Overload Stability Problem in Submarine Cable Systems

Experience gained while laying the Oahu-Guam SD Submarine Cable System shows that a bidirectional single cable system whose repeaters have a common amplifier for both directions of transmission is potentially unstable. So far, this instability has occurred only when an abnormally high-gain repeater section is present. When the instability occurs, noise levels corresponding to repeater overload make communication in either direction completely impossible. The instability results from the feedback of intermodulated signals or noise in overloaded repeaters. Low-band power traveling in one direction through the system is partially converted to high-band power which propagates in the opposite direction, and vice versa. In this manner, a configuration of feedback loops develops. Loops in which repeaters are separated by less than nominal loss are especially critical. 1853 1854 THE BELL SYSTEM TECHNICAL JOURNAL, JULY-AUGUST 1969 H. T H E FEEDBACK MECHANISM The SD and SF repeaters consist of a common amplifier and directional filters. This configuration, shown in Fig. 1, is well suited to undersea applications because of the reduced number of components, greater reliability, and lower power drain compared with the more conventional twin amplifier arrangement. With typical pre-emphasis, the low-band signal levels at the amplifier output are about 10 dB below those of the high band. Thus, the low-band signals comprise a small portion of the total multichannel repeater load. The common amplifier configuration has a severe limitation when one considers the effect of a very large overload.