GB548599A - Improvements in frequency modulation systems for radio communication - Google Patents

Abstract

548,599. Frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 29, 1941, No. 11090. Convention date, Aug. 31, 1940. [Class 40 (v)] The carrier wave is frequency-modulated by a sub-carrier or tone frequency which is itself varied or modulated by the signal; and at the receiver certain side-band groups are separately received, detected, and combined as in diversity reception. The object is to avoid selective fading, and to enable frequency modulation to be used on long-distance short-wave systems. Fig. 1 indicates diagrammatically some of the side frequencies resulting from the frequency modulation of a carrier f0 by a sub-carrier frequency. These side-frequencies are shown in full lines and are spaced from the carrier fo and from each other by a distance representing the subcarrier frequency. When the sub-carrier is frequency-modulated by the signal, groups of sidefrequencies are created as shown in dotted lines. At the receiving station, certain of these groups are selected by geographically separated aerials 1 ... 4 tuned to their mean frequencies and after one or more stages of frequency reduction 5 ... 8, are passed through amplifiers 9 and limiters 10 to frequency-modulation detectors 11. The outputs are fed in parallel to the receiving instrument 12, and arrangements may be provided whereby only those receivers giving the best results are utilized. The Specification includes a tabular statement indicating for a particular case the amplitude and phase of the components of the various groups. Those groups are chosen in which the phases of the components do not change, and the amplitudes are comparatively uniform. Groups extending beyond the useful frequency range may be suppressed at the transmitter.