GB554789A - Rochelle salt piezoelectric crystal apparatus - Google Patents

Abstract

554,789. Piezo-electric crystals. STANDARD TELEPHONES & CABLES, Ltd. July 17, 1942, No. 9960. Convention date, May 22, 1941. [Classes 40 (iii) and 40 (v)] A rectangular Rochelle salt crystal element is cut with its major plane at right-angles to the X or Y axis of the natural crystal and the length dimension is at an acute angle to the other two axes, the crystal being provided with sets of electrodes which give uncoupled resonances in a flexural and an extensional mode, the ratio of width to length being between .05 and .5. Fig. 1 shows an X cut plate 1 of Rochelle salt with the major length dimension at an angle of 49 degrees 561 to the Y axis. In this position the longitudinal flexural vibrations depending on the ratio W/L of width to length and the extensional vibrations depending on length are uncoupled to any other modes. When energised by electrodes 10 ... 13 connected as shown in Fig. 5 the second flexural and fundamental extensional modes are excited and may be used simultaneously in a filter circuit such as those described in Specifications 531,662, 542,452, and 553,457. A similar Y-cut crystal is shown, also electrode arrangements, for a second harmonic flexural mode using four pairs of electrodes, with means for varying the relative impedances associated with the two modes of vibration by altering the position and angle of the line of division between the two pairs of electrodes on each crystal face. The crystal may be mounted by suspension or clamping at the nodal points. Specifications 532,925, 533,204 and 554,788 also are referred to.