GB534757A - Improvements in frequency and space modulation systems for carrier-wave signalling - Google Patents

Abstract

534,757. Frequency and phase modulation. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 10, 1939, No. 29795. Convention date, Nov. 12, 1938. [Classes 39 (i) and 40 (v)] [Also in Group XXXIX] Frequency or phase modulated waves are produced by (a) amplitude-modulating the carrier by a current derived from the signal, (b) amplitude-modulating the carrier displaced in phase by 90 degrees by a current derived from the signal and also displaced in phase by 90 degrees, and (c) combining the two outputs to produce a wave represented by sin (wt+ m sin Át), where w and are the angular frequencies of the carrier and signal respectively. For producing phase modulation, m is constant, and for producing frequency modulation m is equal to a constant divided by nÁ. As shown in Fig. 1, the carrier source 1 supplies waves in quadrature to the modulators 3, 4 the combined output of which feeds the aerial 9. The signal m sin Át derived from a microphone 18 is applied across two deflecting plates 16, 17 of a cathode-ray tube 10. The other pair of plates are connected to an ultra shortwave source 15 by which the luminous spot is caused to traverse a vertical line on the fluorescent screen 12. The screen is divided into two sections by a horizontal opaque plate 28, and each section is partially blacked out in the form of two sine curves 30, 31 in quadrature relation as shown in Fig. 2. As the vertical line 29 is traversed to and fro with a horizontal displacement represented by m sin Át. its luminous intercepts on the respective sections of the screen are represented respectively by sin(m sin Át) and cos(m sin Át). This light is focused by lenses 22, 25 on photocells 23, 26 which through amplifiers supply corresponding voltages to the modulators 3, 4. In a modification, Fig. 3 (not shown), strips of varying translucence are employed in place of the opaque strips bounded by sine curves. In Fig. 4 the fluorescent screen is replaced by grid targets having their vertical elements 36, 37 displaced by a quarter of the distance between them. The targets are connected to amplifiers 40, 41 feeding the respective modulators 3, 4. In order to produce frequency modulation, the microphone currents are fed to the deflecting plates 16, 17, Fig. 1, across a condenser 21 in series with a resistance 20, so that the voltage on the plates varies inversely as the frequency of the signal current. Specification 416,707 is referred to.