GB848019A - Improvements relating to electro-mechanical transducers - Google Patents

Abstract

848,019. Piezo-electric ceramic elements. CLEVITE CORPORATION. Sept. 17, 1957 [Sept. 17, l956], No..29213/57. Class 40(8). A piezo-electric resonator element comprises a disc of ferro-electric ceramic material, e.g. barium titanate, which is polarized to impart thereto piezo-electric properties and is adapted for mechanical resonance in its radial mode at a preselected frequency. Transmitting and receiving electrode means are conductively attached to the disc; said means including a circular spot electrode at the centre of one face of the disc, an annular electrode on the disc concentric with said spot electrode, and at least one additional electrode on the other face of said disc. A resonator 10, for the selective passing and voltage transformation of a predetermined band of frequencies in an electric wave filter circuit is shown in a test circuit in Fig. 3. The spot electrode 14 and annular electrode 18 on one face of the disc conform in area and location to the respective electrodes 16 and 20 on the other face. The resonator 10 is initially axially polarised in the regions lying between corresponding electrodes by connecting leads on opposite sides of the disc to opposite D.C. terminals. The A.C. input voltage is applied between electrodes 18, 20 from signal generator 32 and the output signal taken off between centre electrodes 14 and 16. Variation of the frequency of input signal through a range including the design pass frequency gives response characteristics from VO/V1 versus frequency plot, Fig. 12, where V1 is input voltage across resistor 33 and V0 output voltage across 35. The lower face of the disc may be entirely covered by the electrode, Fig. 4 (not shown) but the area of the spot electrode 14 and the location and area of the annular electrode 18 are such in relation to the distribution and phase of vibrational stresses in the disc that the cancellation of opposing fields between the output electrodes is minimized and the ratio of the areas of input and output electrodes is selected to obtain the desired capacitance and impedance ratio between input and output terminals whereby a voltage transformation of frequencies in said band is obtained. The electrode radius is about 2/5 of the disc radius and the annular electrode 18 is of inner radius just greater than electrode 14 and covers almost all the remaining upper face of the disc. In Fig. 6 the annular electrode 36 extends over the peripheral edge 34 of the disc 12b and the disc is polarized axially between the spot electrodes and radially throughout the remainder of the disc. The polarization may be effected radially throughout the disc. Specifications 780,673, 818,710, 843,919 and 843,920 are referred to.