Patent GB-918127-A (GB)
Title: Improvements in or relating to electromechanical filters
IPC: H03H9/09
Filed: 1959-06-01
Abstract: 918,127. Electro-mechanical filters. SIEMENS & HALSKE A.G. June 1, 1959 [May 30, 1958; April 14, 1959], No. 18598/59. Class 40 (8). An electro-mechanical crystal filter, e.g. operable in the range 4 to 800 kc/s. with bandwidths from approximately 0.5-6%, comprises a plurality of longitudinal or bending resonators constructed, and mechanically coupled together by at least one coupling member, in such a manner that when the resonators oscillate the coupling member is subjected to shear stress, wherein at least the outermost resonators consist of piezo-electric crystal material, electrodes are connected to each of the outermost resonators at spaced points in a nodal plane, and the coupling member, or each coupling member is disposed symmetrically about a nodal plane. Generally the thickness of a coupling member is the same as that of the bending oscillators. Each resonator can be disposed symmetrically about a nodal plane and there can be two coupling- members disposed symmetrically about spaced nodal planes. A11 the resonators can consist of piezo-electric crystal material, and the coupling- member or members can consist of non-piezoelectric material. The resonators and coupling- members can be held together by springs acting in the nodal plane, or resonators of quartz can be connected to coupling-members of polystyrol by glue such as " Araldite " (Registered Trade Mark). The resonators and coupling- members can consist of a single sheet of piezoelectric crystal material, e.g. quartz crystal, subdivided by recesses therein. The resonators can oscillate in the fundamental mode or a harmonic mode. Cuts can be made in the crystal which render the filter operable at a desired optimum frequency-temperature relationship (lowest temperature coefficient) by appropriate orientation of the crystal with respect to its axes. The outermost resonators are coated with a conductive material to receive the electrodes which can serve both as electrical connections and as supporting means for the resonators. In the construction shown in Fig. 8, three longitudinal oscillators 11, 12, 13 are connected by bridges 14, 15, the oscillators and bridges consisting of a single plate of quartz. All the oscillators 11, 12, 13 are coated on both sides with a thin film of gold, the coatings on the oscillators 11 and 13 serving as connections for the filter input and filter output. Short pieces of spring wire 16, 17, 18, 19, bent at rightangles, are soldered to each gold coating on the oscillators 11, 13 in the nodal plane. The pieces of spring wire are fixed to the carrying system which serves as an electrical connection and carries the quartz plate. The carrying system consists of a cover-plate 20 in which are fixed two leads 23, 24 to which are welded metal strips 25, 26, respectively. Two pairs of wires 28, 29 and 30, 31, are respectively welded to the metal strips 25, 26 and to the two lateral lobes of a sheet-metal loop 27 welded to the cover-plate 20. The spring wires 16, 17, 18 and 19 are respectively soldered to the wires 28, 29, 30 and 31. The assembly is inserted into a metal can which is soldered to cover-plate 20. Electrical input and output are effected by leads 23, 24 and the metal can (or cover-plate 20). The metal loop may be insulated from the coverplate 20 and a separate lead connected to the loop 27 which can be brought through an insulated bush in the plate 20. Two threaded bushes 34, 35 are soldered or spot-welded to the outside of the can 32.