Power Supplies for Coaxial Systems

Abstract

TYPE “L” BROAD-BAND carrier telephone systems over coaxial cables require repeaters spaced every eight miles or less along the cables. The large number of repeaters involved makes it desirable to transmit 60-cycle power from main and terminal stations to auxiliary repeaters over the same coaxials as used for carrier transmission. The power circuit shown in Figure 1 for a typical span of about 100 miles between adjacent power sending stations involves primaries of powerpacks at each repeater arranged in series with the inner conductors of the coaxials to form a series loop from each sending station. Power separation filters at each repeater separate the carrier frequencies for amplification and return to the coaxials without affecting or being affected by the power circuit. The power packs are individually adjusted to give proper filament and plate power based on a constant current system with a line current of approximately 500 milliamperes. This current is regulated at the sending end by means of an automatically-controlled motor-driven variable transformer which raises and lowers the voltage as required to compensate for changes in line impedance with temperature or for variations in the power source voltage. Automatic regulation provides protection against overload such as might occur if a coaxial should open or become short-circuited. Where the overload is insufficient to blow the supply fusing, the controls reduce the line current to normal by dropping the voltage to the necessary value. Where the short circuit is near enough to the main station to blow a supply fuse, the control circuit functions to turn the variable transformer to minimum voltage so that replacement of the blown fuse cannot cause a line current surge.