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The low level RF control system consists of a 101MHz signal generator, three feedback loops, an interlock and a protection system. The stability of control system is one of the most important indicators in the cyclotron design, especially when the whole system has a high current. Due to the hugeness of the RF system and the complexity of control objects, the low level RF control system must combine the basic theory with the electronic circuit to optimize the whole system. The major obstacles in the research, which rarely exist in other control systems, lay in the coupling of beam and resonant cavity, requiring to be described by the transfer function between beam and cavity, the complex coupling between microwave devices and the interference signals of all loops. By introducing the three feedback loops (tuning loop, amplitude loop and phase loop) and test results from some parts of electric circuits, this paper unfolds the performance index and design of low level RF control system, which may contribute to the design of cyclotron with a high and reliable performance.
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