US 3189852 A
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June 15, 1965 HIDETOSHI TSUCHIYA 3,
ELECTED-MECHANICAL FILTERS Filed April 9, 1963 Fig, 2
FREQUENCY (KC) FREQUENCY (K C) United States Patent 3,189,852 ELECTRO-MECHANICAL FILTERS Hidetoshi Tsuehiya, Suzaka-shi, Japan, assignor to Kabushiki Kaisha Toko Radio Coil Kenkyusho,
Suzaka-shi, Japan, a ioint stoelr company of Japan Filed Apr. 9, 1963, Ser. No. 271,736 Claims priority, application Japan, Apr. 14, 1962, 37/ 14,980 1 Claim. (Cl. 333-72) This invention relates to mechanical filters which are used in intermediate frequency circuits of radio receivers and other various communication devices and circuits.
Generally, electro-mechanical filters comprise essentially a chain of resonating elements having a configuration of circular discs or rectangular plates and coupling elements interconnecting the resonating elements and are characterized by having sharp filtering properties. On the other hand, however, especially when a rectangular plate is employed, there is a large proportion of spurious responses generated. 7 i
It is an object of this invention to minimize said spurious responses in a flat plate type resonating element system comprising resonating elements having the same size and resonating at the same frequency and coupling elements interconnecting said resonating elements and formed integrally therewith from a single plate thereby providing narrow band filters of the higher order having sharp cut-off characteristics.
The invention comprehends selecting predetermined longitudinal lengths for the resonating elements with respect to the transverse widths, namely 2.5 to 2.9 and 3.1 to 3.7 times, whereby the generation of spurious responses, notably the second harmonic and possibly the third harmonic, are avoided. If such lengths exceed four times the width, there is the possibility of an accompanying bending oscillation.
The invention together with additional objects and advantages thereof will be best understood from the following description of a specific embodiment, when read in connection with the accompanying drawings, in which:
FIG. 1 is a perspective view of one embodiment of this invention, and
FIGS. 2 and 3 show plots of the response characteristics of the filter of this invention.
Referring now to FIG. 1 there is shown a plate resonating element of the shape of the letter H, block style, comprising a pair of spaced rectangular resonating elements 1 and 2 having identical dimensions and coupling element 3 integral with and interconnecting said resonating elements, with said elements being formed by cutting or stamping out a flat metal plate. On one side of said resonating elements 1 and 2 are bonded thin ceramic plates 4 and 5 possessing electrostrictive or piezoelectric properties (for the sake of brevity, hereinafter referred to as ceramic plates). These ceramic plates are silvered on their two fiat surfaces, and respectively sandwiched between pairs of electrode plates 1 and 6 and 2 and 7. In the embodiment shown in FIG. 1, the resonating elements 1 and 2 also serve as the electrode plates. While the electrical connections are not shown, it will be understood that one side (1, 2, 3) forms a common electrode which can be grounded, and electrode plates 6 and 7 are utilized as the output and input terminals for the electrostrictive drive and pickup for the ceramic plates 4 and 5, respectively. It will further be understood that the output or input connections can be made directly to the ungrounded silvered electrode on the ceramic plates in lieu of the electrode plates 6 and 7 in FIG. 1.
It is an object of the invention to provide a mechanical filter of predetermined characteristics and high efliciency by properly selecting the ratio between the longitudinal length and the transverse width of the resonating element. As illustrated in FIG. 1, the following symbols are used for the dimensions:
a denotes the longitudinal length of the resonating element, b the transverse width of the resonating element, 0 the width of the coupling element, d the length of the coupling element, and t the thickness of the metal plate.
It was determined that of these various dimensions, the relation between a and b has the greatest efiect on the spurious response characteristics. The center frequency of the filter is ascertained almost solely by the longitudinal length a of the resonating elements. It was found for a configuration with constant dimensions for a, c, d and t, the degree of the effective coupling between the resonating elements 1 and 2 has a tendency to increase with the increase in width b so that is an increase in the spacing between the two peaks in the response characteristics which results in the broadening of the pass band and the impairing of the filtering properties.
The amplitude characteristics are shown in FIG. 2 wherein curve M is that for a filter with high Q having excellent pass band response, and curve N for a low Q filter having a broad band response and poor selectivity.
Furthermore, the results of experiments show that as the value of the transverse width b of the resonating element approaches a/2 or one-half of the longitudinal length or exceeds this value, then, in addition to the normal fundamental oscillation, the undesirable spurious responses are generated. It has also been found that as the value of b decreases below a/Z such spurious response gradually decreases to substantially zero at about a=5.3 mm. c=0.5 mm. t=0.6 mm. b=1.9 mm. d:1.2 mm.
Dimensions of ceramic plates 4 and 5: 5.1 mm. x 1.9 x 0.2
Metal plate: a ultra-high-strength duralumin plate Input voltage=l00 mv.
Output voltage=38.8 mv. (at 19 C.)
Load=1K In this case, the ratio a/b=2.8, and no spurious response was noted. On the other hand, when a=5.3 mm. b=2.65 mm. a/b=2.0
a certain amount of spurious response was noted as indicated in curves B and Ba.
With respect to the dimension of coupling element 3, its length d is suitably determined from the standpoint of machining, but it was found that it is advantageous to make this length d smaller than the resonator width b and to select its width 0 in the range from a/ 8 to a/ 12 by experiment.
It has been learned by experiment that there are definite relationships between the product of the resonating frequency and the length of the resonating element, that is,
the efiective frequency constant, the ratio between the thicknesses of the metal plate and the ceramic plate and the width of the coupling element for achieving the desired response characteristics. The thickness t of the metal plate is preferably selected to be smaller than the width c of the coupling element for obtaining sharpness in the filter characteristics.
While in the foregoing disclosures one embodiment of this invention has been described wherein the mechanical filter comprises two resonating elements and one coupling element, with ceramic plates bonded to the resonating elements, it will be obvious that the same elfect can be provided by a filter of higher order formed by the connecting in a chain N resonating elements and (N-l) coupling elements where N represents an integer above 2, and ceramic plates bonded only to the resonating elements at both ends of the chain.
In summary, the present invention resides in a plate type tuning element system wherein identical rectangular resonating elements resonating at the same frequency and a coupling element interconnecting said resonating elements are formed from a single plate, with identical rectangular shaped ceramic plates bonded along the longitudinal lengths, the longitudinal length a of said resonating elements being selected to be larger than twice, preferably between 2.5 to 2.9 or 3.1 to 3.7 times their transverse width b, whereby a mechanical filter having extremely small spurious response, sharp cut-off characteristics with respect to the longitudinal oscillations and narrow band characteristics is provided.
In accordance with the provisions of the patent statutes, I have explained the principle and operation of my invention and have illustrated and described what I consider to represent the best embodiment thereof. However, I
desire to have it understood that within the scope of the appended claim, the invention may be practiced otherwise than specifically illustrated and described.
What is claimed is:
An electromechanical filter comprising a plate resonating element system including a plurality of spaced identical rectangular resonating elements and coupling elements interconnecting adjacent resonating elements to provide a chain, said resonating elements and coupling elemerits being formed from a single duralumin plate, an
individual thin plate of a material selected from the group consisting of electrostrictive and piezoelectric materials bonded onto one surface of each of the resonating elements located at the respective ends of the chain, electrode plates respectively bonded on the outer surface of the individual thin plates, said resonating elements, individual thin plates and electrode plates being coextensive in length and width, and the ratio of the longitudinal length to the transverse width of the resonating elements being greater than 3.
References Cited by the Examiner UNITED STATES PATENTS 2,276,013 3/42 Bohannon 333-42 2,814,78 11/57 Burns 333 72 3,015,789 1/62 Honda 333 72 3,051,919 8/62 Foulk ass- 72 3,064,213 11/62 Mason 333-71 OTHER REFERENCES Electronics, H-Shaped Ceramic Filters, Feb. 7, 1964, pages 5557.
HERMAN KARL SAALBACH, Primary Examiner.