GB474701A - Improvements in or relating to piezo-electric crystals - Google Patents

Abstract

474,701. Piezo -electric crystals. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Nov. 2, 1936, No. 29770. Convention date, Oct. 31, 1935. [Class 40 (v)] A crystal is so cut as to be capable of vibrating at two predetermined fundamental frequencies in the direction of its thickness. A section 3, 5, Fig. 3, normal to the optic axis Z is first cut from the mother crystal and a portion 5, Fig. 4, is taken from this. Sections 9, 7 having their thicknesses perpendicular to a mechanical or crystallographic axis Y and an electric axis X, respectively, have two different natural frequencies for vibrations in planes parallel to their thicknesses, but an intermediate section 11 exhibits two natural frequencies simultaneously. The ratio of the latter two frequencies bears a linear relation, Fig. 1 (not shown), to the angle between the faces m, n and the plane normal to the axis X. Plates 11 whose faces m, n are inclined at less than 1‹ to the planes normal to the axes X and Y respectively will not vibrate satisfactorily. Plates 11 whose faces are inclined at an angle of cut lying between 8¢‹ to 29‹ to the plane normal to the axis X can be used for generating oscillations at both frequencies, but plates 11 for which this angle is less than 8¢‹ will oscillate only if they are driven. The invention consists in (1) choosing the angle of cut by the aid of the graph in Fig. 1 (not shown) so that the two frequencies shall bear a predetermined ratio to one another, and (2) choosing the thickness of the crystal, by the aid of the graph in Fig. 2 (not shown), so that one of the frequencies may have a predetermined value. The value of the other frequency then conforms.