Patent GB-345269-A (GB)
Title: Improvements in piezo-electric resonators
IPC: H03H9/19
Filed: 1929-12-18
Abstract: 345,269. Piezo-electric crystals. STANDARD TELEPHONES & CABLES, Ltd., Columbia House, Aldwych, London.- (Assignees of Marrison, W. A. ; 732, Ridgewood Road, Maplewood, New Jersey, U.S.A.) Dec. 18, 1929, No. 38783. Convention date, Dec. 19, 1928. [Class 40 (v).] Relates to means of cutting and mounting a piezo-electric crystal oscillator so that it has a temperature-coefficient which is substantially zero over a given range of temperature. The crystal may be rectangular or annular, with a central hole of circular, elliptical, or other shape ; or there may be several holes. A plate of crystal cut in the plane of the optical and electrical axes, and of considerable thickness, is found to have a small negative temperature-coefficient. By gradually reducing the thickness, whilst maintaining the cross-sectional area constant, the temperature coefficient rises from negative, through zero, to positive values. Similarly by reducing the diameter of a disc-shaped crystal, of such thickness that it had originally a positive temperature-coefficient, the latter can be reduced first to zero and then to negative values. An annular crystal section has a temperature-coefficient less positive than that of a solid disc of equal diameter and thickness, the coefficient becoming less positive, or more negative, as the ratio of thickness to crosssectional area increases. In this way it is possible to obtain a resonator of greater diameter (having a zero temperature-coefficient) to vibrate at a given frequency, than if the crystal is either rectangular or disc-shaped. The hole in the centre of the ring section or toroid may be of various shapes, as shown in Figs 3, 4 and 6. A crystal may also be cut to have zero temperature-coefficient, over a limited temperature range, by cutting the crystal so that vibrations along one axis are harmonics of the vibrations along the other axis. The same methods of shaping down a crystal to a given frequency are also applicable when the crystal section is cut out at an angle to the principal axes. Different frequency adjustments may also be made bv cutting-out sections, such as A, Fig. 5, in the direction of the optical axis, or B in a direction perpendicular to that axis. Fig. 7 shows a mounting for a toroidal crystal 9, with an edged hole, as in Fig. 4, and a central one-point support 2. A metallic screw-cover 5 forms one electrode, and the base 1 a second electrode, the two being separated by spacers 4, which may have a temperaturecoefficient opposite to that of the crystal. Figs. 8 and 9 show two methods of applying a pair of electrodes E to a toroidal oscillator Q. The value of the temperature-coefficient can also be controlled, particularly as a final adjustment, by varying the degree of excitation along the different axes. For instance, as shown in Fig. 11, a potentiometer P allows the exciting voltage applied across one pair of electrodes E to be varied with respect to that applied across the other pair of electrodes E<1>. A similar result is obtained by varying the spacing between the electrodes and the crystal along the different axes; or by varying the size of the electrodes ; or by adjusting the impedance of the circuit to which the circuit is coupled.