TRAVELING WAVE TUBE RE-ENTRANT AMPLIFIER SERRODYNE SYSTEM

摘要

Communication satellite transponders can be simplified by using a traveling wave tube (TWT) in a re-entrant mode for frequency conversion and amplification. Frequency conversion can be accomplished either by use of a crystal RF mixing scheme in the re-entrant loop or by serrodyning the TWT. The crystal mixing method is straight forward for a 2kMc conversion whereas the serrodyning technique is not. A serrodyned TWT system would eliminate the losses associated with crystal mixing as well as one of the rejection filters in the re-entrant loop;however, serrodyning with linear amplification has only been accomplished at frequencies up to 28Mc. In serrodyning, the TWT helix is modulated with a linear sawtooth, adjusted to deviate the phase (tube transit time delay) 2N77 radians, so that all but one of the first sidebands are suppressed and displaced by N times the fundamental modulating frequency. The upper limitation on the serrodyne frequency, linearity and intermodulation distortion is controlled (1) by the inherent TWT characteristics, namely the helix, and (2) by the sawtooth generator, namely the flyback time. The rejection of the re-entrant loop band pass filters establishes the required TWT rej ection characteristics and vice versa. The loop gain should be less than one. The rejection of the filters should be at least -30db. The Q of the rejection filters is inversely proportional to the bandwidth and the skirt-bandwidth should be no more than twice the information bandwidth. The stability of the TWT re-entrant serrodyne system is also governed by the power supply versus the TWT characteristics. The noise figure for the re-entrant TWT can be obtained by considering the re-entrant loop as a system of two cascaded amplifiers with filters interposed between, which are dependent upon the input power levels and filter insertion loss. Manufacturers of both TWT's and filters conjecture that the state-of-the-arts can be advanced to result in a practical TWT re-entrant system.