GB553457A - Piezoelectric crystal apparatus - Google Patents

Abstract

553,457. Piezo-electric crystals. STANDARD TELEPHONES & CABLES, Ltd. April 3, 1942, No. 4479. Convention date, Dec. 12, 1940. [Class 40 (v)] A piezo-electric quartz crystal element has its major plane parallel to the YZ plane of the mother crystal and its major axis at an angle of -47.5 degrees to the Y axis of the mother crystal, the ratio of width W to length L being in the range of .83 to .89. This gives a crystal with uncoupled vibrations in a longitudinal amode dependent on the dimension W, and a double shear amode dependent on the ratio W/L. Thus the two frequencies can be independently controlled, and the impedance of the crystal is determined by the thickness. A graph is given, Fig. 4 (not shown), relating frequency to the ratio W/L which shows that the two amodes have the same frequency for a value W/L = .86 and different frequencies for other values. Fig. 6 shows a suitable arrangement of four electrodes (only 10, 11 visible) for exciting the crystal, the angle a being preferably such as to cause the gap between the electrodes to be along the Y axis of the mother crystal. Variations in α vary the relative impedance of the two amodes of vibration and the equivalent internal capacities. Further adjustments can be made by removing parts of the electrodes as at 16. In a modification, Fig. 9 (not shown), one face is covered by a single electrode having connected to it a thin screening strip which extends round the edge of the crystal plate and lies in the gap between the electrodes on the other surfaces to reduce mutual capacity. In a further modification, Fig. 11, each surface of the plate has four electrodes such as 10A, 10B, 11C, 11D which are so cross-connected that one quarter of the crystal is connected in series with another quarter. This reduces the internal capacity for a given total impedance. In all the embodiments, nodal points exist at the positions 5 and the crystal may be mounted by clamping at these points or by the attachment of spring-mounted lead wires. Specification 542,452 is referred to.