GB600320A - Improvements in wave filters - Google Patents

Abstract

600,320. Electric filters. WESTERN ELECTRIC CO., Inc. March 21, 1945, No. 7097. Convention date, March 23, 1944. [Class 40 (viii)] An impedance transforming wave-filter comprises a lattice network together with a series inductance at one end and a shunt inductance at the other. The Specification considers a lattice 5, Fig. 2, with impedance transforming ladder-type half-sections 6, 7 at either end, each having cut-off frequencies f1 and f4 and the same mid-band frequency as the lattice, and assumes an ideal transformer inserted between the lattice and the end section 7. The transformer and capacity CA are then replaced by an equivalent network as described in Specification 266,055, [Class 40 (iii)]. By suitably choosing the impedance transformation ratio of the transformer it is shown the combination of the transformer and end section 7 can be reduced to section 12, Fig. 5, in which the capacities have the same values C1 and C2 as in the end section 6. The inductances in the two sections are also shown to be equal. The capacities C1 and C2 are then incorporated into the lattice to produce the network shown in Fig. 7. The transformation ratio of the final network is shown to be equal to f<2>m/ (f1 - -f4)<2>, where f<2>m = f1f4. To produce a balanced network, the series inductance may be inserted half in each leg as shown in Fig. 1. A certain proportion of the shunt capacitance CX may be retained outside the lattice as a trimmer. The arms of the lattice may be crystals, subject to certain restrictions on. the percentage band-width of the filter.