1295692 Electro-mechanical filters WESTERN ELECTRIC CO Inc 5 Feb 1970 [10 Feb 1969] 5565/70 Heading H3U A bandpass filter circuit comprises a plurality of monolithic crystal filter structures FS1 ... FS4 operating in the thickness shear mode, each structure consisting of a piezoelectric wafer 14, 48, 50 or 52 and each wafer forming two resonators by means of two pairs of electrodes. Thus, wafer 14 has electrode pairs 10, 12 and 18, 20 forming resonators 16, 22. The two resonators of each -structure FS1 ... FS4 are "control coupled" by means of the respective piezoelectric wafer and each structure is so dimensioned that the two resonators are more loosely coupled than the maximum "controlled coupling condition". "Control coupling" means that the two resonators affect one another by only a small controlled degree and the loosely coupled condition ensures that F A -F B is less than the maximum Fa A -F A or Fa B -F B , where F A , F B are the resonant frequencies of the two resonators and Fa A , Fa B are their antiresonant frequencies. Similarly, the resonators of the adjacent structures FS1 ... FS4 are loosely coupled by a degree less than the maximum controlled coupling condition by means of a capacitor. Thus, resonators 22, 54 of structures FS1, FS2 are coupled by a capacitor C1, the structures FS2 ... FS4 being similarly coupled by capacitors C2, C3. According to another embodiment (Fig. 18, not shown), four coupled monolithic crystal structures have, respectively, five, three, two and three resonators. In another embodiment (Fig. 19, not shown) the coupling capacitors are replaced by inductors. The Specification gives an analysis of the circuit and also a synthesis.