Direct RF Sampling based LLRF Control System for C-band Linear Accelerator

TL;DR

This paper presents a novel C-band LLRF control system using direct RF sampling with RFSoC, eliminating analog mixers and simplifying LINAC control architecture, with detailed architecture, optimization, and performance characterization.

Contribution

The paper introduces a direct RF sampling LLRF control platform based on RFSoC for C-band LINACs, demonstrating simplified architecture and improved performance.

Findings

Achieved stable phase and amplitude control in C-band RF signals.

Optimized RFSoC data converter configuration for high-frequency sampling.

Validated system performance with RF pulse testing and solid-state amplifiers.

Abstract

Low Level RF (LLRF) control systems of linear accelerators (LINACs) are typically implemented with heterodyne based architectures, which have complex analog RF mixers for up and down conversion. The Gen 3 Radio Frequency System-on-Chip (RFSoC) device from AMD Xilinx integrates data converters with maximum RF frequency of 6~GHz. This enables direct RF sampling of C-band LLRF signal typically operated at 5.712 GHz without any analogue mixers, which can significantly simplify the system architecture. The data converters sample RF signals in higher order Nyquist zones and then up or down convert digitally by the integrated data path in RFSoC. The closed-loop feedback control firmware implemented in FPGA integrated in RFSoC can process the base-band signal from the ADC data path and calculate the updated phase and amplitude to be up-mixed by the DAC data path. We have developed a C-band LLRF control RFSoC platform with direct RF sampling, which targets Cool Copper Collider (C3) and other C or S band LINAC research and development projects. In this paper, the architecture of the platform will be described. We have optimized the configuration of the data converter and characterized performance of them with RF pulses. The test results for some of the key performance parameters for the LLRF platform with our custom solid-state amplifier, such as phase and amplitude stability, will be discussed in this paper.