GB579677A - Electric signal communication system - Google Patents

Abstract

579,677. Phase modulation. STANDARD TELEPHONES & CABLES, Ltd. Dec. 10, 1943, No. 20746. Convention date, Sept. 19, 1942. [Class 40 (v)] For obtaining secrecy in communication, a carrier wave is modulated by a sub-carrier and one of its harmonics, and a signal is sent by varying the phase of the harmonic. The generator 12 of the sub-carrier is connected to a frequency-multiplier 16 producing a harmonic, preferably the third. This harmonic is phasemodulated at 22 by a signal from the source 18, and is added to the sub-carrier in the element 26. The resultant is impressed on an amplitude or frequency modulator 28 interposed between the carrier generator 32 and the radio transmitter 30. At the receiver, the carrier is demodulated and the sub-carrier is separated from its harmonic by filters 40, 44. The subcarrier passes to a frequency multiplier 42 comparable with that at the transmitter, and the harmonic thus produced is fed, together with the phase-modulated harmonic from the filter 44, to a phase-comparing device 46, the signal resulting therefrom being fed to an amplifier 41 and reproducer 50. When the third harmonic is used, its amplitude should not be more than 30 per cent of the fundamental, and the phasing should be such that the peak of unmodulated harmonic B coincides with that of the fundamental A, Fig. 2. The effect of phase-shifting the harmonic B to B<1> is to shift the combination C to C<1>.