Design approaches for the X band LLRF system

The low-level RF (LLRF) system regulates disturbances over a limited bandwidth in accordance with its capabilities and the RF loop parameters. The disturbances usually originate in the RF system or can be coupled to the RF system from the environment. In this paper a general overview of the possible design approaches for a digital LLRF system operating in X band is presented. Firstly, the possible design approaches of the RF front/back ends are presented and reviewed. We also define the main design parameters for the RF front/back ends. Parameters like isolation between channels, noise, gain, linearity and number of IF stages are put into the perspective of machines using RF components in the X band. An important part of the LLRF system is the local RF timing generation and distribution, which is also treated in the paper. In the second part of the paper the main design approaches in the digital signal processing part of the LLRF system are presented. The emphasis is on the algorithms that are machine specific. Some standard processing algorithms like adaptive feed-forward and arbitrary shaping of feed-forward pulses are presented. Finally, a suggestion for the X band LLRF design is given.