Patent GB-748736-A (GB)
Title: Improvements in or relating to radio communication systems
Inventor: BENNETT ADRIAN MICHAEL
IPC: H04B7/08
Filed: 1953-11-12
Abstract: 748,736. Radio signalling; electric discharge tube circuits. STANDARD TELEPHONES & CABLES, Ltd. Nov. 5, 1954 [Nov. 12, 1953], No. 31429/53. Classes 40 (5) and 40 (6). In a communication system with two independent paths carrying the same signal, automatic means at the receiving station select the signal on the path with least noise and an overriding control changes over to the other path when the signal level is abnormally low on the path in use. To ensure a high degree of reliability, some of the components of the radio system described are duplicated, both parts normally being in operation simultaneously. The two radio paths may operate on different carrier frequencies or different series of frequencies. The duplicate radio receivers 1A, 1B, Fig. 1, on the first path feed a common output transformer 3 and balancing resistor 5 while the second path has similar components 2A, 2B, 6, 8, respectively: The transformers 3, 6 are connected to the outgoing line 9 through gate circuits 4, 7, respectively, and a common transformer 10 and balancing resistor 11, the gate circuits 4, 7 being controlled so that when one is open the other is closed, by an electronic switch 12 which produces a control current of reversible polarity. Conventional means are provided for replacing the radio receiver in use on each path by the other of the pair if failure occurs. Duplicate logarithmic noise detectors 13A, 13B for the first path and 14A, 14B for the second are fed from the centre taps of transformers 3, 6 respectively and include filters responsive to a frequency band above the signal frequency band. The paralleled outputs of each pair are connected in series opposition to the switch 12 over conductors 19 so that if the output of detectors 13A, 13B is greater than that of detectors 14A, 14B, the switch is set to close gate 4 and open gate 7 and vice versa. Additional rectifiers in the noise detectors operate an alarm 21 to indicate the failure of one of the detectors. Another alarm 22 connected across the conductors 19 indicates an abnormal difference in output between the pairs of noise detectors. The pairs of pilot detectors 15A, 15B and 16A, 16B supplied from the same points as the noise detectors respond to a pilot frequency transmitted on both paths just below the signal frequency band. The pilot detectors normally operate relays 23A, 23B, 24A, 24B, respectively, to keep their contacts open, the contacts being connected in two series pairs to the switch 12. If the signal level on either path falls by a piedetermined amount, the corresponding pair of contacts closes to produce an overriding control of switch 12 which is thereby set to render the other path operative. If one pilot detector fails the operation of the circuit is not upset. A polarized relay 25 with a third unoperated position of its moving contact and connected in the control circuit of the gates 4, 7, operates lamps to indicate which path is in operation and an alarm if the control circuit is interrupted. The gate circuits may be of known type or of the type, Fig. 4 (not shown), described in Specification 748,746. The switch 12 comprises a multivibrator, having two stable states, composed by the two pairs of parallelconnected pentodes 56, 57 and 58, 59, Fig. 3, the valves being duplicated for reliability. Between the control current leads 19 and the multivibrator is a cathode follower stage comprising the parallel-connected pentode pair 60, 61. The anode and screen voltages of valves 60, 61 and the screen voltages of valves 56 ... 59 are derived from a pair of gas-filled voltagestabilizing tubes 64, 65, connected in parallel. The cathodes of valves 60, 61 feed the trigger voltage to the grids of valves 56, 57 through isolating rectifiers 74, 75 and conductor 77 so that if one of valves 60, 61 fails the other continues to operate isolated from the faulty one. A double-wound relay 73 in their individual cathode circuits gives an alarm if one cathode current falls below a predetermined value. A similar alarm relay 100 monitors the voltage stabilizers 64, 65. Other similar alarm relays 83, 89 whose double windings are in the individual anode circuits of the valve pairs 56, 57 and 58, 59 respectively operate buffer relays which have hold contacts and change-over contacts 83X, 89X which so connect resistors 94, 95, 96 in the common cathode circuit of all four valves that on the failure of any one valve the common cathode potential is maintained unaltered. The control current for the gate circuits 4, 7 flows between the common anodes of the two multivibrator stages over conductors 26A, 26B, 27A, 27B, its direction depending upon the state of the multivibrator which depends primarily upon the polarity of the triggering current in the input leads 19. The overriding signal amplitude control for the multivibrator is provided by the contacts 23AX, 23BX of relays 23A, 23B, Fig. 1, and the contacts 24AX, 24BX of relays 24A, 24B. If the elements of one of these pairs are closed a fixed positive or negative potential, as the case may be, is applied to the triggering conductor 77 to effect the desired switching: Specifications 438,839 and 630,964, [Group XL (b)], also are referred to.