GB296678A - Improvements in or relating to radio and the like receivers - Google Patents

Abstract

296,678. Marconi's Wireless Telegraph Co., Ltd., (Assignees of Usselman, G. L.). Sept. 1, 1927, [Convention date]. Receiving systems and apparatus ; thermionic detectors ; multiplex signalling.-Wireless receivers for frequency - modulated waves are Tendered insensitive to amplitude modulation by applying the incoming wave to a pair of circuits tuned on either side of the operating frequency range, the voltages developed in these circuits being combined in phase opposition, as by them to the grids of push-pull connected rectifying valves. Automatic means' may be provided for maintaining the tuned circuits at their correct frequencies in spite, of accidental variations, e.g., in the mean carrier frequency. As shown in Fig. 1. the incoming wave is applied to two differently-tuned circuits 4, 6 which are connected in push-pull relation to the grids of two leaky-grid detectors 12, 14. The circuits 4, 6 are tuned to frequencies F1, F2, Fig. 2, on opposite sides of the mean carrier frequency Fm. The modulation range is indicated by the shaded portion CD on Fig. 2. As the input frequency varies, the voltage response rises in one of the tuned circuits, and falls in the other, producing corresponding changes in the plate current of the valves. As the plates are connected to opposite ends of the primary coil of the output transformer 16 an additive effect is produced in the secondary coil. The combination of the two resonance curves gives a symmetrical and substantially straight resultant characteristic, thereby avoiding distortion, harmonic production and intermodulation. Variation of amplitude of the incoming wave would cause the grid voltage to change in the same sense in both valves, and no effect would be produced in the secondary of transformer 16. The plate current variations flow to the transformer 16 through condensers 32 which are in shunt to electromagnets 30 through which the plate direct current passes. These magnets act oppositely on an armature 34 which functions as a reversing relay for an electromotor 40. If the tuning of the circuits 4, 6 is not equidistant from the mean received frequency, the magnets 30 are unbalanced and the motor 40 is started up in the correct direction to adjust both tuning condensers (or other tuning elements) until a balance is restored. The condensers can be adjusted separately by hand by loosening a slot connection 24, or bv movement of the normally-fixed condenser vanes In a slightly modified circuit (Fig. 3, not shown) anode-bend detection is employed, the condensers 32 are omitted, and the HT supply B is taken to the inner ends of both magnet coils 30, which are directly in parallel with the transformer 16 and the bye-pass condenser 18. The modulation imposed on the carrier wave may be a complex wave composed of several lower frequencies, each of which constitutes a separate signalling channel. At the receiving end the frequency of the carrier wave may be reduced by heterodyning before it is applied to the tuned circuits 4. 6.