Transistor Surface Effects Responsible for Anomalous Third-Order Intermodulation Distortion in Undersea Cable Telephone System

The deviation from perfect linearity in a repeater amplifier for a high-capacity, long-haul system is very small by normal standards. However, when the signal passes through many repeaters, the effect of nonlinear distortion can accumulate to a significant value. The resulting modulation noise of many signals is similar to white noise and is an important factor in the specification of such system parameters as repeater spacing and power-handling capability. Permissible third2025 order harmonic distortion at the output of a repeater, for a milliwatt of fundamental, is typically 10"10 to 1(T13 mW. Measurements of the third-order intermodulation product for the SG submarine cable amplifier showed unusual behavior as power was increased.1 The deviation from the expected 3-dB increase in thirdorder power for a 1-dB increase in the three fundamental powers was largest for the case where the fundamental frequencies were slightly lower than the product frequency, as illustrated in Fig. 1. Distortion measurements on the open loop amplifier produced similar results, so the feedback network was not a cause. When the bias voltage and current were varied separately for each of the signal-carrying bipolar transistors in the amplifier, the collector voltage nonlinearity showed -50 SLOPE 3/1