The L3 Coaxial System

The equalization and regulation problems of the L3 system are described and a theory of equalization of complex systems is outlined. The location and function of the various equalizers are explained including the roles and design of the various fixed, dynamic and manual equalizer networks. The analog computer used in the regulation system is described together with the cosine‐equalizer adjusting technique used with manual equalizers. Finally the circuits and operation of the regulation system and its components are presented.

The L3 Coaxial System

Ketchledge¹,

Finch²

1953

The L3 Coaxial System

Morris¹,

Lovell²,

Dickinson³

1953

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.

The L3 Coaxial System

Rieke¹,

Graham²

1953

Television terminals are required at circuit ends of the L3 coaxial system; at the transmitting end to condition video signals for carrier transmission and at the receiving end to detect the transmitted signals. Special signal characteristics, e.g., a degree of modulation which exceeds the value commonly referred to as 100 per cent modulation, require departures from standard modulating and detecting processes. The high degree of modulation requires both careful control of transmitted wave form and at the receiver product demodulation with phase synchronous carrier (homodyne detection). Carrier regeneration requirements result in the choice of one step frequency translation from the video frequency spectrum to the allocated vestigial sideband carrier spectrum. The one step process using a single modulator results in unusual balance requirements for the modulator itself and an unusual circuit configuration. Transmission quality objectives for the terminals are such as to permit six pairs of television terminals to operate in tandem in a transcontinental circuit. This permits a degree of interconnecting flexibility in operation with other systems, e.g., L1 coaxial or microwave systems. These objectives place severe requirements upon the transmission stability of various fillers and other circuits within the terminals. New network techniques both in design and fabrication are brought to bear in the effort to achieve required performance. The transmitting and receiving terminals are described, illustrating the functional operation and mechanical and electrical arrangements of the equipment.