GB550193A - Improvements in and relating to multiplex frequency-modulation systems - Google Patents

Abstract

550,193. Multiplex wireless signalling. MAR. CONI'S WIRELESS TELEGRAPH CO., Ltd. June 19, 1941, No. 7756. Convention date, June 19, 1940. [Class 40 (v)] In a multiplex radio system employing a plurality of frequency-modulated sub-carriers all modulated on a main carrier, (a) any sub-carrier is modulated to a greater frequency swing than any sub-carrier of lower frequency, and .the main carrier is modulated to the same extent by all the sub-carriers; or (b) all sub-carriers are modulated to the same frequency swing and the main carrier is modulated to a greater extent by any sub-carrier than by a sub-carrier of lower frequency; or (c) the sub-carriers are modulated as in (a) and the main carrier as in (b), and the products of the sub-carrier and main carrier deviations are proportional to the sub-carrier frequencies. The purpose is to maintain the same favourable ratio of signal to noise on all the channels. Fig. 3 shows a transmitter in which five signal sources 1 ... 5 are coupled to the respective sub-carrier generators 6 ... 10 to produce frequency-modulated sub-carriers. These are combined in the valves 11 ... 15 and used to modulate the main carrier source 18. In the (a) system, all the sub-carriers are of the same voltage and therefore modulate the carrier 18 to the same extent, but the signals are adjusted in strength so that the frequency swing of each subcarrier is made proportional to the frequency of that sub-carrier, the combined sub-carriers forming an approximately continuous spectrum. In the (b) system, all the sub-carriers 6 ...10 are given the same frequency; but the voltage of each sub-carrier is proportional to its frequency, so that the higher sub-carriers modulate the main carrier more deeply than the lower. Automatic frequency control circuits may be provided for the sub-carrier and main carrier generators (Fig. 8 not shown). At a receiving station, the receiver carrier is demodulated and the resulting sub-carriers are assigned to their respective channels by filters, and are subjected to a further demodulation (Fig. 5, not shown). The system may be used for transmission between relaying stations, signals from distant stations being picked up by receiving aerials, demodulated, and used as the signal sources 1 ... 5 of Fig. 3 (Figs. 6 and 7 hot shown). Some of the signals 1 ... 5 modulated with different matter, and the higher may comprise a plurality of tone frequencies each tones may give a greater frequency swing to the sub-carrier than the lower Phase modulation followed by frequency multiplication may be used instead of frequency modulation. Specification 533,479 is referred to.