GB555400A - Piezoelectric crystal apparatus - Google Patents

Abstract

555,400. Piezo-electric crystals. STANDARD TELEPHONES & CABLES, Ltd. July 24, 1942, No. 10367. Convention date, July 25, 1941. Class 40 (v)] A rectangular piezo-electric crystal element is cut from a crystal of the Rochelle salt type so that its major surfaces are substantially perpendicular to one of the principal axes of the crystal and so that adjacent edges of the major surfaces are parallel to the other two principal axes, and comprises sets of independent electrodes for causing vibration in a plurality of independently controllable modes at frequencies depending on the dimensions of the major surfaces, one of the modes being the first shear face mode of vibration. Fig. 1 shows crystals 1, 1A cut with the principal plane at right-angles to the electric axis X of the natural crystal and with the major length dimensions L respectively parallel to the Z and Y axes respectively. Similar pairs of crystals are described having the major planes at right-angles to the Y and Z axes, Figs. 2 and 3 (not shown). With these orientations, the first and second face shear mode vibrations are substantially free from coupling to other modes and have a low temperature co-efficient of frequency. The provision of two electrodes on each major face enables the crystal to be driven simultaneously in the first and second face shear modes which may be used in the filter circuits described in Specifications 531,662 and 542,452. Specifications 532,925, 533,204, 553,457 and 554,788 also are referred to. The crystals may be mounted by suspension or clamping at the nodal points. The frequencies are determined by the ratio of width to length, and the impedance by the thickness.