Improvements in and relating to frequency modulation receivers

摘要

594,769. Frequency modulation. BRITISH THOMSON-HOUSTON CO., Ltd. April 26, 1944, No. 7770. Convention date, April 27, 1943. [Class 40 (v)] In order to provide steady increase and decrease of signal strength, and to reduce distortion, when tuning a frequencymodulation receiver through a desired station, a balanced discriminator is employed and each rectifier feeds the receiving instrument through a separate channel. These channels each include a valve, and bias potentials derived from the rectifier circuits operate on the valves to diminish the signal response from that rectifier which, owing to mistuning, tends to take the greater load. As shown, a discriminator network 11, 14, 15 of the phasing type feeds oppositely-poled diodes 20, 29, each of which is connected to a divided load resistance 22, 23 and 26, 27, the division points being earthed. The signal is fed from a tap 40 on resistance 22 through a condenser 42 to the grid of a valve 44 and from a tap 41 to the grid of a valve 45. The valve anodes are connected in parallel to the signal output circuit 60 and as the alternating signal voltages are in phase, their effects add in the output circuit. A bias voltage for the valve 44 is provided from the mid-point of resistances 55, 56 connected between the unearthed ends of the load resistances 23, 26 ; and bias voltage for the valve 45 from similar resistances 53, 54 connected between the unearthed ends of load resistances 22, 27. In this way, the channel which, owing to mistuning, would otherwise be operating on the hump of its discriminator curve E or E2, Fig. 3, is reduced in effectiveness, while the other channel is improved. The coupling afforded by the condenser-shunted resistance 48 in the common cathode lead of the valves 44, 45 enhances this reciprocal action. In a modification (Fig. 5, not shown), the triodes 44, 45 are replaced by oppositely-poled diodes, the signal and bias voltages being applied to their anodes and cathodes.