GB696356A - Improvements in or relating to multichannel single sideband radio transmission systems - Google Patents

Abstract

696,356. Multiplex radio signalling. WESTERN ELECTRIC CO., Inc. June 6, 1952 [June 8, 1951], No. 14291/52. Class 40 (v). In a two-way multi-channel single side-band radio transmission system substantially the same range of frequencies is used for signalling in both directions, the two oppositely directed carriers being continuously transmitted during periods of operation with a frequency spacing sufficient to avoid interference therebetween. At the western terminal of the system the land lines 11 of subscribers A, B, C, D, are connected to the transmitter 12 and receiver 13 through a vodas 10, which when a signal comes in from a line disconnects the receiver from that line and connects the transmitter, similar apparatus being assumed at the eastern terminal. In the transmitter each channel has two stages of modulation, a first modulator 14 followed by a filter 15 which passes one of the side-bands, and a second modulator 16 followed by a filter 17 passing the desired side-band, the carrier being suppressed to the required degree. The oscillators 18 derive the frequencies fa, fb, fc and fd from a common stable oscillator 22, and the carrier oscillator 23 may derive the frequency f1 from the oscillator 22. The outputs of the filters 17 are combined in the amplifier 24. Incoming signals are applied to the amplifier 32 of the receiver 13 and thence to a common first detector 33, the resultant intermediate frequency being amplified at 34 and the channel side-bands being separated by filters 35. The original signal is recovered in each channel by a second detector 39 and applied to the land-line through a vodas 10. A filter 41 passes only the frequency f2 of the carrier which is applied to a frequency control circuit 42 to control the frequency of the oscillator 43, and to an automatic gain control circuit 45. The oscillators 46 derive their frequencies fa to fd from a stable oscillator 50. If the two carriers were of the same frequency the A.F.C. circuit 42 and A.G.C. circuit 45 would be affected by the locallytransmitted carrier, and to avoid this and at the same time use substantially the same bands of frequencies in both directions, the carrier frequencies are separated sufficiently to prevent the locally-transmitted carrier from drifting into the pass-band of the filter 41. Since the carriers are separated by only a few hundred cycles the audible note produced may be selected by a filter 51 tuned to the frequency f2 - #1 and applied to the oscillator 23 of the transmitter to maintain the frequency difference constant. Figs. 2, 3 and 4 (not shown), illustrate alternate arrangements for the frequency spacings of the two carriers and the side-bands.