Oscilent Corporation - Technical References
Introduction to Quartz Frequency Standards

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Introduction to Quartz Frequency Standards - Other Effects on Stability

Ambient pressure change (as during an altitude change) can change a crystal oscillator's frequency if the pressure change produces a deformation of the crystal unit's or the oscillator's enclosure (thus changing stray capacitances and stresses). The pressure change can also affect the frequency indirectly through a change in heat-transfer conditions inside the oscillator. Humidity changes can also affect the heat-transfer conditions. In addition, moisture in the atmosphere will condense on surfaces when the temperature falls below the dew point, and can permeate materials such as epoxies and polyimides, and thereby affect the properties (e.g., conductivities and dielectric constants) of the oscillator circuitry. The frequency of a properly designed crystal oscillator changes less than 5 x 10-9 when the environment changes from one atmosphere of air to a vacuum. The medium and long term stability of some oscillators can be improved by controlling the pressure and humidity around the oscillators [40].

Electric fields can change the frequency of a crystal unit. An ideal AT-cut is not affected by a DC voltage on the crystal electrodes, but "doubly rotated cuts," such as the SC-cut, are affected. For example, the frequency of a 5-MHz fundamental mode SC-cut crystal changes 7 x 10-9 per volt. Direct-current voltages on the electrodes can also cause sweeping, which can affect the frequencies of all cuts.

Power-supply and load-impedance changes affect the oscillator circuitry and, indirectly, the crystal's drive level and load reactance. A change in load impedance changes the amplitude or phase of the signal reflected into the oscillator loop, which changes the phase (and frequency) of the oscillation [41]. The effects can be minimized through voltage regulation and the use of buffer amplifiers. The frequency of a ''good'' crystal oscillator changes less than 5 x 10-10 for a 10% change in load impedance. The typical sensitivity of a high-quality crystal oscillator to power-supply voltage changes is 5 x 10-11/V.

Gas permeation under conditions where there is an abnormally high concentration of hydrogen or helium in the atmosphere can lead to anomalous aging rates. For example, hydrogen can permeate into ''hermetically'' sealed crystal units in metal enclosures, and helium can permeate through the walls of glass-enclosed crystal units.