Survey of Mechanical Filters and Their Applications

Since the inception of mechanical filters several years ago, their many desirable features have resulted in many applications. As filtering requirements have become more stringent, the improved performance of mechanical filters has become more pronounced. Q's in the order of a hundred times better than those of comparable electrical circuits are possible. These high Q's allow the use of lossless filter design for narrow bandwidth flat-top filters with more than one section. The mechanical filter allows compact design which is consistent with the miniaturization of modern equipment. Although numerous mechanical filters structures have been built, three types have found the most application. These are the ladder type with resonant plates interconnected by fine wires, the cylindrical rod structure machined to produce alternate necks and slugs, and a cylindrical arrangement with disk resonators interconnected by coupling wires. The center frequency, bandwidth, and filter skirt selectivity are a function of element sizes, spacing, and number of elements used. Proper selection of resonator size and shape, and proper arrangement of driving and coupling elements will suppress spurious responses, which are a major problem in mechanical filter design. Transducers used with mechanical filters provide for the converting of electrical to mechanical energy or mechanical to electrical energy and impedance matching of the filter. Of the four kinds of transducers that have been employed-electromagnetic, electrostatic, magnetostrictive, and piezoelectric-the magnetostrictive have been the most promising in regard to high frequency of operation, stability, efficiency, and economy.