Patent GB-1151275-A (GB)
Title: Subscriber Carrier System
IPC: H04M19/08
Filed: 1967-05-12
Abstract: 1,151,275. Automatic exchange system; transistor amplifier circuits. INTERNATIONAL STANDARD ELECTRIC CORP. 12 May, 1967 [19 May, 1966], No. 22159/67. Headings H3T and H4K. In a subscriber carrier system, in which substations are connected to a central office by a carrier system, a terminal device is provided between the central office and the substation to tap off D.C. power, ringing and communication signals for the subscribers from the common line, the terminal device including arrangements to D.C. isolate the subscriber' ringing and communication signals from the line, and a D.C. to D.C. converter to convert the D.C. supply current from the line, which may be at a high D.C. voltage relative to earth due to series connection of a number of subscribers as regards D.C. supply, to a D.C. supply at a convenient and safe level. Fig. 1 shows a terminal device which is connected to the central office over line 11 to receive ringing and communication signals and in which D.C. power is tapped off between the centre tap of winding 12, connected to the central office, and the centre tap of winding 13, connected to the following subscribers' terminals. Communication and ringing signals are tapped off transformer T1 by secondary 14 and fed to the receiver 15 from which communication signals are fed, via a compander, 19 and a hybrid 21 to the contacts of a ringing relay K10 and then to the subscriber's set over lines 53 and 54, while ringing signals are fed to a ringing detector 28, to operate ringing relay K10, and to a ringing amplifier 27 feeding the subscriber's line via transformer T3 and the ringing relay contacts. D.C. power for the subscriber's set is obtained from the D.C. on the main office line via a D.C. to D.C. converter 31, fed via a current limiter 32, choke 33 and a subscriber's loop detection relay K20 which, on completion of the subscriber's loop by an off-hook condition, inhibits over line 35 the operation of the ringing signal detector 28 and so terminates the subscriber ringing. Signals from the subscriber are fed via hybrid 21, compander 22, and transmitter 23, to the outgoing line 26. Circuits of the ringing detector, D.C. to D.C. converter, and ringing relay are shown in Fig. 2, and a Darlington pair push pull amplifier for the ringing amplifier is described with respect to Fig. 3. In Fig. 2, transistors Q1 and Q2 are connected in a multivibrator circuit to switch the current in transformer T4, the secondary output of which is rectified in bridge CR1 to CR4 to produce to D.C. feed to the subscriber's set via a current stabilizing circuit Q3, Q4, to compensate for different subscriber's line lengths. The ringing detector circuit comprises transistors Q5 and Q6 with a transistor Q7 which is conductive when the subscriber is onhook but is rendered non-conductive when contacts K21 are closed, due to the loop current flow in the loop relay coils 41 and 42, when the subscriber goes off-hook so as to inhibit the operation of the ringing relay K10. Operation of the ringing relay K10 inserts winding W1 and W2 of transformer T3 in series in the subscriber's line and also closes contacts K13 in the ringing amplifier, Fig. 3, to feed ringing current via a phase splitter Q8 and amplifiers Q9 and Q10 to the Darlington push-pull amplifier including transistors Q11, Q12, Q13, and Q11<SP>1</SP>, Q12<SP>1</SP>, Q13<SP>1</SP>. Feedback is provided over the amplifier by resistors 47 and 47<SP>1</SP> and temperature stabilization is provided by silicon diode CR12, CR13 and CR14.