GB554787A - Rochelle salt piezoelectric crystal apparatus - Google Patents

Abstract

554,787. Piezo-electric crystals. STANDARD TELEPHONES & CABLES, Ltd. July 9, 1942, No. 9542. Convention date, May 23, 1941. [Class 40 (v)] A rectangular element is cut from a crystal of the Rochelle salt type so that its major surfaces are parallel to the plane containing two of the principal axes of the crystal and so that the longest edge of the element cuts one of the crystal axes at an angle of 22.5 degrees, the dimensional ratio of width to length being in the range of .567 to .822. Fig. 1 shows a plate of dimensions W, L, T cut with its major plane at right-angles to the X-axis of the natural crystal, and the major length dimension marking an angle of 22.5 or 67.5 degrees to the Y-axis. This gives equal excitation and minimum coupling for the two modes, fundamental longitudinal dependent on dimension W, and double shear mode dependent on the ratio W/L. These may be excited by two pairs of electrodes 10, 11 and 12, 13. Fig. 5 and the crystal may be supported at nodal points 5. The impedance of the crystal is determined by the thickness, while the relative impedances of the two modes may be varied by varying the angle of the dividing line 7 between the electrode, relative to the axes of the crystal plate. A small earthed electrode strip may be inserted between the adjacent electrodes to reduce the mutual capacity. Also the lower pair may comprise a single electrode. Similar arrangements are described for Y-cut and Z-cut crystals. Specifications 531,662, 532,925, 533,204, 542,452, 553,457, 554,788 and 554,789 are referred to.