Digital self-excited loop for a superconducting linac

A self-excited loop based RF control with signal processing carried out primarily in the digital domain, has been developed for the amplitude and phase stabilization of RF fields in the superconducting resonators of BARC-TIFR linac, having a resonant frequency of 150 MHz. The system employs direct sampling and the subsequent signal processing has been carried out in a Field Programmable Gate Array. The signal processing has all been carried out in the baseband using the in-phase and the quadrature components only. Limiter, one of the key elements of the signal processing, has been implemented as a feedback loop, which keeps the magnitude of its output constant without affecting the phase. Using a first order low pass filter with gain as the controller, good steady state and adequate dynamic characteristics have been obtained for the limiter. The paper describes the signal processing modules with emphasis on the analysis of the limiter. The test results with the BARC-TIFR linac are presented. The results are encouraging and establish the suitability of the signal processing scheme for this and similar systems.