GB485652A - Improvements in and relating to telephone systems and the like - Google Patents

Abstract

485,652. Exchange systems. TELEPHONE MANUFACTURING CO., Ltd., and PADDLE, L. H. Oct. 20, 1936, No. 28506. [Class 40 (iv)] [Also in Group XL] In an alternating current signal receiver for use in telephone and like systems and which discriminates against current of signal frequency present in the speech or other band transmitted over the system, the receiving relay has two windings controlled respectively by a circuit tuned to the signalling frequency and by a circuit over which the whole of the transmitted band is fed. In the particular arrangement described, the winding controlled by the second of the above circuits is also differentially controlled by a further circuit tuned to the signal frequency. The receiver comprises a valve V1, which may be a pentode, having two transformers T1, T2, in its anode circuit, the primary of T1 being tuned to the signalling frequency whereas T2 is responsive to a broad band including the signalling frequency. The output of secondary S1a of T1 is rectified and applied directly to a winding of relay SR whereas the outputs of S1b and S2 are rectified and used to control the bias of valve V2 in opposite senses, the output from S1b tending to increase anode current. On arrival of signalling frequency containing not more than 10 per cent harmonics in addition, the left hand winding of SR is energized and the effects of the rectified outputs of S1b and S2 neutralize and leave normal anode current over the right-hand winding of SR which pulls up. If more than the above proportion of extraneous frequencies is present, the rectified output of S2 preponderates and produces a net negative change of bias, SR remaining inoperative. The relay may be differentially wound if the polarities of rectifiers M2, M3 are the reverse of those shown. To avoid difficulties due to different time constants of the circuits of rectifiers M2, M3, condensers C2, C3 may be replaced by a single condenser. The normal bias of valve V2 may be zero, the effect of positive increase of grid bias being reduced by grid current so that increase of negative bias has a greater effect.