Computer analysis of the effect of a spectrum anomaly on the RF system for the Cassini Mission

Summary form only given, as follows. An anomaly was observed in the output of the flight TWTA (traveling wave tube amplifier) for the 32 GHz experiment scheduled to fly on the Cassini Mission to Saturn which will be launched in October 1997. The anomaly was an apparent intermodulation product in the neighborhood of 35 GHz which was manifest only when the TWTA was driven in a range from 10 to 15 dB below saturated drive. This effect was not observed until 18 months after the production run of the TWT (the Hughes 955H) had been terminated, and the tube and its power supply had been packaged for night. There was, therefore, opportunity for only very limited testing of the TWTA and no chance of building and qualifying a replacement prior to launch. The effect was observed to occur only outside the expected mission performance profile, however, system engineers had serious concerns that the anomaly might interfere with the 32 GHz experiment if changes in tube performance were to occur during the mission. An analysis of the TWT was conducted using the Detweiler and MAFIA computer codes which produced an explanation of of the observed phenomena, a generic defect inadvertently incorporated in the slow wave circuit design, and predicted that the most likely outcome of TWT aging would be to reduce or eliminate the anomaly.