Patent GB-797123-A (GB)
Title: Radio diversity receiving system
Inventor: JACOBSEN BENT BULOW
IPC: H04B7/155
Filed: 1956-04-10
Abstract: 797,123. Automatic phase control systems. STANDARD TELEPHONES & CABLES. Ltd. April 10, 1956, No. 10856/56. Class 38 (4). [Also in Group XL (c)] In a two-antenna space diversity reception system the antenna outputs are differentially phase-modulated at a low frequency, and combined through individual adjustable phasing means before demodulation, the envelope of the amplitude modulation due to the said differen- . tial phase modulation being derived, and the phasing means adjusted according to the phase of the said envelope, with reference to the source of the low-frequency energy. In the system described the antennµ 1 and 2, Fig. 1, are located for space diversity reception over the total frequency band of three channels A, B and C. Output from antennµ 1 and 2 respectively to phase modulators 4 and 7 is phase-modulated in reversed phase indicated by the cross in line 8 at a frequency F of the order of 10-100 c.p.s. by L.F. source 5, a differential phase modulation of  10 to  15 degrees being set up between the outputs of the two antennae. The outputs of modulators 4 and 7 are applied to known channel-dropping devices 11, 12 and 13 such as described in Specification 753,886, [Group XL (c)], which respectively cover only one of channels A, B and C and provide an output for each antenna to diversity receivers 14, 18 and 20 respectively. (1) The receiver 14 for channel A, shown in detail in Fig. 3, is fed by aerial 1 over lines 15 connected to phase-shifter 31a adjustable over a range of 3# radians, the output of 'which is connected to " Magic T " hybrid network 33, to which aerial 2, through line 16, is also connected. The symmetrical arms of network 33 are connected to balancing impedance 37 and mixer unit 38 so that all the energies of the two antennae are added at the mixer unit, frequency changed, and fed to preliminary I.F. amplifier 40 without A.G.C. Part of the output of amplifier 40 excites the main I.F. amplifier 41 whose A.G.C. circuit removes the residual amplitude modulation caused by the differential phase modulation of circuits 4 and 7, Fig. 1, and whose output is amplified and detected by known circuits 43. A further part of the still amplitude modulated output of amplifier 40 is further amplified by auxiliary I.F. amplifier 44, and thence fed to envelope detector 45 whose lowest. frequency output component is a wave only of frequency 2F under ideal conditions, but which will contain also a wave of frequency F whose phase is dependent on the error in setting phase shifter 31. Part of this output waveform is applied to phase discriminator 47, which also receives a reference input wave of frequency F from source 5, Fig. 1, to give a D.C. output the sign and magnitude of which is determined by the phase difference between the input from rectifier 45 and that from source 5. The D.C. output is applied by relay 51 to motor 52 which corrects the phase-shifter adjustment. Thus any change in transmission conditions resulting in a change in the mean phase relationship between the outputs of antennµ 1 and 2 will be compensated. In the event of fading in one antenna a device 53 operates the motor 52 to set the phase-shifter 31 centrally, in order to enable a correct adjustment to be made quickly. When only one antenna is active there is no output from amplitude detector 55, and relay 51 remains unoperated, contact 50 being connected to contact 56, device 53 then supplying a continuous current to operate the motor to charge the phase adjuster by #/2 radians, the direction of rotation being controlled by a mechanical reversing contact over line 57.