The desire to send multiple voice messages on a single telephone line resulted in the introduction of carrier telephone systems in 1916. The crystal filter has been a particularly critical element in the development of narrowband communications systems. The history of the development in crystal filter technology, from the initial concepts of Cady to the current wide range of products, provides a fascinating chapter in the development of today’s highly complex electronic products. A short discussion of measuring and modeling problems is included and the final section covers the applications for crystal filters that drove their technology. This paper will discuss the progression of these developments in the crystal filter art and the accompanying work on filter crystals which was essential to their success. This was a forerunner of the 2-pole filter elements in wide use today. In 1962 a paper was presented describing the first practical monolithic crystal filter elements and included a cascaded 6-pole design. In the early 1960’s the more general filter design process based on “insertion loss” theory was adapted for use with crystal filters and made possible the fabrication of filters with true Chebyshev, Butterworth, and other traditional filter characteristics. In the mid 1950’s new designs were developed for “intermediate-bandwidth” filters which covered the bandwidth region between the earlier narrowband and wideband designs. However, the narrowband design concepts were used with very little modification for the next 20 years. The wideband designs had very limited applications because extremely low impedance crystals were required which were only available at certain frequencies. In Mason’s paper he describes a “narrowband” design which is useful for bandwidths up to about 0.4% of center frequency and a “wideband” design for bandwidths of about 2% to 6% of center frequency. Mason published the results of his work on crystal filters which were developed for use in frequency-division-multiplex telephone systems. The use of multiple crystals in ladder and lattice configurations was proposed in the late 1920’s and in 1934 Walter P. The first use of quartz crystals as filter elements was suggested by Walter Cady in 1922 in his paper “The Piezo-Electric Resonator”, although in the initial conception a single crystal only was used as a very narrow bandwidth coupling element. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists or to reuse any copyrighted component of this work in other works must be obtained from IEEE. Personal use of this material is permitted. Proceedings of the 1998 IEEE International Frequency Control Symposium © 1998 IEEE.
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