Development of a high-performance RF front end for HEPS 166.6 MHz low-level RF system

Abstract

Digital low-level radio frequency (LLRF) system has been proposed for 166.6 MHz superconducting cavities at High Energy Photon Source. The RF field inside the cavities has to be controlled better than 0.03% (rms) in amplitude and $$0.03^\circ $$ (rms) in phase. A RF front end system is required to transform the RF signal from the cavity to IF signal before inputting into the digital signal processing (DSP) board, and up-convert the IF signal back to RF to drive the power amplifier. Connectorized off-the-shelf microwave components were used to realize the RF front end system. The local oscillator generation and distribution, choices of main components and design of down-/up-conversion channels have been elaborated in detail with a focus on minimizing nonlinearity and signal interferences among channels with optimized signal distribution loss. The RF front end has been incorporated with the existing DSP board and tested on a warm 166.6 MHz cavity in the laboratory. Excellent channel isolations and good linearities were achieved on the RF front end system. The RF field inside the cavity was controlled with a residual noise of 0.004% (rms) in amplitude and $$0.002^\circ $$ (rms) in phase, well below the HEPS specifications. The sensitivity to ambient environment changes have also been studied and presented in this paper. This demonstrates a first high-performance 166.6 MHz RF front end system and provides valuable insights into HEPS LLRF system development in the future.