OVERLOAD STABILITY PROBLEM IN SUBMARINE CABLE SYSTEMS

Modern submarine cable systems usually provide bidirectional transmission over a single cable with repeaters which use a common amplifier for both directions of transmission. The signals for the two directions of transmission occupy separate frequency bands. Under normal loads, the amplifier is highly linear and there is negligible interaction of the signals in the two bands. However, when approaching overload, intermodulation in the repeaters may transfer appreciable power between bands. It has been discovered that this feedback, especially in the presence of large misalignments, can result in a system that maintains itself in overload. Such a system, once excited by a momentary signal or noise peak, generates sufficient intermodulation noise to keep itself overloaded, even in the absence of any further external signal. This paper describes the occurrence which drew attention to this phenomenon, presents an analytical approach used to predict stability margins for any given repeater, and describes the action taken to ensure the stability of the SF Submarine Cable System. © 1969 The Bell System Technical Journal