Patent GB-369984-A (GB)
Title: Electrical impedance networks
IPC: H04B1/58
Filed: 1930-11-17
Abstract: 369,984. Impedance networks ; multiplex signalling. TELEFONAKTIEBOLAGET L. M. ERICSSON, 33, Kungsgatan, Stockholm. Nov. 17, 1930, No. 34600. Convention date, Nov. 18, 1929. [Classes 40 (iii) and 40 (v).] Comprises filter networks for use in multiplex carrier-wave signalling and two-way amplifiers, and for other purposes. The network shown in Fig. 1 is built up of two networks A, Fig. 2, and B, Fig. 3, having, equal arms 11, 12 in common while the quadripoles 17, 18 belong to A and B respectively. The component filters A, B have the same transmission zones and propagation constant, and their characteristic impedances X1, X2 at cd and ef respectively are inverse to one another. One is preferably of T type and the other of *type. The quadripoles 17, 18 may be closed by impedances or instruments R1, R2. The Specification shows that the characteristic impedances facing the terminals 1, 3 are both equal to # (X1, X2), provided that R1, R2 be matched to the filter. It then shows how to build up filters A, B so that A and quadripole 17 are of T type while B and quadripole 18 are of # type, Fig. 5 (not shown), the filters A, B being of constant-k type and inverse to one another. The 8-pole network suppresses all frequencies between the terminals 5, 7. Between terminals 1, 5 or 7 and between terminals 3, 5 or 7 the characteristic is of the same type as that of each of the component filters A, B which may have band-pass, low-pass or other characteristics. Between terminals 1, 3 the characteristic is of the band-suppressing type. In a modification, the terminal impedances R1, R2 are not matched to the filter but are nevertheless inverse to one another ; in this case the filters are composed of non-uniform sections. In a further modification, a second component filter similar to B is added so as to make the filter symmetrical, and five pairs of terminals are then obtained, Fig. 4 (not shown). Fig. 6 shows an embodiment of the invention in which filter A comprises the quadripole 17 and the balanced and tightly coupled primaries L1, while filter B comprises quadripole 18 and the whole transformer L1, L5. The characteristics are determined essentially by the quadripoles 17, 18 owing to the tightness of the coupling. In modifications, Figs. 7 and 8 (not shown), one pair of the coils L2 may be omitted and replaced by a direct connection. In the arrangement shown in Fig. 9 the filter B comprises the parts LB, CB, 11, 12, while filter A comprises the parts TA, CA, 11, 12. The system is so dimensioned that the impedances at 1, 3 are equal, while that at 5 is twice as great and that at 7 is half as great, the impedances of the externally connected apparatus being matched. Multiplex signalling. A series of networks of the type shown in Fig. 1 and having different transmission zones may be connected in cascade by their terminals 1, 3 and terminated at one end by a line and the other by a matched impedance. Sending and receiving instruments are connected at 5, 7. Signals for the second and subsequent networks pass through the first and other earlier networks from the line. The system shown in Fig. 10 is for multiplexing both audio-frequency signals and carrier signals. Audio-frequency signals pass between lines L2 and L through the amplifiers V, network I, and through those terminals of networks II, III, IV which correspond to 1, 3 in Fig. 1, a line-balance N<1> being provided at the first pair of terminals -1. For one carrier channel, the receiver MIV and transmitter SIV serve the line L and the network IV prevents them from'affecting one another, in the manner described above. Outgoing signals of one carrier frequency are sent to line L by transmitter SII in the network II and incoming signals of another carrier frequency are received from that line by receiver MIII in network III. The transmission zones are so calculated that signals cannot penetrate further to the left (in Fig. 10) than the apparatus to which they pertain.