GB595421A - Frequency shift keying in carrier signalling systems - Google Patents

Abstract

595,421. Radio signalling. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Fob. 9, 1945, No. 3355. Convention date, Feb. 11, 1944. [Class 40 (v)] In a system in which signalling is effected by shifting the frequency of a carrier wave, the signalling currents control the valve coupling between one or two fixed frequency carrier sources and a source of oscillations controlled by the fixed source or sources. In one embodiment, Fig. 2, the outputs of crystal-controlled oscillators 10, 20, of marking and spacing frequencies respectively, are selectively applied through valves V2, V3, respectively, and potentiometer R6, to synchronize oscillator VI. The anodes of valves V2, V3 are connected together and to R6 and one or other valve is rendered conductive selectively by a positive potential applied to its screen grid by the signal relay RE, the screen grids having a standing negative cut-off bias applied from source B. When the relay contact 21 is in its neutral position during the change-over from mark to space, the oscillator V1 operates at an intermediate frequency determined by the constants of the resistance-capacity feed-back network C1 ... C4, R1 ... R5. The synchronizing voltage is applied to its screen grid. In a second embodiment, Fig. 3, the controlled oscillator V1 is a modified Hartley type, with a tuned circuit L1, CT, connected to the anode, screen grid and cathode and synchronizing signals are applied to its normal control grid, being fed to tappings on a grid resistance 27. H. T. voltage for the screen grid is fed through resistance 25 shunting the feed-back condenser N. Some current from the H.T. supply is passed through another grid resistance 29 for biassing purposes. When the relay contact 21 is in its neutral position, a third controlling oscillator 70, synchronizes oscillator V1 at a frequency intermediate those of the marking and spacing oscillators 10, 20. Steady bias for the switching valves V2, V3 is provided by current from the H.T. source fed to a tapping on the common grid resistance R9. In a similar arrangement, Fig. 5 (not shown), one of the oscillators 10, 20 and valves V2, V3 is omitted and the controlled oscillator is arranged to be stabilized at the spacing frequency by oscillator 70 when the single contact of the signalling relay is open. In another embodiment, the signalling relay is replaced by a flip-flop multivibrator V4, V5, Fig. 4. The standing negative screen-grid biasses for the control valves V2, V3 (see Figs. 2 and 3), are provided from source B2 through tappings on resistances 45, 46 respectively and these biasses are replaced alternately by positive potentials from the anodes of valves V4, V5 when these valves are alternately blocked. In the absence of signals, corresponding to spacing periods, V5 is conducting and V4 is blocked by a biassing voltage from source B3 with switch S in position S1. Source B3 may not be necessary and switch S is then in position S2. A.C. or D.C. signals applied to terminals 71, 73 in the grid circuit of valve V5 are adapted to block this valve which results in the unblocking of valve V4 during the marking periods. Arrangements for rendering the controlled oscillator more stable in frequency, when no control oscillator is provided for the no-signal condition, are described, Figs. 8 and 9 (not shown), and include diminishing the normal L /C ratio of the tuned circuit, making the grid leak and anode supply resistances low and equal and inserting inductances, of half the value of the tank circuit inductance, between the ends of the tank circuit and the normal connection points. Specification 595,200 and U.S.A. Specification 2,326,314 are referred to.