Patent GB-1433589-A (GB)
Title: Arrangement for the signal-to-noise ratio of signals picked up  by aerials comprising a plurality of elements
IPC: H04B7/08
Filed: 1967-07-30
Abstract: 1433589 Diversity reception COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS CIT-ALCATEL 30 July 1967 [22 Feb 1967] 50438/69 Divided out of 1433588 Heading H4L An arrangement for improving the signal/ noise ratio of signals received on at least two aerials 1 1 , 1 2  in the instance where the useful signals S occupy a narrow band of frequencies centred about a mean value Fo and where the noises B 1 , B 2  received on the two aerials are at least partially cross-correlated, comprises compensating delay networks 3 1 , 3 2  to ensure that two absolutely identical useful signals S are obtained at the outputs 4 1 , 4 2 . The outputs 4 1 , 4 2  are connected to identical band pass filters 5 1 , 5 2  centred on the frequency Fo. The output # 1  of filter 5 1  is equal to S+B 1  and the output # 2  of filter 5 2  is equal to S + B 2 , these outputs being applied to sub-assemblies E1 1  and E2 1 . The signals having the improved signal/noise ratio are obtained from the output of an amplifier 76 in a final sub-assembly E2 2  whilst a further sub-assembly E1 2  derives signals # 1 , # 2  from signals u, v obtained in sub-assembly E1 1  and supplies those signals to the subassembly E2 1 . Signals # 1 , # 2  and V obtained in sub-assembly E2 1  are applied to sub-assembly E2 2 . Sub-assembly E1 1  comprises a polarity coincidence correlator 114 preceded by limiters 115, 85 providing square-wave outputs of equal amplitude and of +ve or -ve sign depending upon the polarity of the inputs # 1 , # 2 . The correlator 114 produces an output voltage equal to unity having + ve or -ve polarity depending on whether the two inputs have the same or different signs. The output of correlator 114 is connected to a low-pass filter 116 of time constant T of the same order of magnitude as the slow variations of the noises B 1 , B 2 . The output of filter 116 is applied to a corrector 117 to compensate for the effect of the limiting so that the output signal of the corrector represents the cross-correlation coefficient, R(0), of the noises B 1 , B 2 , this signal having a value less than 1. The circuit member 118 produces the difference between a constant voltage C equal to unity which is applied thereto and the voltage R(0), this difference signal, 1-R(0), being denoted by u. The cross-correlation coefficient, J(0), of the noises B 1 , B 2  when they are in phase quadrature with respect to each other is obtained, similarly, by a correlator 86 supplied with signals from limiters 85, 84; however, the limiter 84 is preceded by a phase shifter 11 which shifts the phase through The correlator output is applied to a filter 13 and corrector 119, the output v of the latter representing J(0). Sub-assembly E1 2  comprises an auxiliary generator 44 supplying a voltage of frequency F where F is not equal to Fo. A phase shifter 45 shifts the phase through the sense of this shift being the same as that of phase shifter 11. Modulators 47, 46, receive the signals u, v of very low frequency and also signals of frequency F from the generator 44 and supply their outputs to filters 120, 121, the latter producing at their outputs voltages x, y, respectively, with variable amplitudes proportional to the voltages u, v. A summing member 48 produces a signal #, equal to x+y and a difference member 74 produces a signal # 2  equal to x-y. Sub-assembly E2 1  comprises limiters 122, 123 producing voltages # 1 , # 2 . Modulators 87, 88 multiply, respectively, # 1 , ## 1 , # 2 , ## 2  and these products are added in member 9 connected to a band-pass filter 75 producing an output voltage V of frequency Fo+F. Sub-assembly E2 2 , comprises an amplifier 76 having its gain controlled to compensate for the modification given to the signal S by the preceding circuit members. The gain control voltage W is derived from a circuit including an adder 130, band-pass filter 131 and envelope detector 132.