GB577247A - Improvements in diversity reception of frequency modulated signals - Google Patents

Abstract

577,247. Wireless receiving circuits. DEHN, S. G. (Press Wireless, Inc.). April 13, 1944, No. 6842. [Class 40 (v)] A diversity reception system for frequency modulation has means for reducing the efficiency of the channel bearing the weaker signal, the means being operative at a stage prior to the frequency discriminator. In Fig. 1 separate diversity aerials A, B feed two channels comprising radio receivers 1a, 1b, limiters .2a, 2b, and discriminator detectors 6a, 6b, the detected outputs being fed to a common combining network 7 and reproducer 8. Part of the I.F. output from each channel is fed to amplifier-rectifier-filter units 3a, 3b to produce direct voltages corresponding to carrier strength which are compared in a differential comparison network 4 to give an output which is zero when the carrier strengths are equal and has opposite sign according as which channel bears the stronger signal. This output is fed through D.C. amplifiers 5a, 5b in a sense to quench the channel bearing the weaker signal. As the A.V.C. circuits of the receivers la, 1b are tied the difference in the I.F. outputs is already made greater than the difference in the R.F. inputs. High speed electronic or other switching may be used to connect the reproducer to the channel having the stronger signal. In a modification, Fig. 2 (not shown), the limiter and the discriminator-detector are common to both channels and are arranged after the combining network. Alternatively the networks 3a, 3b may be dispensed with, the differential comparison network being fed directly from the respective A.V.C. circuits. (Fig. 3 not shown). Fig. 4 shows in more detail, the arrangement of Fig. 1 adapted, for receiving telegraph signals from a transmitter T including a generator G which is keyed to give two carrier frequencies corresponding to marking and spacing signals. The diversity receivers Ra, Rb produce, after heterodyning, the two audio frequencies corresponding to the signals which are selected by band pass filters, Fa, Fb. The two signal channels comprise limiters 2a, 2b, discriminatordetectors 6a, 6b feeding a common combining network 7 and reproducer 8. The units 3a, 3b include amplifiers 12a, 12b and detectors 14a 14b each producing a smoothed output proportional to signal strength. The outputs are fed to the grids of tubes 16a, 16b of a differential comparison unit 4 to produce differential control voltages across cathode resistances 19a, 19b. These voltages are connected to the cathodes of valves 20a, 20b connected as shown to valves 22a, 22b arranged so that for equal signals in both channels valves 22a, 22b are biassed to cut off. If one'end of resistances 19a, 19b is positive relative to the other end, the valve 22a or 22b in the other channel conducts and shunts the output of the corresponding limiter 2a or 2b to disable the weaker channel. Specification 537,689 is referred to.