GB794787A - Improvements in or relating to electric wave filters - Google Patents

Abstract

794,787. Impedance networks. STANDARD TELEPHONES & CABLES, Ltd. June 19, 1956, No. 18936/56. Class 40 (8). An electric wave filter of the differential lattice type comprises two transformers 8, 9, Fig. 3, each having inductively coupled windings 12, 13 or 14, 15, the windings 13, 15 each having N times as many turns as the windings 12, 14, where N is greater than 1, two loop circuits being formed by the windings 13, 15 in series with an impedance 16 and the windings 12, 14 in series with an impedance 17, and the connections being such that the mutual inductance of the windings 12, 13 is opposite in sign to that of the windings 14, 15, and the input is fed to a coil 10 of the transformer 8 whilst the output is derived from a coil 11 of the transformer 9. The impedances 16, 17 may comprise piezo-electric crystals, or resonant networks containing inductors and capacitors, or a combination of such elements. The electric wave filter may be rendered equivalent to a lattice filter having impedances X, Y arranged as in Fig. 1, by making N equal to #p/q, where the value A of the impedance 16 is equal to pX and the value B of the impedance 17 equal to qY, p and q being different numerical instants. The adjacent terminals of the windings 12, 13 and 14, 15, respectively, may be connected together, one of the conductors 18, 19 then being omitted. A filter equivalent to two or more of the lattices of Fig. 1 connected in cascade may be obtained by connecting one or more transformer or transformers across the junctions of impedances connecting the windings 12, 14 and 13, 15. The windings 22, 25 and 23, 26 of two transformers 20, 21, Fig. 4, are connected across the junctions of impedances 16, 24, 28 and 17, 27, 29, neither transformer 20, 21 having a third winding.