Patent GB-988227-A (GB)
Title: Improvements in or relating to electric pulse modulating arrangements
Inventor: LEAKEY DAVID MARTIN
IPC: H04Q11/04
Filed: 1960-09-16
Abstract: 988,227. Pulse-modulation circuits. POSTMASTER-GENERAL, and GENERAL ELECTRIC CO. Ltd. Sept. 18, 1961 [Sept. 16, 1960; Oct. 31, 1960], Nos. 32002/60 and 37338/60. Heading H4L. In a circuit producing amplitude modulated pulses the input modulating signal is supplied via terminals 1, Fig. 1 (Prov.), to the primary winding of a transformer 3 so that the current flowing across the secondary winding and through a rectifier 7 varies above and below the mean level provided by the battery 6 when there is no input signal. A normally non- conducting transistor 13 is connected in a common base circuit to the secondary winding 11 of a pulse transformer 9 and is caused to conduct by the application of positive-going pulses from a source 12. During each pulse a voltage pulse is induced in the primary winding 8 of the transformer 9 of such polarity as to block the rectifier 7, the capacitor 10 having a value such that the rectifier 7 remains blocked throughout the period of the pulse from 12. Since the total current which flows into the transformer 9 cannot change instantaneously the break in current flow through the winding 8 is compensated for by a flow of current in the winding 11 so that a current pulse of the same duration as the pulse from 12 and amplitude modulated in accordance with the signal supplied at 1 flows as emitter current of the transistor 13 and is supplied to the output lead 14. A resistor may be connected between the winding 11 and the emitter of the transistor 13 to damp unwanted oscillations. In a modification, Fig. 2 (Prov.), the modulating signal is supplied via terminals 16 to the primary winding of a transformer 18 so that a current varying in accordance with the modulating signal flows through the inductor 23 while the rectifier 22 is conducting. A normally non-conducting transistor 25 has negative-going pulses from a source 12 supplied to its base to render it conducting so that a negative-going pulse appears across the inductor 23 and blocks the rectifier 22, which due to the capacitor 10 remains blocked for the duration of the pulse. A current of magnitude equal to the immediately preceding instantaneous value flows through the inductor 23 as emitter current of the transistor 25 for the duration of the pulse and corresponding amplitude modulated pulses flow in the output lead 27 of the transistor. Variations on these two circuits are described, Figs. 1 to 4 (Comp.) (not shown). Multiplex system. Fig. 3 (Prov.) shows the circuit of Fig. 1 (Prov.) incorporated in a telephone exchange in which the subscribers are divided into groups of one hundred and each group is sub-divided into sub-groups of ten. A subscriber's line wires are connected via terminals 30 to line windings 31, 32 of a hybrid transformer 33 balanced so that speech signals across windings 31, 32 from the subscriber's line appear only across winding 38, and speech signals applied to the winding 39 appear only across the windings 31, 32. The winding 38 is connected through a rectifier 41 to the winding 42 of a pulse transformer 43, the winding 46 of the pulse transformer being connected via a diode 48 to a junction point 49 common to the ten subscribers of a sub-group. The junction point 49 is connected via a damping resistor 50 to the emitter of a transistor 51. A diode 53 connected between the emitter and base limits the reverse voltage to a safe value. The winding 46 of the transformer 43 is supplied via terminal 47 with gating pulses from a source 81 and amplitude modulated pulses are produced as described previously. The collector of the transistor is connected to a junction point 55 which is common to ten sub-groups which supply a " Go " highway 56. Incoming time division multiplexed amplitude modulated pulses over a common " RETURN " highway 60 are supplied to a junction point 61 common to the ten sub-groups of a group and pulses of the appropriate sub-group are passed via a diode 62 to the emitter of a transistor 63. A diode 65 limits the base-emitter reverse voltage. The collector of the transistor is connected to a junction point 67 common to the ten subscribers of a sub-group. A rectifier 69 limits the collector potential when the transistor conducts and a rectifier 70 clamps it when the transistor is non-conducting. The pulse source 81 is common to a group of subscribers and over leads such as 84, 85 supplies appropriately phased gating pulses to terminals 54 and 66 of each sub-group. Gating pulses are also supplied over leads such as 82, 83 to the terminals 47 and 74 of each subscriber's line circuit. A large resistance is present across terminals 30 before the subscriber lifts his handset and the diode 36 is non-conducting but when the handset is lifted the resistance across terminals 30 has a low value, the diode 36 conducts and a substantial direct current flows through the diode 41 and winding 42 which varies in accordance with the speech signals. Demodulator. The subscriber's line circuit, Fig. 3, includes a pulse transformer 71 whose primary winding 72 is connected via a diode 73 to the junction 67, the signals appearing in the secondary winding 75 being amplified by a transistor 76. The transistor 76 remains non- conductive for the duration of each pulse flowing in the winding 72 and then conducts for a period which is much longer than that of the pulse, the speech signal modulation from the resulting lengthened pulses being derived by a low-pass filter 79 and passed via the winding 39 of the hybrid transformer to the subscriber's line.