Quartz Tuning Fork Crystal Using Overtone Flexure Modes

Abstract

Summary We have developed high frequency quartz tuning fork crystals using different overtone flexure modes over the frequency range from 15OKHz to lMHz. These quartz tuning fork crystals were produced by using photolithographic, chemical etching and laser frequency adjustment manufacturing techniques, and packaged in a relatively small ceramic package. With its ultraminiature size and ability to withstand high mechanical shock, these crystals have important applications in this frequency range. In this paper we shall first describe the motions of overtone flexure modes of a tuning fork and their frequencies related to the fundamental flexure mode. One of the problems in obtaining a high quality factor of the quartz crystal in the overtone flexure modes tuning fork designs, is the energy loss due to the crystal mounting in the package. The motions of two crystal tines tend to couple to the displacement of a portion of the base of the tuning fork in longitudinal and lateral directions which cause a great energy loss of the flexure motion of two crystal tines. We shall show the effect on the crystal Q factor and frequency shift in different mountings and modification of tuning fork shape. With proper crystal tuning fork shape and mounting, we have achieved Q of better than 2.5 x lo5. The turning point temperature of overtone flexure modes is much higher than fundamental flexure mode. With the change in crystal orientation, one can design a quartz crystal tuning fork which has a turning point temperature over a very wide range.