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To ensure that an oscillator will meet the performance requirements in the final application environment, it is necessary to make sure that the manufacturer understands the details of the application. These include the expected vibration levels and frequencies and the required performance required under these conditions.

Manage Quartz Crystals Under High Vibration, Fig. 4

Crystal oscillators are available in a multitude of different configurations, running the gamut from simple clock oscillators costing less than $1 to precision ovenized oscillators for specialized applications that may cost thousands of dollars. While the overall frequency stability of the different types of oscillators may vary by more than 10,000 to 1, the acceleration sensitivity of any type of crystal oscillator will rarely vary from 1 ppb/g by more than a factor of 4 (ranging from around 0.25 ppb/g for some selection of specialized crystals to as high as 4 ppb/g for some typical standard low-cost crystals.)

Manage Quartz Crystals Under High Vibration, Fig. 5

As Fig. 5 shows, the vibration-induced phase noise of any type of typical crystal would fall within about 30 dB of any other type under the same conditions. It should not be assumed that just by specifying a more precision oscillator with better frequency stability and static phase noise that the performance under vibration will likewise be improved. In many cases, a more precision oscillator will use a crystal that still has similar acceleration characteristics to a less-expensive one. Figure 4 shows the degradation in phase noise that occurs on a high-reliability 10-MHz OCXO with a precision “SC” cut crystal under a flat random vibration profile of 0.05 g2/Hz from 20 Hz to 2 kHz (Fig. 6). Γmax for this oscillator is measured at 7 × 10-10/g, which is typical for a standard 10 MHz third-overtone SC-cut crystal in a round HC-37 holder with a four-point mount.

Manage Quartz Crystals Under High Vibration, Fig. 6

This overall vibration profile is commonly used for oscillator evaluation since the effects due to the oscillator itself over the vibration frequency range are more apparent. Even though the oscillator shown in Fig. 4 is a very stable device with overall stability of better than 0.01 ppm, it can be seen that even this relatively modest vibration causes significant degradation in the short-term stability and phase noise.

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