Patent GB-727442-A (GB)
Title: Four-terminal impedance network with variable attenuation
IPC: H04B3/14
Filed: 1953-03-12
Abstract: 727,442. Attenuator equalizers. TELEFONAKTIEBOLAGET L. M. ERICSSON. March 12, 1953 [March 12, 1952], No. 6872/53. Class 40 (8). A four-terminal impedance network for regulating the attenuation in a transmission line comprises a first four-terminal network I, Fig. 1, terminating in at least one series circuit consisting in an impedance Z, and a variable resistance r1, a second four-terminal network II terminating in at least one parallel circuit consisting of an admittance Y2 and a variable resistance r2, and a third four-terminal network III whose input and output terminals are connected in parallel or in series respectively with the inputs of networks I and II, the ratio between Z1 and Y2 and the design of the network being such that the variation of the attenuation is independent of the product Z1Y2 for certain values r10, r20 of r1 and r2 but dependent on this product for values of r1, r2 which differ from r10, r20. It is thus possible to dimension the average attenuation, defined as the attenuation (as a function of frequency) obtained with resistances r10, r20, and the attenuation variation, obtained by varying r1, r2 from their normal values, r10, r20, as two different independent functions of frequency. The transmission admittance Y12 of the whole network may be evaluated using the equivalent circuit of Fig. 2 in which the generator circuit E, ZA is replaced by a shunt impedance fed with a current i from a source of potential having a very high internal resistance. This shunt impedance, the load impedance ZB and the shunt impedances of the equivalent #-network of network III are comprised within networks I, II, whilst the series impedance of the equivalent #-network are represented by the impedance Zo. It is shown in the Specification that it is possible for certain conditions to make Y12 independent of Z1 and Y2 for values r1 = r10 and r2 = r20, in which case where Y120 is the special value of Y12 obtained for values r10, r20, a1, a2 being functions only of the properties of networks I and II. If the condition is met with (where b1, b2 are functions of networks I, II) then for values r1 # r10, r2 # r20 it is possible to write where #Y represents the divergence of Y12 from the normal value Y120. It is shown that #Y is determined by Z1 and Y2 by means of the product whereas the condition (5) is determined by the ratio Z1/Y2. c1 and c2 are further functions of networks I and II. Thus for given values of r1 and r2, #Y may be arbitrarily varied by suitable choice of Z1 and Y2 and for given values of Z1 and Y2 the magnitude and sign of #Y may be changed by varying r1 and r2 around the normal values r10 and r20. If the resistances r1, r2 are varied equally and in the same direction they may take the form of indirectly heated thermistors whose heaters pass the same current. If a number of frequency ranges are to be independently regulated, the network I is termined by n series circuits (r11 + Z11) ... (r1n + Z1n) in parallel and the network II by n parallel circuits in series (Fig. 3, not shown). The resistances (r11, r21), (r1n, r2n), &c., are ganged in pairs. Fig. 7 shows a circuit suitable for two independent frequency ranges. RO, RI, RII are resistive elements comprised in networks I, II, III; L11 corresponds to Z11 and C21 to Y21; the series circuit L12, C12 corresponds to Z12 and the parallel circuit L22, C22 to Y22. The Specification gives curves showing the variation of log(Y12) with frequency for various combinations of values of r11, r12, &c., greater or less than their normal values r110, r120, &c. A modification is briefly described in which the shunt-connected networks I, II are replaced by series-connected networks (Fig. 4, not shown).