Patent GB-1365386-A (GB)
Title: Electromechanical filters
IPC: F15B15/14
Filed: 1971-09-17
Abstract: 1365386 Electromechanical filters SIEMENS AG 17 Sept 1971 [29 Sept 1970 30 Sept 1970] 43390/71 Heading H3U An electromechanical filter; comprising a plurality of bending resonators 1, 2...n formed of a steel with low thermal expansion coefficient, coupled at points of maximum amplitude oscillation by mechanical couplers K 12 , K 23 ,..., also formed of steel or a nickeliron alloy with metallic additions, termed "Thermelast" and having anomalous dependence of its modulus of elasticity (E) upon temperature, Figs. 8, 9 (not shown), the end resonators having electrostrictive input and output transducers W, W<SP>1</SP> directly applied thereto, the side remote therefrom being metallized; is trimmed after assembly with its output terminals 6, 6<SP>1</SP> either shorted by an ammeter, or effectively open circuited, a voltmeter being connected across them, or terminated with a reactive network of substantially short circuit, or open circuit impedance, by comparing the response of the assembled filter when a variable frequency source is applied across its input terminals 5, 5<SP>1</SP> with the desired response, under control of computer PR and, in dependence on the deviation, utilizing trimmer A to sandblast material from the resonators, and/or couplers. The ends of the resonators may either be mechanically free, or clamped. Alternatively material may be evaporated off by laser radiation, or "Thermelast" couplers are annealed. Trimming process. Fig. 6.-Transmitter 16 excites the filter F whose response via receiver 18 and device Amp are fed to element PE which, under control of computer PR simultaneously trimming further filters via lines 12, adjusts transmitter 16 in frequency steps until all filter resonances are found, whence PR on the basis of the deviations of the actual from the theoretical resonances controls the trimmer A via circuit T to reduce the deviations to within a given tolerance. In Fig. 5 (not shown) a frequency meter (FM) indicates the actual filter frequencies. The trimming may take place in several coarse followed by fine trimming steps. The resonators are preferably trimmed individually to their respective desired resonance frequencies f 1 , f 2 ...f n  prior to assembly. Trimmer A.-Should trimming be achieved by sandblasting (not shown), the couplers are formed to an oversize to allow for material removed during trimming. When annealing of "Thermelast" is used for trimming, an initial heat treatment of the couplers is made to allow for the modulus of elasticity to be increased only upon trimming (Fig. 9), or a higher initial treatment temperature may be selected to allow for either increase, or decrease of the modulus. In Figs. 10, 11 (not shown), the resonators (50) are coupled by a member (60) forming the set of couplers, and each coupler is heated under control of unit 120 to a predetermined trimming temperature detected by sensor (110), by means of a current through electrodes (70, Fig. 10) or alternatively the pulse duration, intensity or action time of a laser beam focused by device (160) is controlled (Fig. 11). Theory.-For trimming, the filter may be so terminated that its performance can be gauged in terms of the zeroes of one element of its cascade matrix, to govern the trimming which is carried out uniformly for each filter component.