GB293803A - Improvements in or relating to high frequency signalling systems - Google Patents

Abstract

293,803. Marconi's Wireless Telegraph Co., Ltd., (Assignees of Peterson, H. 0.). July 12, 1927, [Convention date]. Receiving systems ; thermionic modulating systems; fading, preventing.-Signals sent by frequency modulation as distinct from amplitude modulation are received by the heterodyne method. The modulation may be produced by an electromagnet traversed by microphone currents with its armature connected to one plate of a small condenser connected in parallel with the tuning condenser of an oscillation generator (Fig. 1, not shown). For telegraphic signalling the electromagnet may be in circuit with an alternator and a keying-switch, and its armature may be linked to a copper plate adjacent to a coil in the tuned circuit of the oscillator (Fig. 2, not shown). The received waves are reduced in frequency by heterodyning with a local oscillator 64. and the resulting intermediate frequency is passed through one or more amplifiers 66 and to a detector 68. The amplifier 66, cr the input circuit of the detector 68. is slightly mistuned to the intermediate frequency so as to work on the slope fm instead of at the resonant peak fr, Fig. 4. The frequency modulation is thus converted into an amplitude modulation and gives a rectified signal through the planes 70. The extent of the frequency modluation may be about 500 cycles per second. The system of modulating by varying the wave frequency is stated to help in the elimination of fading. The subject-matter of Specification 295,957 is disclaimed.