A history of crystal filters

The first use of quartz crystals as filter elements was suggested by Waiter 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. The use of multiple crystals in ladder and lattice configurations was proposed in the late 1920's and in 1934 Walter P. Mason published the results of his work on crystal filters which were developed for use in frequency-division-multiplex telephone systems. 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. The wideband designs had very limited applications because extremely low impedance crystals were required which were only available at certain frequencies. However, the narrowband design concepts were used with very little modification for the next 20 years. 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. 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 1962 a paper was presented describing the first practical monolithic crystal filter elements and included a cascaded 6-pole design. This was a forerunner of the 2-pole filter elements in wide use today. 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. A short discussion of measuring and modeling problems is included and the final section covers the applications for crystal filters which drove their technology.