The L3 Coaxial System

Abstract

The line amplifiers for the L3 coaxial system are designed to compensate for the loss of the four miles of cable which separate the repeaters; the flat amplifiers are used to compensate for equalizer loss and as transmitting amplifiers. The two types are basically similar, consisting of two feedback amplifiers in tandem separated by an inter-amplifier network; in the line amplifier this network is variable and is automatically adjusted to compensate for variations in cable temperature and for small deviations from the nominal four-mile spacing. Coupling networks employing high-precision transformers are used to connect the amplifers to the coaxial cable through the required power separation filters. The low impedance windings of the transformers are center-tapped and a balancing network provided in order to match the cable impedance over the transmitted frequency band. The amplifiers are equipped with plug-in tubes of high figure of merit which were developed for this application. A double-triode output stage is used to obtain improved system signal-to-noise performance. Provision is made for preventive maintenance of vacuum tubes and for a controlled adjustment of gain on an in-service basis. All important components of the amplifier are subject to quality control procedures to assure that the average gain of groups of amplifiers will be held within narrow limits and that individual amplifiers will form a normal distribution around the average. This approach is essential in order to meet system equalization and signal-to-noise objectives. Careful mechanical design and rigid control of the mechanical aspects of manufacture are necessary to minimize gain variations which might be caused by variations of parasitic circuit elements and unwanted feedback effects. Special measures were required to keep the temperature rise within the sealed die-cast housing within tolerable values.