GB547601A - Electrical circuit arrangements for obtaining substantially constant relative phase shifts over a range of frequencies - Google Patents

Abstract

547,601. Impedance networks. STANDARD TELEPHONES & CABLES, Ltd., and HODGSON, K. G. Jan. 31, 1941, No. 1284. [Classes 40 (iii) and 40 (v)] Relative phase shifts which are substantially constant over a range of frequencies are obtained between the currents in two or more branches of a circuit by including in each branch a network whose insertion phase angle approximates to a linear function of the logarithm of the frequency. The constantresistance lattice network in Fig. 2, in which L11/C21 = L22/C12= R<2> and L11/L22 = C21/C12 = 4K1<SP>2>, gives a phase shift of 180 degrees at fo, the resonant frequency of one impedance arm and the anti-resonant frequency of the other arm, and has an approximately linear phase characteristic with reference to the logarithm of the frequency for values of K1 greater than 2, as indicated in Fig. 4. Two such networks having the same value of K, and with their critical frequencies relatively displaced will therefore give a substantially constant difference in phase. Greater accuracy may be obtained by the use of lattice networks having three or more elements in each arm, such as that in Fig. 3 which gives phase shifts of 180 degrees and 360 degrees at its two critical frequencies. When a phase difference of 90 degrees is required, it is advantageous to employ a network similar to Fig. 2 in one branch of the circuit and a network similar to Fig. 3 in the other branch with their parameters suitably chosen, or, more generally, one network with n elements in each arm and one network with n+1 elements in each arm, with their critical frequencies forming a geometrical progression. Instead of lattice networks, the equivalent bridged-T networks may be used, that in Fig. 2 for example being equivalent to the network in Fig. 7 in which the coupling between the inductances L0 is dependent on the parameter K1. The phase characteristic can frequently be improved by the addition of resistances, as in Fig. 9, or by modifying the terminating impedances. If the resistances r1, r2 are equal, the attenuation characteristic is symetrical about the critical frequency, but it is usually desirable to make them unequal in order that the attenuation shall be more uniform over the working range. Application.-Networks giving a phase difference of 90 degrees over a range of frequencies may be employed in modulating and demodulating arrangements. Fig. 12 shows a known arrangement in which the carrier is fed to modulators 1, 11 with a phase difference of 90 degrees through simple networks 3, 31, while the modulating frequencies are fed through networks, 5, 51 of the kind described. By suitable poling either side-band may be produced in the output circuit, the other being balanced out. Figs. 13, 14 show modifications, which in addition to producing a single side-band have the property of rejecting modulating frequencies above or below the carrier frequency according to the poling. This feature is of importance in the elimination of second channel interference in receiving circuits. Instead of balancing out one side-band, the output circuits shown in Figs. 16, 17 may be used to produce one side-band in the load 4 and the other in the load 41.