Patent GB-442123-A (GB)
Title: Improvements in or relating to modulated carrier wave signalling systems
IPC: H04B14/08
Filed: 1934-04-26
Abstract: 442,123. Wireless signalling. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Electra House, Victoria Embankment, London.-(Assignees of Beverage, H. H. ; Riverhead, Long Island, New York, U.S.A.) April 26, 1934, No. 12673. Void [Abridged as open to inspection under Sect. 91 of the Acts, Oct. 27, 1934]. [Class 40 (v)] In a modulated carrier wave system, signals are transmitted in one or more side bands spaced a substantial distance from the carrier wave in the frequency spectrum. Fig. 1 shows the transmitter diagrammatically. Speech signals of say 0 to 5 KC from the microphone M are amplified and modulated at 2 with an oscillation of 20 kc, the lower side band 15-20 kc being selected by a filter 3. Further modulation at 4 by a 25 kc oscillation is followed by a filter 5 selecting the lower side band ranging from 5-10 kc representing the original signal raised 5 kc in frequency. This is used to modulate at 6 a 200 kc oscillation, the upper side band and the carrier being selected by a filter 7 and applied to modulate at 8 a 10,000 kc oscillation. The upper side band which comprises a carrier of 10,200 kc and a band ranging from 10,205 to 10,210 kc is selected at 9 and radiated after passing an amplifier 10. At the receiver, Fig. 2, the received waves are reduced in frequency at 22 by a source 27 of frequency 10,100 kc, giving a wave of 100 kc and a signal band of 105-110 kc. After amplification at 23, the signal band is selected by a filter 24 and passed to a detector 25. The 100 kc wave is selected by a sharply-tuned filter 29 such as a piezo-electric crystal filter as disclosed in Specifications 293,446, [Class 40 (v)], and 400,404. The filter 29 is followed by a phase detector 31, which reacts on the frequency of the local source 27 so as to maintain the intermediate frequency wave precisely at 100 kc. Another local source 28 supplies waves of 105 kc to the detector 25 thus giving the original signal band of 0-5 kc in the audio-frequency amplifier 26. In a modification, the transmitter filter 7 is adjusted to pass the 200 kc carrier and both side bands. The radiated wave would then consist of two side-bands, 10,190-10,195 kc and 10,205- 10,210 kc, symmetrically placed about a carrier of 10,200 kc. The receiver in this case, as shown in Fig. 3, comprises a frequencyreducing stage 22 fed by an oscillator 27 of 10,100 kc. Of the resultant frequencies, the 100 kc wave is selected by a crystal filter 29 and the upper and lower side bands by filters 37, 38 respectively. The side bands are passed to separate detectors 39, 41 where they are mixed with waves of 95 kc and 105 kc respectively, obtained by combining at 33 the 100 kc wave from filter 29 with a 5 kc wave generated at 32. The 95 kc wave and the 105 kc wave are separated from each other by filters 43, 44. The output from detector 39 or from detector 41 comprises the original signal, and the two outputs may be used individually or combined as shown in an amplifier 42. In a further modification, the 5- 10 kc band emerging from stage 5 at the transmitter is used directly to modulate the carrier, and the transmission is received by the circuit shown in Fig. 3. The reduction of the signal currents to a single octave at stage 5, facilitates their use for frequency modulation of a carrier. The tendency in frequency modulation to over-modulate at low signal frequencies is further compensated by passing the signals through a resistance-inductance series combination, and utilizing the voltage across the inductance for frequency modulation, as in Specification 411,644. A corresponding correcting network is embodied in the audio-frequency circuit of the receiver. Phase modulation also is referred to.