GB594160A - Improvements relating to electric signalling systems - Google Patents

Abstract

594,160. Electric signalling systems. SIEMENS BROS. & CO., Ltd., CRUTCH, L. S., MOLE, J. H., and NEWMAN, W. C. May 11, 1945, No. 11914. [Class 40 (iv)J [Also in Group XL] In an electric signalling system, such as a telephone system, using alternating current for signalling, the input and output circuits TRL, TRC of a limiting device LIM are linked by a first coupling including TRL(II), and a second coupling TRL(III) connected in opposition to the output TRC so as to give, by virtue of their special characteristics, a combined output characteristic in which there is no substantial increase in output for increases of input above a critical value. The first coupling comprises winding TRL(II), a series resistance YC and an output shunt arm comprising equal resistances YA, YB and oppositely connected rectilier elements RA, RB biassed by a suitable battery current through resistance YD. The characteristic of this coupling comprises a comparatively steep first portion and a less steep second portion. The second coupling consists in winding TRL(III) which has a linear characteristic. In alternative arrangements, either the windings TRL(II), (III) may be continuous or a resistance and capacity may be inserted between YC and the rectifier elements and the resistance YB reduced to zero. The limiter is described in conjunction with a signal responsive apparatus suitable for use when alternating currents of two separate speech frequencies are used for the transmission of calling and supervisory signals over a conversational circuit. Low impedance highspeed relays X, Y are normally operated and each relay is released when the anode current of the corresponding pentode VA or VB drops on receipt of a pure signal of the appropriate frequency. The normal control grid is given a positive bias by connection to positive battery through resistance YPX. The suppressor grid is used for control purposes and is given a negative bias which is controlled by a non-linear ballast resistance BRX in the cathode lead. The screen grid circuit includes the primary windings of transformers TRX, TRXB, the former sharply tuned and the latter broadly tuned to the same signalling frequency. The signal through TRX(II) and a voltage doubling rectifier RX produces a potential difference across the resistance YX which tends to reduce the anode current to release X. This voltage is countered by a similar potential difference across YXG produced by rectifier RXG according to the differential effects of TRX(III) and TRXB. For a pure signal, this effect is zero and relay X releases. The alternating current signals may be transmitted as modulations on a carrier current. The Provisional Specification states that the signal discriminating arrangements described in Specification 594,159 may be employed and that the circuits may be so arranged that the signal relay is operated by a signal, being normally released.