201,591. Robinson, E. Y. March 28, 1922. Heterodyne receiving systems; wired wireless systems. - Modulated waves for telegraphic and telephonic signalling are received by apparatus which separates the carrier wave component from the sideband components, amplifies the former relatively to the latter and re-combines them to effect the de-modulation or detection of the signals. The carrier wave component is amplified by highly-selective amplifiers with little or no damping, so that their amplified amplitude is substantially constant. The amplifiers of the side-band components preserve their relative amplitude, for example by employing oscillatory circuits having substantial decrement. Fig. 2 shows a circuit arrangement suitable for wireless telephony. The received oscillations are applied to the grid of an amplifying-valve 8 which has its anode circuit coupled to the grid circuit 13 of the detecting- valve 14. A second valve 9 also has its input connected across the receiving-circuit 7, and its anode circuit is coupled to a valve 11 which has its grid circuit 10 tuned to the carrier frequency and is given a low decrement by a reaction coil 12. The amplified carrier oscillations in circuit 10 are superposed on the mixed incoming waves by coupling the circuit 10 to the circuit 13. The valve 11 may be of the dimmed filament type, so as to maintain constant the amplitude of the oscillations in the circuit 10. The valve 8 may be direct-coupled to the aerial inductance, and the tuned secondary circuit 7 connected only to the valve 9. When the modulation frequencies represent a considerable fraction of the carrier frequency, as may happen with telephony on relatively low carrier frequencies, the aerial may be tuned to a band of frequencies by means of several oscillatory circuits 24, Fig. 4. The received oscillations are amplified by a cascade of valves and detected in the valve 30. The carrier waves are received on a separate aerial 25, which has its decrement reduced by a reaction coupling from the first amplifying-valve, and are caused to excite a tuned circuit 27 which has a back coupling from the second valve. The output circuit 28 of this valve is coupled to the input circuit of the detector 30, or of one of the earlier valves. Fig. 5 shows a modification for wired wireless systems. The mixed oscillations received in the line 31 are applied in phase opposition to the grids of two valves 34, 35 which have their anode circuits coupled jointly to the telephone circuit 37. The carrier oscillations are applied by a transformer 32 to an amplifier 40 and thence to a lower decrement circuit 41 connected to the grid of a valve 43. The plate circuit of this valve is back-coupled to the grid circuit, and is also coupled to a coil 38 in a common branch of the grid circuits of valves 34, 35. Fig. 6 shows an arrangement in which the same set of valves is used for amplifying the carrier waves and the side-band waves. The aerial coil 44 is coupled to a number of circuits 47 tuned to side-band frequencies, and to a circuit 45 tuned to carrier frequency. All the circuits are in series in the grid circuit of the first valve, and a reaction coil 46 is coupled to the circuit 45 only, so as to reduce the decrement of this circuit. Any other negative resistance device may be used for this purpose. The amplified oscillations are transferred by couplings 48, 50 to the grid circuit of the next valve, and the circuit 49 is tuned to the carrier frequency and is given a low decrement by the back coupling 51. The side-band oscillations are then applied in phase opposition by a coupling 52 to the grids of valves 57, 58, and the carrier oscillations are applied to the same grids in phase agreement by the transformer 54, the secondary of which is tuned to carrier frequency and given a low decrement bv a back coupling 55 from a valve 56. The detected signals are heard in a telephone circuit coupled jointly to the anode coils of the valves 57, 58. The carrier oscillations and the side-band oscillations may for the purpose of detection 'be applied to two separate grids in a single valve, the anode circuit of which contains the telephone transformer. In re-combining the two sets of oscillations, their phase relation should be the same as when received, and for this purpose the constants of the various circuits may be adjusted, as, for instance, by varying the condenser 28<a>, Fig. 4. The circuits described as of low decrement may be on the verge of oscillation or oscillating so feebly that the amplitude and phase of their oscillations is determined by the incoming waves. Grid potentiometers may be provided for the back-coupled valves so that the grid potential may be adjusted to the point of inflexion or the straight part of the characteristic. If the frequency of the carrier wave varies slightly according to the degree of modulation, the low-decrement receiving circuits should be slightly mis-tuned in such sense that when the received carrier waves are weak they are more nearly in tune with the low-decrement circuits, thereby tending to keep the resulting oscillations constant. Reference has been directed by the Comptroller to Specifications 148,380 and 174,312.