GB506306A - Improvements in or relating to modulated carrier wave systems - Google Patents

Abstract

506,306. Wireless transmitting and receiving systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 21, 1938, No. 15229. Convention date, March 8. Addition to 472,214. [Class 40 (v)] In a modulated carrier wave receiver or transmitter either sideband is reduced or suppressed, generally for the purpose of reducing interference caused by overlapping sidebands of adjacent carrier waves, by means of two coupled circuits fed simultaneously but with different phase from the same modulated carrier source. In the single sideband transmitter shown in Fig. 3, the carrier from source 1 is modulated at 4 and passed to the amplifiers 26, 27 whose inputs are connected in parallel. Valve 27 is resistance-capacity coupled to a further valve 8. Valve 26 is coupled by tuned circuit 18, 19, to valve 7, reaction being applied by coil 31. The output circuits of valves 7, 8 consist of the two coupled circuits 9, 10, 11 and 12, 13, 14, tuned to the carrier frequency, the coupling being provided by the mutual inductance between coils 11 and 12. Circuit 9 10, 11, due to the tuned circuit 18, 19 is excited with amplitude J and phase #1 as shown in Fig. 4, where fo is the carrier frequency, and circuit 12, 13, 14 is excited with approximately constant amplitude and phase #2. As a result in one circuit the upper sideband and in the other circuit the lower sideband is accentuated, as indicated by curves J and J2. The output from either circuit may be selected by switch 17 and passed to any suitable utilization means. Various degrees of sideband suppression may be obtained by varying the damping of tuned circuit 18, 19,-by adjustment of the reaction,- and by adjustment of the potentiometer 23. If a fixed additional phase-shift is introduced between the amplifiers 7 and 8 by means such as a capacity-resistance bridge, Fig. 6 (not shown), the characteristics of the coupled circuits are no longer symmetrical but a sharp cut-off is produced in one of the circuits at a frequency controlled by the initial phase displacement. This arrangement may be employed to provide ultra-selective band-pass circuits by providing two such circuits in cascade with appropriate cut-off frequencies. The single side-band suppression obtained with the symmetrical characteristic circuits may be improved by connecting a number of such circuits in cascade, one circuit of a group being fed via a valve from one tuned circuit of the preceding group, the other tuned circuits of each group being fed from the same source.