Patent GB-1074292-A (GB)
Title: Improvements in or relating to electromechanical filters
IPC: H03H9/50
Filed: 1965-01-29
Abstract: 1,074,292. Electromechanical filters. SIEMENS A.G. Jan. 29, 1965 [Jan. 30, 1964; May 12, 1964; Sept. 21, 1964 (2); Sept. 30, 1964], No. 3935/65. Heading H3U. A filter, Fig. 1, comprises two resonators 10, 11 coupled through means 12. The resonators are of steel, but may be of glass, and are divided up by piezo-electric plates 13 ... 20, e.g. of a lead-ceramic material. The plates are incorporated into the resonators so that gaps 21 are formed, these gaps being situated in the central planes of the resonators and being mutually parallel. Two opposite corners of the resonators 10, 12 have chamfers 22, heads 27, 271 connected to a terminal 23 are attached to the ends of resonator 10 and to the central part is attached a lead 28 connected to a terminal 24. The resonator 11 similarly has leads 29, 291 connected to a terminal 25 and a lead 30 connected to a terminal 26. The leads 28, 30 make an angle of 45 degrees with the upper and lower surfaces of the resonators and are of such stiffness to enable them to secure the filter in a housing (not shown). The plates 13, 14 are polarized by prior D.C. processing in the direction of arrows 31, 32 and the plates 15, 16 are polarized in the opposite sense indicated by arrows 33, 34. Thus on the application of an oscillating voltage U 1  to the terminals 23, 24 the plates 13, 14 are expanded during one halfcycle and the plates 15, 16 contract, this process being reversed during the following halfcycle. This expansion and contraction causes bending oscillations in the resonator 10 indicated by double arrow 1 when its natural frequency corresponds with the frequency of voltage U 1 . The asymmetry of the resonator 10, due to the chamfers 22, causes a bending oscillation indicated by the double arrow 2 to be simultaneously excited and, due to the square cross-section of the resonator, the frequency of this oscillation coincides with the frequency of oscillation 1. The resonator thus executes two bending oscillations in mutually perpendicular directions, these oscillations being transmitted to the resonator 11 through the coupling means 12 situated at tho antinodes. For the oscillations 2 the means 12 acts as an axial coupler and produces bending oscillations 3, and due to the chamfers 22 bending oscillations 4 are simultaneously produced. The piezo-electric plates 17, 18 are oppositely polarized in the directions of arrows 36, 37 and the plates 19,20 are similarly polarized in the directions of the arrows 38, 39. Thus the resulting expansion and contraction of the plates 17 ... 20 due to the oscillations 4 produces an oscillating voltage U 2  at terminals 25, 26. In addition, the coupling means 12 serves as a bending coupler to couple the oscillation 1 and produce the oscillation 4 in a manner by-passing the oscillations 2, 3. The oscillations 1 and 4 may be in phase or in antiphase with one another and similarly may be the oscillations 2 and 3. As shown, the coupling means 12 is of rectangular cross-section, the degree of axial coupling being determined by the area of this cross-section and the degree of bending coupling being determined by the second moment of area. Since both coupling effects can be determined independently of each other, the band width of the filter and also the spacing of the attenuation poles occurring above and below the pass-band may be arbitrarily selected. Fig. 8 shows the amplitude x of the output voltage U 2  plotted against the frequency ratio f/f 0  where f o  is the centre frequency, the two resonators simulating a 4-circuit band filter. Fig. 9 shows attenuation a B  plotted against f/f 0 , the interval B #  between the pole frequencies f #1  and f #2  being arbitrarily selected by the degree of the bending coupling. Thus an n-circuit filter may be produced using only n/2 resonators by means of which 2(n/2 - 1) poles of attenuation are formed. In a modification, Fig. 10, the asymmetry is  introduced by crescent-shaped recesses 66 and the coupling means 57 is of circular crosssection. In another modification (Fig. 11, not shown) four resonators (70 ... 73) are interconnected through coupling means (74 ... 76), the end resonators (70, 73) being provided with piezo-electric plates (77 ... 84) as in Fig. 1, the assembly being mounted by anchoring wires (85) attached to the resonator (72). The input oscillating voltage U 1  is applied to terminals (94, 95) connected by wires (98, 98<SP>1</SP>, 99) to the resonator (70) and due to the chamfers (101) bending oscillations (1 ... 8) are set up in the resonators so that an output oscillating voltage U 2  is produced at terminals (96, 97) connected by wires (109, 109<SP>1</SP>, 110) to the resonator (73). The assembly functions as an eight circuit band filter having six poles of attenuation. In modifications, Figs. 13, 14 (not shown), the coupling means between a pair of resonators comprises a pair of wires in " X " configuration. In a modification, Fig. 15, four resonators 170 ... 173 are interconnected by pairs of wires 175, 176 arranged at right-angles to each other, the wires 175 of greater diameter acting as bending couplers and the wires 176 as axial couplers. The end resonators 170, 173 have piezo-electric plates as in Fig. 1. In modifications, Figs. 16, 17 (not shown), the coupling means comprise, respectively, a channel-section member and a thin-walled tube. Fig. 19 shows wire coupling means 307, 3071 bent in to U-shape. In a modification, Fig. 26, the asymmetry of the resonators 407, 408 is introduced by chamfers 405, 406 which extend only part way along the resonator length and are perpendicular to one another. Wires K1, K2 serve as the coupling means. In a modification, Fig. 27, the resonator comprises three steel sections 415 ... 417, piezo-electric plates 420 ... 423 being dispersed between sections 416, 417 and piezo-electric plates 424 ... 427 being disposed between sections 415, 416, these plates being constructed as in Fig. 1. Silver layers 430 ... 433 are disposed, respectively, between the plates 420 and 421, 422 and 423, 424 and 425, 426 and 427. Leads 435, 436 connect the layers 430, 431 to a terminal 439 and leads 437, 438 connect the layers 432, 433 to a terminal 440, the three sections 415 ... 417 being interconnected by leads 453. The input oscillating voltage U is applied to terminals 439, E and the output voltage U a  at terminals 440, D.