DESIGN AND IMPLEMENTATION OF A PULSED DIGITAL LLRF SYSTEM FOR THE RAL FRONT END TEST STAND

Design, implementation and some practical results of the pulsed digital LLRF system (amplitude, phase and tuning loops) of the RFQ for the RAL (Rutherford Appleton Laboratory) FETS (Front End Test Stand) are presented. The design is based on a fast analog front-end for RF-baseband conversion and a model-based Virtex-4 FPGA unit for signal processing and PI regulation. Complexity of the LLRF timing is significantly reduced and the LLRF requirements are fulfilled by utilizing the RF-baseband conversion method compared to the conventional RF-IF approach. Validity of the control loops is ensured practically by hardware-in-the-loop cosimulation of the system in MATLAB-Simulink using an aluminium mock-up cavity. It was shown through extensive tests that the LLRF system meets all the requirements including amplitude and phase stability, dynamic range, noise level and additionally provides full amplitude and phase control range and a phase margin larger than 90 degrees for loop stability.