Crystal filters for radio receivers

Crystal filters can be effectively employed to give highly selective i.f. channels in communication receivers. The first example is the pair of 465-kc/s single-section bridged-T crystal filters designed for the Army receivers R206 and R201 during the early part of the war. The second is the set of channel and carrier filters for the Post-Office two-channel single-sideband receiver. These are of recent design. Foster´s Theorem for two-terminal reactance networks is described and applied briefly to the solution of the properties and elements of a lattice network¿the starting point of filter design. Properties of the particular forms of lattice network necessary in crystal filters for radio receivers are discussed. Resistance compensation is explained. The application of the equivalent circuit of a crystal resonator to filter networks is discussed and the properties of some applicable quartz cuts are outlined. The method of calculation of L-and C-element values is given. Finally, the design of a new type of two-section half-lattice channel filter is brieflv described.