The frequency of a quartz crystal is determined by its vibration mode (fundamental or overtone), the harmonics of that mode, and the physical dimensions of the quartz plate. Utilization of the harmonics and modes enables the crystal engineer to design crystals spanning a frequency range of 1.0 MHz. To 250 MHz.
AT-Cut crystals at and above 1.0 MHz. are typically shear vibrators whose frequency is determined by the thickness of the quartz plate. Like a guitar string, the thinner the quartz plate the higher the frequency. Another factor that must be considered is the D:T ratio or diameter to thickness. Since the vibrational energy takes place in the center of the quartz plate, confining the energy into a small area makes it difficult for the plate to be set into vibration. Optimum D:T is equal to or greater than 70:1. At ratios of 40:1 or less, the impedance of the plate begins to increase and make it difficult for the device to vibrate. The D:T at a given frequency has a direct bearing on the size of the holder that can be utilized. Generally, lower frequency devices in a given mode require larger packages and higher frequency devices can be placed in smaller packages at the same mode.
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