Concepts in Communication Amplifier Redundancy Systems

Redundancy is a major concern to satellite communication amplifier systems. This paper examines the various types of redundant systems used in satellite communication systems from a system reliability viewpoint. To overcome the ambiguities involved with comparing MTBFs, the paper discusses amplifier reliability based on static reliability concepts. Both traditional switched redundant systems and switch-less systems are compared. Amplifier Reliability Redundant systems of long been employed in communication amplifier systems as a means of increasing the amplifier system reliability. Redundant systems first appeared in communication systems with the introduction of the klystron amplifier. Generating high power microwave energy places tremendous electrical stress on the amplification device. Microwave klystron and traveling wave tubes (TWTs) operate at extraordinarily high cathode temperatures. These high operating temperatures result in relatively low MTBFs (Mean Time Between Failures) and thus a corresponding high failure rate. The high voltage power supplies required to operate traveling wave tubes also have a history of high failure rates. In recent years solid state power amplifiers (SSPAs) have made significant improvement in MTBF but still typically fall short of meeting the reliability expectations of satellite communication links. Satellite transponder time is extremely expensive and operators cannot afford to have a satellite link off the air for any period of time, no matter how short in duration. In many instances satellite equipment is installed in remote locations which are not easily accessed for maintenance. Therefore it is imperative that any amplifier system used in satellite communication be equipped with some form of automatic backup or redundancy. The goal of any redundant amplifier system is to achieve a system reliability that is greater than the reliability of an individual amplifier. Static Reliability Theory To simplify the understanding of amplifier redundancy concepts, the static reliability principles are applied throughout this paper. This removes the time varying probability density functions that can be based on many different types of distribution. The reader can choose to apply the probability density functions of choice to the results of the static reliability models to arrive at a Mean Time Between Failure (MTBF) figure of merit. The basic static reliability theory requires that series systems be modeled using the following equation.