Electromechanical Filters for Single-Sideband Applications

Abstract

In discussing some of the basic properties of electromechanical filters, both longitudinal and torsional modes of vibration are considered. The torsional mode has advantages which make it superior to the longitudinal mode in most applications. The limits of the frequency range, at the present state of the art, are from approximately 50 to 600 kc. From a production standpoint, however, the preferred frequencies are from 200 to 250 kc. The high Q and inherent thermal stability of electromechanical filters make them ideally suited for filter type single-sideband equipments. In view of their size, weight, rugged construction, and reduced spurious responses, torsional mode filters designed to operate at a carrier frequency of 250 kc are an excellent choice for this application. A typical 250 kc 9-resonant section torsional sideband filter provides 300 cycles of audio cutoff, carrier rejection of 27 db, and a 60 to 6 db shape factor of 1.56. A 7-resonant section filter of the same type provides 20 db of carrier rejection and a shape factor of 1.85. Carrier filters of 250 kc have been built having a bandwidth of 200 cycles and a shape factor of 3.