an investigation of the long-term frequency stability of wafer-scale encapsulated silicon MEMS resonators. Two aspects of stability were examined: long-term stability over time and temperature-related hysteresis. Encapsulated resonators were tested over a period of 8,000 hours in constant environmental temperature of 25/spl deg/C /spl plusmn/ 0.1/spl deg/C. No measurable drift, burn-in time, or other changes in resonant frequencies were detected. Another experiment was performed to investigate the stability of the resonators with temperature cycling. The resonant frequency was measured between each cycle for more than 450 temperature cycles from -50/spl deg/C to +80/spl deg/C. Additional data is presented for short-term hysteresis measurements -10/spl deg/C to +80/spl deg/C temperature cycle. No detectable hysteresis was observed in either of the temperature cycle experiments. These series of experiments demonstrate resonant frequency stability of wafer-scale silicon based MEMS resonators.

/pdf/frequency-stability-of-wafer-scale-encapsulated-mems-10pzcp1ao2.pdf

Frequency stability of wafer-scale encapsulated MEMS resonators

Acoustic sensor sensitivity is often reported in units of Hz per unit of measurand. It has been frequently stated or implied in the sensor literature that higher-frequency sensors are preferable to lower frequency sensors because the frequency change per unit of measurand is higher, i.e., the higher-frequency sensors are more "sensitive". This paper reviews the resonator instabilities that lead to the conclusions that: 1) sensitivity expressed as Hz per unit of measurand is not a good measure of sensor quality, and 2) when compared on the bases of accuracy, reproducibility and resolution capability, "good" low frequency sensors are often superior to "good" high frequency ones.

A review of sensor sensitivity and stability

The literature on the frequency versus temperature characteristics of quartz crystal resonators is reviewed. Three papers that deal with frequency versus pressure hysteresis are included, as these may possibly have relevance to frequency versus temperature hysteresis. It is seen that the causes of hysteresis are not well understood. The evidence to date is inconclusive. The mechanisms that can cause hysteresis include: strain changes changes in the quartz, contamination redistribution, oscillator circuitry hysteresis, and apparent hysteresis due to thermal gradients. The results to date seem to indicate that lattice defects are somehow related to thermal hysteresis. Stress relief in the mounting structure can also produce significant hysteresis. As crystal processing techniques have improved. contamination has become less of a problem. >

Hysteresis in quartz resonators-a review

The subject of thermal hysteresis is reviewed. A review of the literature which deals with frequency vs pressure and frequency vs. temperature hysteresis is presented. Typical hysteresis curves are shown. Hysteresis mechanisms such as contamination redistribution, strain changes, and changes in the quartz are discussed. The results seem to indicate that lattice defects are somehow related to thermal hysteresis. Stress relief in the mounting structure can also produce significant hysteresis. >

J.A. Kusters

Papers

Hysteresis in quartz resonators-a review

Thermal hysteresis in quartz resonators-A review (frequency standards)

Selected Topics in Quartz Crystal Research

Measurement Techniques for Quartz Crystals