GB540839A - Improvements in or relating to telegraph systems - Google Patents

Abstract

540,839. Wireless receiving system. STANDARD TELEPHONES & CABLES, Ltd., and EARP, C. W. April 30, 1940, No. 7801. [Class 40 (v)] In a radio telegraph receiving arrangement in which a signal path and an interference path are derived subsequent to the second detector stage, the signal path and interference paths have opposite effects upon the telegraph receiving relay. Fig. 1 shows a receiver in which interrupted carrier wave transmission is received by an aerial AN from where it passes through H.F. amplifier A1, frequency changer D1, I.F. amplifier A2 to a detector D2 having two outputs. One output is connected to a low pass filter F to provide a signal path to a conversion circuit CC and thence to a telegraph relay circuit RC, security apparatus SA and recorder TP. The second output from D2 is connected to a band-pass filter F1, amplifier A3 and detector D3 the output from which passes through a low pass filter F2 over an interference path IP to a second input of conversion circuit CC. The circuits are arranged so that when interference is present, the voltage applied to CC over IP is opposite to that applied over SP so as to produce a spacing signal, i.e. interference produces the same effect as fading. Fig. 2 shows circuits RC and CC. RC comprises reciprocally coupled valves V1, V2. With no externally applied potential, VI is biased to cut-off by negative bias from R13, while positive potential applied from the anode of VI via resistance R2 to the grid of V2 causes current to flow in anode circuit of V2. This causes negative bias to be applied to grid of V1 via resistance R5 thus maintaining the cut-off or "-rest" condition. However a strong negative potential applied to grid of V2 changes the arrangement to the other condition of equilibrium or " disturbed " condition in which V1 passes anode current and V2 is cut off. The external control for V1, V2 is applied from the conversion circuit CC comprising valves V3, V4 in response to voltages applied over paths SP and IP. Non interrupted radio frequency energy causes constant negative potential to be fed over path SP to grid of V3 to cause it to become more negative and grid of V4 more positive. This causes anode of V4 and grid of V2 to become negative thus enforcing the " disturbed " condition of V, V2. The potential from path IP however causes grid of V4 to become more negative thus opposing the effect of the potential over path SP. It is seen that V1, V2 assumes the "rest" position in the absence of a signal with or without interference, and also in the presence of a signal and interference. A signal without interference causes V1, V2 to assume the "disturbed" condition and give a marking signal. In one modification, two frequency changes are employed while in another, the two channels are separated before the final detector stages. In a further modification a single interference path is used to control a series of signal paths in a multi-channel system. The safety apparatus SA is as described in Specification 514,853, while the recording apparatus TP may be of the kind described in Specification 526,680.