Piezoelectric ceramic tuning fork filters

Within the past year, marked progress has been made in the development of a class of low frequency ceramic tone filters offering significant advantages over existing filters in the 50 to 9000 cps range. These filters utilize a pair of piezoelectric ceramic Bimorph elements mounted and connected so that stress cancellation occurs in the base region and the operation approaches that of an ideal double cantilever structure. When operated as a three terminal filter device an asymmetrical response characteristic is obtained with a pole of attenuation on one side of the passband or the other. This pole can be placed on either side of the passband. In addition, this pole can be moved arbitrarily close to the passband. An equivalent circuit is presented which explains both of these features. A 2000 cps filter can be mounted in a package which is 0.25" in diameter and 0.62" long, has a volume of 0.03 cubic inches, and weighs only 0.01 ounces. A 200 cps filter would be about 1.8" long. Typical response curves will be presented showing 6 db bandwidths of about 4% and minimum power insertion losses ranging from 3 to 8 db. With minor modifications these piezo-electric tuning forks can be used as resonant reed relays which require 100, 000 times less power than magnetic resonant reed relays. In addition, piezoelectric resonant reed relays offer substantial savings in size and weight as well as immunity to stray magnetic fields. Both the filters and the resonant reed relays are made from a filter ceramic whose temperature dependence and aging characteristics have proven satisfactory in a wide variety of commercial and military applications.