Patent GB-1051575-A (GB)
IPC: H03H9/54
Abstract: 1,051,575. Electro-mechanical filters. KOKUSAI DENKI KABUSHIKI KAISHA. Nov. 2, 1964 [Nov. 2, 1963 (2); Dec. 28, 1963], No. 44624/64. Heading H3U. A piezo-electric oscillator for use in an electro-mechanical filter comprises a discshaped piezo-electric body 1 having silver electrodes 2, 3 on the opposed plane surfaces along diameters of which metal wires 4, 5 are adhered, by soldering for example, in such a manner that the wire 4 on the electrode 3 is perpendicular to the wire 5 on the electrode 2, the wires 4, 5 being connected to lead wires 6, 7. The effect of any asymmetry of the disc 1 is eliminated by the point symmetry of the cross wires 4, 5 with the result that asymmetrical oscillations which are near the desired oscillating mode are effectively eliminated. An electro-strictive body may be used instead of the piezo-electric body. Two such oscillators can be connected to form an electro-mechanical filter by extending the wire 4 across from one oscillator to the other (Fig. 3, not shown). In the embodiment of Fig. 5(A), a multi-stage filter is constructed by connecting two discshaped piezo-electric oscillators OSC and two disc-shaped resonators 1a by means of a coupling wire 4b. Each resonator la comprises a metal disc and is coupled to the wire 4b at its peripheral points a, b and at a central point c. Coupling at the central points c has been found to remove the undesirable oscillations which would be caused by coupling the resonators 1a to the wire 4b at only the two points a and b. Rod-shaped torsional resonators may be coupled in series between the oscillators OSC and their adjacent resonators 1a (Fig. 6, not shown). Although the frequency of the desired torsional mode of these resonators coincides with the frequency of the desired radial mode of the resonators la, since the frequency distributions of the undesired oscillations differ, undesired oscillations can be suppressed. In another embodiment (Fig. 8, not shown), a plurality of intermediate resonators 1a are successively coupled by a plurality of coupling wires 4c so as to be superimposed. A further embodiment (Fig. 11(B)) has a plurality of rod-shaped resonators 13, 13a disposed along opposite sides of a coupling wire extending between two oscillators OSC. An A.C. voltage at the input terminals 6, 7 of one of the oscillators OSC excites the latter in the radial direction and this causes excitation of the coupling wire in its longitudinal direction, whereby the resonators 13, 13a attached to the coupling wire are excited so as to carry out torsional oscillations. The intermediate resonators 13 are designed so as to carry out torsional oscillations of third order at the rated frequency of the filter and the terminal resonators 13a are designed so as to carry out torsional oscillations of second order at the rated frequency. Since the oscillation mode orders of the resonators 13, 13a are different, subsidiary undesired resonating frequencies of the two kinds of resonator are different, whereby response produced from the intermediate resonators 13 is suppressed. This construction enables the distance between adjacent coupling points of the resonators to be made smaller than the diameter of the resonators.