Patent GB-355339-A (GB)
Title: Improvements in or relating to electric high frequency signalling systems
IPC: H03L7/02
Filed: 1930-05-23
Abstract: 355,339. Wireless transmitting and receiving systems. STANDARD TELEPHONES & CABLES, Ltd., Columbia House, Aldwych, London.-(Western Electric Co., Inc.; 195, Broadway, New York, U.S.A.) May 23, 1930, No. 15896. [Class 40 (iv).] In a high-frequency signalling system in which the carrier wave is suppressed at the transmitter, the frequency of the locally generated wave at the receiver is maintained constant by a control frequency sent from the transmitter between impulses constituting the elements of a signal. At the transmitter T1, part of the signal currents from 8 are branched through an amplifier-rectifier AR1 to energize relay 9 which disconnects a 1000-cycle control frequency generated at 5 from the transmitting apparatus 6, a delay circuit D1 being provided to cover the time lag of the relay. By this means the control frequency is transmitted at all intervals when no signal currents are flowing in circuit 8. At the receiver R1, the incoming wave, consisting of a single side band, is amplified and detected at 12, the necessary local wave being supplied from source 11. The output from 12 is passed to the utilizing circuit 25 when the contacts of a relay 16 are closed. Across line 14 is connected a circuit comprising delay circuit D2, low-pass and high-pass filters LPF, HPF, and amplifier-rectifiers AR3, AR4. Filter LPF passes frequencies below 998 cycles and HPF1, all frequencies above 1002 cycles. Associated with the output circuits of these filters are electromagnets 17, 18, which, when energized, attract spring-controlled discs 19, 20 keyed to shaft 21 but free to slide thereon. Also associated with line 14 is a circuit compressing a band filter BF1 which passes all frequencies except those between 990 and 1010 cycles; the output from the associated amplifierrectifier AR2 is fed to relay coils 15, 16. The action is as follows:-In the intervals between signal elements the 1000 cycle control frequency 5 modulates the transmitter 6. The single side band is heterodyned and detected at 12 and passed to line 14. If the resulting frequency lies between 998 and 1002 cycles no current passes through HPF1, LPF1. If, however, the local frequency varies so that the resultant frequency exceeds 1002 cycles electromagnet 18 is energized, thus attracting disc 20 until it engages with a continuously rotating disc 22 driven by motor M. The shaft 21 is thus rotated and a frequency determining element of source 11 is thereby varied through gearing 23 until the frequency is brought within the prescribed limits. Similarly if the frequency falls below 998 cycles, electromagnet 17 is energized and the frequency of source 11 is varied in the opposite sense. During the period when the control frequency is received the utilizing circuit 25 is open as relay 16 is only energized by frequencies outside the band 990 to 1010 cycles which is rejected by BF1. Similarly the circuit feeding HPF, LPF, is opened by relay 15 when signals are being received. In Fig. 2 (not shown) a system employing a supersonic control frequency is shown, the corrector system LPF, HPF, being modified by the provision of an auxiliary local frequency to provide sum and difference frequencies for the actuation of a corrector means such as enclosed by the dotted rectangle of Fig. 1. Specification 253,595, [Class 40 (iv), Telephones &c.], is referred to.