A vibrating quartz crystal has a number of modes being excited simultaneously . These additional modes or overtones are at odd multiples of the fundamental mode/frequency, ie: 3X, 5X, 7X, 9X….etc. Understandably they are called the 3rd, 5th, 7th, 9th and so on overtones or modes. For example, a crystal manufactured to operate at 10 MHz. fundamental mode will also have vibrational modes at 30 MHz, 50 MHz, 70 MHz, 90 MHz, and so on.
By using the overtones/modes the crystal engineer is able to overcome the physical limitations of plate geometry to extend the frequency range to 250 MHz. And higher while, at the same time, insuring the size of the device remain practically small.
Typically fundamental modes using flat blank technology and mechanical lapping processes are practical to about 40 MHz. The corresponding overtones would be 3rd overtones to 120 MHz 5th overtones to 200 MHz. 7th overtones to 280 MHz. and so on.
Fundamental modes to 350 MHz. are made possible through ring-supported resonator technology, i.e. chemical milling. Fundamental modes to approximately 200 MHz. are practical with theoretical corresponding 3rd overtones to 600 MHz. State-of-the Art measurement techniques limit the practical frequency range of fundamental & overtones resonators to about 250 MHz.
INVERTED-MESA RING SUPPORTED RESONATOR GEOMETRY
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