Introduction to Quartz Frequency Standards - Aging

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"Aging" and "drift" have occasionally been used interchangeably in the literature. However, recognizing the "need for common terminology for the unambiguous specification and description of frequency and time standard systems," the International Radio Consultative Committee (CCIR) adopted a glossary of terms and definitions in 1990 [16]. According to this glossary, aging is "the systematic change in frequency with time due to internal changes in the oscillator," and drift is "the systematic change in frequency with time of an oscillator." Drift is due to aging plus changes in the environment and other factors external to the oscillator. Aging, not drift, is what one denotes in a specification document and what one measures during oscillator evaluation. Drift is what one observes in an application. For example, the drift of an oscillator in a spacecraft might be due to (the algebraic sum of) aging and frequency changes due to radiation, temperature changes in the spacecraft, and power supply changes.

Aging can be positive or negative [17]. Occasionally, a reversal in aging direction is observed. At a constant temperature, aging usually has an approximately logarithmic dependence on time. Typical (computer-simulated) aging behaviors are illustrated in Figure 16, where A(t) is a logarithmic function and B(t) is the same function but with different coefficients. The curve showing the reversal is the sum of the other two curves. A reversal indicates the presence of at least two aging mechanisms. The aging rate of an oscillator is highest when it is first turned on. When the temperature of a crystal unit is changed (e.g., when an OCXO is turned off and turned on at a later time), a new aging cycle starts. (See the section concerning hysteresis and retrace below for additional discussion of the effects of temperature cycling.)

Figure 16. Computer-simulated typical aging behaviors; where A(t) and B(t) are logarithmic functions with different coefficients.

The primary causes of crystal oscillator aging are stress relief in the mounting structure of the crystal unit, mass transfer to or from the resonator's surfaces due to adsorption or desorption of contamination, changes in the oscillator circuitry, and, possibly, changes in the quartz material. Because the frequency of a thickness-shear crystal unit, such as an AT-cut or an SC-cut, is inversely proportional to the thickness of the crystal plate, and because a typical 5-MHz plate is on the order of 1 million atomic layers thick, the adsorption or desorption of contamination equivalent to the mass of one atomic layer of quartz changes the frequency by about 1 ppm. Therefore, in order to achieve low aging, crystal units must be fabricated and hermetically sealed in an ultra-clean, ultra-high-vacuum environment. As of 1992, the aging rates of typical commercially available crystal oscillators range from 5 ppm to 10 ppm per year for an inexpensive XO, to 0.5 ppm to 2 ppm per year for a TCXO, and to 0.05 ppm to 0.1 ppm per year for an OCXO. The highest precision OCXOs can age a few parts in 1012 per day, i.e., less than 0.01 ppm per year.

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