GB551293A - Improvements in automatic gain control for electrical transmission systems - Google Patents

Abstract

551,293. Repeater systems. STANDARD TELEPHONES & CABLES, Ltd. March 28, 1942, No. 4171. Convention date, Oct. 19, 1940. [Class 40 (iv)] In a system particularly for carrier transmission. in which a pilot current is employed to control the gain of the amplifiers along a line, a change of signal level tends to produce transient irregularities since a correction is applied at a number of amplifiers more or less simultaneously. To reduce this effect, the line is divided into sections by means of filters such as 12 which block the passage of pilot current, and a new pilot current is applied beyond each blocking filter, the strength being dependent on the integrated variations of the pilot current arriving from the preceding section. In Fig. 1, the outgoing pilot current is applied from a source 13 through a resistance element 15 which has a negative temperature coefficient, e.g. silver sulphide, and which is controlled by a heating coil 16. The incoming pilot current passes through a filter 18, amplifier 19, rectifier 20 to a direct current amplifier 21 the output of which supplies current to the heating coil 16. In the modification in Fig. 2, the outgoing pilot current is derived from the incoming pilot current by taking a part of the output of the amplifier 19 to an overloaded amplifier 31 which supplies current of substantially constant amplitude through a transformer 33. The pilot current controls the feedback path of the amplifiers for example in the manner described in U.S.A. Specification 2,179,915.