Patent GB-547548-A (GB)
Title: Apparatus for connecting balanced and unbalanced networks in electrical systems
IPC: H04B3/46
Filed: 1941-07-11
Abstract: 547,548. Transmission systems. STANDARD TELEPHONES & CABLES, Ltd. July 11, 1941, No. 8782. Convention date, July 20, 1940. [Class 40 (iv)] [Also in Group XXXVI] In an arrangement for connecting networks balanced and unbalanced with respect to earth, particularly in the employment of a single attenuating network for effecting measurements both balanced and unbalanced with respect to earth, a condenser is adapted to be connected between earth and one terminal for neutralising the unbalancing effect of earth capacitance. As shown in Fig. 1, source 10 of alternating current waves is applied to a network 13 comprising a pair of resistance elements with their midpoint earthed at 14. The network 13 is connected by leads 24, 25 to a network 15 comprising a pair of resistances in series with the lead 24, together with a pair of shunt resistances. The network 15 is enclosed in a shield 16. Equivalent condensers 17 represent the distributed capacitances between the shield 16 and the network 15, and an equivalent condenser 18 represents the capacitance to earth of the terminal 20 common to the lead 25, one side of the network 15 and the shield 16. The network 13 is balanced and if it is connected alone to the source 10 equal currents flow to earth through both its branches. If, however, the unbalanced network 15 is connected as shown the balanced condition of the network 13 is upset and unequal currents flow in its branches. The effect of the equivalent condenser 18 is neutralised and the balanced condition restored by connecting a condenser 21 of equal capacitance between the lead 24 and earth. Fig. 2A shows an arrangement for measuring far-end cross-talk between a disturbing pair 28 and a disturbed pair 29 contained in a multi-conductor cable 30. A source of alternating current waves 31 is applied through an input transformer 32 whose secondary winding 33 is connected to the near end of the disturbing pair 28, of which the far end is terminated by a network 34. The disturbed pair 29 is terminated at the near end by a network 35 and at the far end by a network 36 to which are connected terminals 37 of a two-way switch 38. Both pairs 28, 29 are balanced. Across the secondary winding 33 and shunting the pair. 28 is connected the input side of an attenuating network 39, the output side of which is applied through a shielded transformer 40 to a terminating network 27 connected to terminals 41 of the switch 38. The blade terminals 42 of this switch are connected through an amplifierdetector 43 to an indicator 44. A shield 16 surrounds the attenuating network 39 and the primary winding of the transformer 40. A condenser 21 is connected through a switch 59 from the terminal 60 of the input circuit of the attenuating network 39. For making measurements on balanced apparatus such as the pairs 28, 29 the switch 59 is closed so that the condenser 21 compensates for the unbalancing effect of the equivalent condenser 18. For measurements on unbalanced apparatus the switch 59 is open. In a modification the oscillator is connected to the circuit through a transformer having two secondary windings so arranged that the input circuit of the attenuating network is in series with the disturbing pair. Fig. 5 shows an arrangement whereby a valve amplifier 85, which is inherently unbalanced may be connected without an input transformer to a balanced conductor pair 83, 84 in a multi-conductor cable 87, the terminals 88 of which may be connected to a source of alternating current waves. The output stage of the amplifier 85 is connected through an output transformer 89 to a coaxial pair 90 having terminals 91 which may be connected to a load. A shield 92 surrounds the output transformer 89, while another shield 16 encloses both the shield 89 and the amplifier 85. Choke coils 94, 95, 96 prevent the shield 16 from being effectively earthed through the power supply for the amplifier 85. A terminal 98 common to the conductor 83, one side of the input of the amplifier 85 and the shield 16, is connected to earth through the equivalent condenser 18 representing the earth capacitance of the shield 16. A strap 99 connects the shields 16, 92. An equivalent condenser 100 represents the capacitance between the shields 16, 92 and the outer conductor of the coaxial pair 90. A variable condenser 101 is connected between the outer conductor of the pair 90 and a terminal 102 common to the conductor 84 and the other side of the input of the amplifier 85, and neutralizes the effect of the condenser 100. A balancing network 103 neutralizes the earth admittance of the shield 16 due to the choke coils 94, 95, 96 and the condenser 18.