GB582798A - Improvements in and relating to secret carrier wave signalling systems - Google Patents

Abstract

582,798. Frequency modulation. BRITISH THOMSON-HOUSTON CO., Ltd. (General Electric Co.). Nov. 25, 1943, No. 19732. [Class 40 (v)] In a frequency-modulated radio signalling system, secrecy is effected by imposing on the carrier a frequency modulation by a distorting signal before it is frequency-modulated by the desired signal. The distorting signal may be a selected combination of a fundamental and its harmonics, and is removed at the receiver by a similar combination synchronized with that at the transmitter. As shown, the carrier oscillator 3 is first frequency-modulated at 5 by a distorting signal arising from an oscillator 11 and harmonics thereof generated at 12. A suitable combination is selected by plugs 30-32 and applied to modulator 5 through an amplifier 13. The distorted output from 5 passes through an amplifier 6 to a second frequency modulator 7 on which the desired signal is imposed through an amplifier 14. The F.M. carrier is further amplified at 8 and is then amplitude-modulated at 9 by the oscillator 11, the output passing to the aerial 4 through a power amplifier 10. At the receiver the aerial 33 feeds a normal superheterodyne converter 35, 36 and the I.F. output is divided into two channels 38, 39. The former responds to amplitude modulation and includes a rectifier 40 and filter 41 which synchronizes the harmonic generator 42 with that at the transmitter. The other channel 39 includes a heterodyne converter 52 for mixing with oscillations from a source 54 which is frequencymodulated at 53 by the same combination of fundamental and harmonics as was selected at the transmitter. The difference frequency output from the converter 52 is thus free of the distortion signal, amd is passed through the normal F.M. signal detector comprising limiter 55 discriminator 56, amplifier 57 and loud speaker 58. When no signals are present, the distortion signal is cut off at the amplifier 13 by a prohibitive bias which is removed by the device 15 as soon as signals start. Discovery of the distortion signal during non-signalling intervals is thus rendered more difficult. In a further system (Fig. 2, not shown), the desired signal before application to 7 is mixed with an oscillation of about 10 kc. which has a wide frequency deviation imposed on it by a selected combination of harmonics from the source 11, 12.