Patent GB-814984-A (GB)
Title: Improvements in or relating to duplex radio communication systems
Inventor: BEANLAND CHARLES PERCY
IPC: H04B1/54
Filed: 1956-06-14
Abstract: 814,984. Automatic frequency control systems. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. March 4, 1957 [June 14, 1956], No. 18411/56. Class 38 (4). [Also in Group XL (c)] A ratio transmitter-receiver comprises means controlled by the received frequency to maintain a predetermined difference between the transmitted and the received frequencies and to adjust the receiver tuning to said received frequency. In the embodiment described a ground transmitter - receiver 1, Fig. 1 co-operates with an airborne station 2. A single side-band transrnitter comprises a known audio amplifier 3, balanced modulator 4 to which is also applied a 100 kc/s signal from oscillator 5 (also in Fig. 2, not shown) to filter 6 passing the upper side band, which feeds balanced modulator 7 also fed with a 1500 kc/s signal from oscillator 8. Amplifier 9 selects the sum frequencies derived from mixer 7 and applies them to mixer 10 to which is also fed an 18,400 kc/s signal from oscillator 11. Amplifier 12 having a passband centred on 20,001 kc/s selects the sum frequencies and applies them to aerial 13 through the contacts of send-receive relay 14. The receiver comprises an R.F. stage 15 a first mixer 16, also fed by oscillator 11, I.F. stage 17, selecting 1,601 kc/s, second mixer 18 fed by oscillator 8, a signal therefrom of 101 kc/s. being applied to the filters 19 and 20. Signals from filter 19 and from oscillator 5 are fed to a carrier reinserting and demodulating network and thence to A.F. stage 22. Signals through the filter 20 are amplified and limited by circuit 23 and applied to an A.F.C. circuit AFC, which compares this frequency with that of oscillator 5 and adjusts oscillator 8 to reduce the difference in the two frequencies to zero. A similar co-operating transmitter-receiver 2 comprises aerial 24, and associated relay 25, the receiver comprising an R.F. stage 26, frequency changer 27, amplifier 29, second frequency changer 30, amplifier 32, third frequency changer 33, and audio stage 35, with pass bands as shown in Fig. 1, and three oscillators 28, 31, 34 corresponding to those in unit 1. The transmitter comprises audio stage 36, balanced modulator 37, band-pass filter 38, mixer 39, to which are also applied signals from the carrier leak circuit 40, amplifier. 41, a second mixer 42, and a final amplifier 43. In operation there is no necessity for elaborate frequency stabilization in the co-operating station because if any oscillator changes its frequency the A.F.C. circuits in the main station will alter the frequency of oscillator 8 to retune the main station. The carrier leak network 40, Fig. 3, which injects into the balanced modulator a component whose level increases as the audio frequency level at amplifier 36 decreases, thus operating the A.F.C. circuits under adverse signal/noise conditions, comprises a valve 49 whose grid bias is derived from the rectified speech input 36a, to deliver a controlled carrier in its output tank circuit 52. In the A.F.C. arrangement, Fig. 4, signals from amplifier 23 are applied to one end of a diagonal of a balanced bridge 53 the opposite end of which is earthed, the other diagonal supplying signals in opposite phase to the grids of push-pull valves 55 and 56, each feeding a pair of pushpull valves 59, 60 and 61, 62 respectively. The 100 kc/s output of oscillator 5 is applied at the mid points of the secondaries of transformers 57 and 58, and the four-phase stator windings of motor 63 are respectively included in the anode circuits of the two pairs of push-pull valves, the motor driving a variable capacitor to tune oscillator 8. The carrier re-insertion network and audio stage 21, 22 and 33, 35 comprise two amplifiers 50, 51, Fig. 5, the output of amplifier 51 being applied to the primary 66 of a transformer, that of amplifier 50 appearing at the electrical centre of the secondary of the transformer, whose ends feed a pair of diodes in a known combining circuit.