Electromechanical Filters for 100-Kc Carrier and Sideband Selection

A general discussion of a torsional type mechanical filter and its termination by mechanical and electrical means is followed by a detailed description of two 100-kc filters, one 50 cycles wide and the other 3.1 kc wide. The design, fabrication, and frequency adjustment of one-piece multiple section filters is described. Frequency stability better than 2 parts per million per degree centigrade is obtained with filters constructed of the metal alloy Ni-Span-C. Magnetostrictive ferrite and Ni-Span-C transducers resonant in the longitudinal mode of vibration are attached tangentially to the end resonators of the torsion filter. The filter is terminated by converting the electrical resistance in low-Q input and output circuits to mechanical damping. Responses due to spurious modes of vibration in the filter are reduced by the use of a pair of transducers in a balanced arrangement at each end of the filter, and by mechanical damping in the end supports.