GB635590A - Radio receiving systems - Google Patents

Abstract

635,590. Radio receiving systems. WESTERN ELECTRIC CO., Inc. Aug. 1, 1947, No. 20989. Convention date, May 23, 1944. [Class 40 (v)] [Also in Group XL (b)] In a radio receiving system more particularly a telegraph system which employs different frequencies for marking and spacing signals, two separate signal receiving paths are arranged so that the paths are cut off on the reception of excessive noise or the gain of the path having the greater noise content is decreased. Fig. 1 shows a telegraph receiver wherein incoming signals are passed through a current limiter CL to two filters 2, 3 which separate the marking and spacing signals. The separated signals are detected by full-wave rectifiers 6, 11 and are impressed on the grids of the " marking " and " spacing " amplifiers 7, 12 connected to corresponding windings of the polarized relays 8. The voltages of the mid-point 17 of resistances 13, 14 bridged across the grids of valves 7, 12 are a measure of the averages of noise currents in the marking and spacing channels so that if during signal reception on a channel the noise level on the other channel is excessive, the voltage of the point 17 increases to operate, through a valve 18, a polarized relay 19 and open the screen-grid circuits and cut-off the valves 7, 12. The arrangement shown in Fig. 1 may be modified for application to a dual diversity receiving system (Fig. 2, not shown), so that the gain of the channels having the poorer signal-to-noise ratio may be decreased while the gain of the other channels is increased. In this arrangement, the mid-points 17 of the pairs of channels are connected to pairs of valves and in the presence of excess noise in the channel, one of these valves reduces the screen voltages on its associated valves 7, 12 to reduce the gain and the other valve brings about an increased gain in the other pair of channels with the result that the combined current supplied to the receiving relay 8 is maintained constant. A system of diversity reception is also described (Fig. 3, not shown), in which the noise-detecting and cut-off circuits shown in Fig. 1 are combined with circuits for compensating selective fading. The output of each amplifier of one channel is supplied through a resistance in series with the screen grid of the corresponding amplifier in the other channel. So that alteration of the signal strength in one channel causes a change in the gain of that channel and a corresponding compensating gain in the other channel, so as to produce constant current operation of the telegraph receiving relay whether both or one only of the channels is receiving signals.