GB469067A - Attenuation equalizers for electric transmission and like systems - Google Patents

Abstract

469,067. Impedance networks. STANDARD TELEPHONES & CABLES, Ltd. Dec. 8, 1936, No. 33693. Convention date, Jan. 30. [Class 40 (iii)] Attenuation equalizers, particularly suitable for use with transmission lines whose characteristics are subject to variation with temperature and humidity, comprise a network N, Fig. 1, having input terminals 1, 2, output terminals 3, 4, and side terminals 5, 6 to which a variable impedance Z is connected. The arrangement is such that the attenuations represented by the ordinates of the normal characteristic A, Fig. 2, are obtained when Z has a particular value Z0, but are increased so as to yield the characteristic B when Z=0, or decreased to yield the characteristic C when Z=#, while intermediate values of Z yield characteristics such as C, C<1>, and the differences between A and C or C<1> bear to the differences between A and B or B<1> a ratio which either is constant throughout the curves or is a simple function of the frequency. General theory. The invention consists in designing the network N so that Z<2>0 = Z5<SP>2>6. Z2# Z20, where Z0 is a datum value of Z chosen in relation to the network, Z56 is the impedance of the network at terminals 5, 6 and Z2#, Z20 are the values of the transfer impedance E/I2 when Z=# and Z = 0 respectively, E being the input voltage and I2 the output current. Then if e<-#> is the ratio of I2 to a fixed reference current equal to E/Z0, # determines the insertion loss, from which it differs by an additive constant only, and where x=Z/Z0, while #=#0+# when Z=#, and # = #0-# when Z = 0. The Specification shows by expanding # in ascending powers of #, that to a close approximation where y = (Z-Z0) /(Z +Z0) Then #0 determines the normal characteristic A, Fig. 2, # determines the extreme departures from it represented by B, B<1>, and y# determines intermediate departures such as C, C<1>. Thus #0 and # are constants of the network including ZS and ZR, while y is determined by Z. Instead of being a variable resistance, Z may comprise a constant-resistance network M, Fig. 3, terminated by a variable resistance R. Then #0, determining the normal characteristic A, is unaffected by the insertion of the network M, but #, and consequently the deviating characteristics B, B<1>, C, C<1> are modified by it. Practical embodiments. In one group of embodiments typified by Fig. 6 (not shown) the variable impedance Z, Fig. 1, is a variable resistance R, Fig. 7, so that Z0 is R0, while the network N together with the terminal impedances Zs, RR, is represented by the pi network shown, in which Z11, Z21 are inverse impedances such that Z11.Z21=R<2>0. Figs. 8, 9 (not shown) show modifications of this arrangement. The network may be so designed that # is independent of frequency, and a constant-resistance network M, Fig. 3, is then inserted to give it the desired frequency variation, Fig. 10 (not shown) ; or the network N, Zs, ZR, Fig. 1, may comprise resistances only, Fig. 11 (not shown). In a further group of embodiments typified by Fig. 12 (not shown) the variable resistance R is in a shunt arm as in the arrangement shown in Fig. 13, which is inverse to that shown in Fig. 7. Modifications are shown in Figs. 14, 15 (not shown). The variable resistance R may be in series with or in shunt to a fixed impedance Z4, Figs. 16, 18 ; particular embodiments of this arrangement are shown in Figs. 17, 19 (not shown). In a further group of embodiments typified by Fig. 20 (not shown) the variable resistance R forms one diagonal of a bridge, the input and output terminals being in adjacent arms. Fig. 21 shows one embodiment comprising resistances and arbitrary shunt impedances forming a bridgedtee section and related to one another as shown. In a modification the resistances a.R0 and the variable resistance R are replaced by condensers, Fig. 22 (not shown) or inductances. Further embodiments of the bridge type are shown in Figs. 23, 24, 25 (not shown), the variable element R being in either the bridging diagonal or the shunt diagonal. Two constant-resistance networks M, Fig. 3, may be interposed between two adjustable resistances R, R<1>, Fig. 26, and two different arms of the bridge ; in this case by varying R<1> a family of extreme curves B, B<1>, Fig. 2, can be obtained, and for each member of the family a family of intermediate curves C, C<1> can be obtained by varying R. The Specification gives specific values for the impedance elements of an attenuator adapted to compensate for the temperature variation of the attenuation in a section of coaxial cable, Fig. 27 (not shown), and also of an attenuator, Fig. 29 (not shown), for use in a feed-back amplifier of the type which yields an overall characteristic inverse to the characteristic of the attenuator, so that the arrangement compensates distortion in a line having the same characteristic as the attenuator. The normal characteristic A, Fig. 2, is made to be that of a cable in which coaxial conductors are held apart by spaced insulators, and the temperature variations of the cable, which are known to produce characteristics of the type represented by C, C<1>, Fig. 2, are compensated by adjustment of the variable resistance R.