Electronically Tunable Circuit Elements

During the past sixty years, attempts have been made to develop electronically-controllable circuit elements to be applied to filter networks in tunable receivers and transmitters. Both the polarization of the dielectric material used in the construction of capacitors and the permeability of the ferrite core material used in inductor design, lend themselves to electronic variation; in fact, the change in the incremental permeability of a magnetizable material in the presence of a superimposed varying magnetic field was patented as early as 1901. Usable circuit elements, however, were not realizable for application in the frequency ranges above 100 kc, nor did the available materials provide the desired characteristics for modern filter design of receivers and transmitters. USASRDL initiated a program in 1948 to investigate the problems associated with electronically variable inductances. New ferrite materials were developed and a better understanding of the over-all problem obtained. This program grew in magnitude, calling upon the efforts of both universities and commercial organizations, in addition to the internal effort within this laboratory. The outcome is a family of controllable inductances, electronically tunable in the frequency range dc to 500 Mc with reasonable Q´s, power requirements, and small in size and weight. Large frequency variations are obtainable and they are usable under environmental conditions of temperature, humidity and vibration encountered in field conditions.