Patent GB-562993-A (GB)
Title: Frequency modulation signal receiving system
IPC: H03D3/24
Filed: 1943-02-11
Abstract: 562,993. Frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 11, 1943, No. 2310. Convention date, Feb. 12, 1942. [Class 40 (v)] In a frequency modulation receiving system, the received wave, preferably after frequency reduction by heterodyning, is applied to lock in a local oscillator at a sub-multiple of the applied frequency, the local oscillator output being fed to a frequency-discriminating detector. The locking in may be effective over only a portion of the frequency deviation, and the mid-frequency of the oscillator may be continuously adjusted by a reactance valve controlled by the detected signal. The system limits interference due to noise or adjacent channel frequencies. As shown, the incoming signal is heterodyned at 7 to produce a suitable intermediate frequency in the tuned circuits 10, 11 which are connected to the inner control grid of a pentagrid oscillator 12 having its frequency-determining anode circuit 13 tuned to a sub-multiple, say, one-fifth, of the I.F. and back-coupled by a circuit 47 to the outer control grid of the valve. The anode of valve 12 is coupled by adjustable resistance 25 and condenser 27 to a frequency discriminator comprising a parallel tuned circuit 14 earthed at its opposite side, and a circuit 15 parallel resonant at the higher end of the deviation range and also forming with the capacity 15<11> across the rectifier 17 a series-resonant circuit at the lower end of the range. A capacity 80 couples the oscillator anode to one end of the circuit 15, to the anode of rectifier 16 and to the mid-point of the load resistance 18, across which is connected a condenser 81 and the output to the A.F. amplifiers. The coupling provided by resistance 25 and condenser 27 may be adjusted so that the valve 12 locks in over the whole range of frequency deviation, or over only a portion of that range. In the latter case, the oscillator tuned circuit is associated with a reactance valve 50, the grid 52 of which is connected to a potentiometer 55 across the A.F. output so that as the input frequency deviates in either direction the tuning of the oscillator is adjusted by the reactance valve in that same direction. Slow frequency drifts may also be provided for by shorting the condenser 71 in the lead to the reactance valve grid. When incoming signals are weak, the effectiveness of the control by the reactance valve may be adjusted by connecting the grid by a switch 75 to an AVC potentiometer 39 connected in the inner grid circuit of the oscillator valve 12. The reactance valve may be put out of action by a switch 72 shorting the cathode bias resistance 84; or by passing through that resistance the anode current of one of the amplifier valves controlled by the AVC, the reactance valve may be rendered inoperative during inter-station tuning. Noise during tuning may be suppressed by applying suitably-derived potentials to one of the control grids of the oscillator 12. A modified arrangement of the discriminator circuits 14, 15 and rectifiers 16, 17 is described, Fig. 1 (not shown). Specifications 379,608, 411,351, 489,094 and 554,914 are referred to.