Patent GB-810168-A (GB)
Title: Electrical identification circuit
IPC: H04Q3/62
Filed: 1957-02-01
Abstract: 810,168. Impedance networks. STANDARD TELEPHONES & CABLES Ltd. Feb. 1, 1957 [Feb. 10, 1956], No. 3565/57. Class 40 (8). [Also in Group XXXIX] In a system in which lines to be identified are connected by coupling impedances via output terminals to registering devices in different combinations, so that A.C. applied to a particular line operates an associated combination of registering devices, an additional network is connected to the said output terminals whereby zero admittance is obtained between any two terminals, one of which is connected to a device to be operated and the other of which is connected to a device not to be operated. By this means a large power transfer can be effected between the A.C. source and the devices to be operated without risk of spurious operation of the remainder of the devices through couplings within the network. It is demonstrated that if the coupling impedances are purely reactive then the power supplied by the A.C. source can be made to supply the devices without loss. In such a network it is shown that impedances of opposite sign must be used, e.g. if the coupling impedances are condensed additional network must contain inductances. Alternatively resistance-capacity networks may be used. Fig. 1 illustrates schematically a typical layout including the requisite additional network BUN. The points A are associated with the lines to be identified and the points B with the signal receiving devices. Each point A is connected to a particular combination of points B by admittances Y 2  and each point B is connected to a common point F by admittances Y 4 . Admittanees Y 1 , Y 3  and Y 5  may merely be due to parasitic capacities to ground, but if these are likely to vary appreciably (due e.g. to different routings of calls in the exchange) then capacitors may be deliberately inserted to swamp them. Different methods of coding may be used, e.g. the points A may be connected to different combinations of n out of the m points B. Alternatively the points B may be divided into p different groups of m points (e.g. p=4 m =10) and each A point connected to one point; or a combination of these methods may be used. Other types of coding will be described later. The Specification shows in the various cases how to calculate the admittance between two points B, B<SP>1</SP> which respectively belong and do not belong to the combination of points B representing A and obtains relationships which must be satisfied between the parameters of the coding network and those of the additional network BUN in order that this admittance shall be zero. Energy transfer between the source connected to A and the device connected to B is also discussed mathematically. The coding methods so far enumerated have been symmetrical. The case is discussed also, where each A point is connected to one of a group of mpoints B and one of a group of m<SP>1</SP> points B<SP>1</SP>, as may be necessary for an incomplete exchange. Such a network together with its additional BUN network is shown in Fig. 12 and it is demonstrated that if the admittances Y 2  are capacitors then Y 6  and Y<SP>1</SP> 6  are capacitors but Y 4  are inductances. These values depend on mand m<SP>1</SP> so that if the capacity of the exchange is increased so that m<SP>1</SP> has to be changed, the components of the network must all be changed which is expensive in the case of the inductances. Accordingly an alternative BUN network. Fig. 16, is proposed and it is illustrated that with such an arrangement the inductances Y 4  and the capacities Y<SP>1</SP> 6  are independent of m<SP>1</SP> and that only the one inductance Y 4o  and the m capacitors Y 6  need be changed. A further type of coding network discussed, is that where the coding is performed in stages. Thus as shown in Fig. 18, each of m 4  (= 10,000 say) points A may be connected to two points D lying in different groups of m<SP>2</SP> (=100 say) points. Each of these is connected to two points B lying in different groups of m (=10 say) points. Each of the points B is connected to a receiving device. The additional network BUN comprises impedances Y 41  connected between the D points and a common point G and impedances Y 42  connected between the points B and a common point F. A simple additional network BUN for use when the coupling impedances of the coding network are capacities, is illustrated in Fig. 26. The multipling arrow x (y+1) is based on the assumption that for any point A there are x terminals B (B 1 ) which are required to be energized and X y  terminals B (B o ) which are required not to be energized. Fig. 27 illustrates the equivalent network for the purpose of calculating the admittance between B 1  and B o , C being the capacity between B 1 , B 0 due to the coding network. Clearly L 4  may be chosen so that the circuit is parallel resonant at the frequency of the applied A.C. so that the admittance is zero. Capacity C 5'  which is parasitic, may be neutralized by shunting it with a suitable inductance. Additional networks giving the required zero admittance at two frequencies may be used, e.g. as shown at BUN, Fig. 29. This reduces to the network shown in Fig. 31 for the admittance between B 1  and B 0  which network has the required zero admittance at two frequencies. This enables two frequencies to be used in the system of identification and a suitable receiver for connection to the point B is illustrated in Fig. 28. In this circuit the network C 33 , L 31 , C 31  is resonant and the network L 33 , C 32 , L 32  is anti-resonant at a first frequency so that receiver Rr is operated, whilst the opposite conditions obtain at a second frequency so that R<SP>1</SP>r is operated. If inductances are to be avoided but energy considerations are not vital, then a resistance-capacity network BUN, Fig. 32, may be used. This together with the capacity C introduced by the coding network gives the equivalent circuit of Fig. 33 between B 1  and B 0 . Conditions are set out for the parameters R 4 , C 5 , k which give zero admittance between points B 1 , B 0 , which give minimum energy loss and which permit the highest possible operating frequency. In an alternative system for a 10,000 line exchange the lines are divided into 1000 line groups, each group having its individual BUN network. Each line of a group may be connected to one out of each of three groups of 10 conductors. These may be directly connected to associated receiving devices or alternatively via rectifiers each to a combination two out of five receivers the rectifiers serving both for rectifying the A.C. and for decoupling the receivers. Fault detection. If a condenser in the coding network goes open circuit an incomplete code will be produced when an identification is attempted and this can be used to raise an alarm. For detecting short circuit of a condenser the receiving circuit may contain a series connected large condenser shunted by an alarm relay which operates to D.C. on the identification wire (which may be a test-wire in the switching circuit). The invention is also applicable to systems in which line finders serve a well-defined group of say 100 lines. Test wires extended to the line finders of a group are connected via condensers to one of 100 multipling points which identifies the first two digits. The test-wires of the lines themselves are connected to points one of 100 multipling points, each point being indicative of the identity of a line within its hundred. The multipling points are then connected to detecting devices according to any of the codings mentioned above. Specifications 551,328 and 683,794, [both in Group XXXIX]. are referred to.