Patent GB-878375-A (GB)
Title: Filter transformer
IPC: H03H9/54
Filed: 1959-09-10
Abstract: 878,375. Electro-mechanical filters; piezoelectric crystals. CLEVITE CORPORATION. Sept. 10, 1959 [Sept. 17, 1958], No. 30898/59. Class 40(8) A frequency-selective impedance transformer comprises a pair of piezo-electric resonators, one resonant at a pre-determined frequency and the other anti-resonant at that frequency, means mechanically coupling the resonators, means for driving one of the resonators, and means for deriving a signal from the other resonator. The device shown in Figs. 1 and 3 comprises a pair of ferro-electric ceramic bodies 12 and 14 of substantially disc form mounted coaxially on a cylindrical rod 16. The discs are provided with annular inner electrodes 18a, 18b and annular or circular outer electrodes 20a, 20b, a small unelectroded margin 22 being provided between the electrodes and the edges of the discs. A similar margin 24 isolates the electrodes from the rod 16 when it or its surface is conductive. Discs 12 and 14 are polarized in the thickness direction either after the assembly is complete by utilizing the electrodes 18a, 20a and 18b, 20b, or by utilizing special poling electrodes which are thereafter replaced by the operating electrodes. The diameter, and length of the rod 16 is small compared with the diameters of the discs 12, 14. One or more intermediate discs 26, Fig. 2, can be provided on the rod 16 as described in Specification 867,038. The intermediate disc or discs 26 can be of the same ceramic material as the discs 12 and 14 but need not be polarized, or can be of a different ceramic material which need not be ferro-electric, or can be of metal such as "Ni-Span-C" (Registered Trade Mark). The end disc 14, as well as any intermediate disc 26, is proportional to exhibit resonance in the radial mode at a pre-determined frequency which is substantially equal to the centre frequency of the desired pass band. The other end disc 12 is proportioned, e.g. by being of slightly larger diameter than disc 14, to exhibit antiresonance at the same frequency. The discs can be tuned by silver plating the edges of the discs and applying a thin layer of solder thereto. The accoustic impedance of the material of the rod 16 should closely match that of the discs. When the discs and rod are of the same material the device is formed in two parts 12a, 14b, Fig. 3. Each part consists of a disc and an integral stub 16a, 16b which can be joined by conductive epoxy resins. In a modification the whole rod 16 can be formed integrally with one disc. In operation an A.C. signal is applied to the electrodes of one end disc and the filtered signal is derived from the electrodes of the other end disc. One electrode of each pair can be connected to a common or ground potential, e.g. the inner electrodes 18a and 18b can be grounded. Where the rod 16 or its surface is conductive grounding can be effected by extending the inner electrodes into contact with the surface of the rod, i.e. by eliminating the margins 24. Additional impedance transformation can be obtained by varying the ratio of the area of the input disc electrodes to that of the output disc electrodes. The device can be mounted between resilient spring members contacting the centres of the outside faces of the end discs. A test circuit for the device is described. The polarizable ferro-electric ceramic materials used in the device usually have a perovskite type of crystal structure and can be barium titanate or solid solutions of lead zirconate and lead titanate as described in U.S.A. Specification 2,486,560 and Specification 780,673. Ferro-electric ceramics in which ageing, and/or temperature dependence are minimized and in the form of chemical modifications of lead zirconate titanate are described in Specifications 818,710, 834,920, 843,919, and 867,039. The ferro-electric material preferred is a modified lead strontium zirconate titanate having the composition:- Pb 0 . 95 Sr 0 . 05  (Zr 0 . 53 Ti 0 . 47 ) O 3 +0À7 weight per cent Cr 2 O 3 .