| MadSci Network: Engineering |
The highest comercially-available frequency I can find for quartz crystals is 200 MHz, though above 20 MHz the long-term reliability degrades due to aging of the crystal. According to the Web site for Piezo Crystal Company , below 20 MHz they are supposed to lose less than 0.5 parts per billion per day.
The cut of the crystal affects the frequency in the same way that the length and diameter of an organ pipe affects the frequency of sound it produces. For a given size and shape, a "resonant frequency" of vibrational waves will travel along the crystal and reflect from the end in just the right way to match with and reinforce the initial vibration. There are usually more than one resonant frequency possible other than the primary one. These are called "harmonics" or "overtones." It is the full set of tones that give each instrument its particular "sound," even when playing the same primary note.
Similarly, quartz oscillators often use an overtone of the crystal rather than the primary frequency. I don't understand all the details of why that is a better operating mode, but apparently it helps reduce aging problems.
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