Advanced topics on RF amplitude and phase detection for low-level RF systems

Low-level radio frequency (LLRF) systems stabilize the electromagnetic field in the RF cavities used for beam acceleration in particle accelerators. Reliable, accurate, and precise detection of RF amplitude and phase is particularly important to achieve high field stability for pulsed accelerators of free-electron lasers (FEL). The digital LLRF systems employ analog-to-digital converters to sample the frequency down-converted RF signal and use digital demodulation algorithms to calculate the RF amplitude and phase. Different sampling strategies and demodulation algorithms have been developed for these purposes and are introduced in this paper. This article focuses on advanced topics concerning RF detection, including accurate RF transient measurement, wideband RF detection, and RF detection with an asynchronous trigger, local oscillator, or clock. The analysis is based on the SwissFEL measurements, but the algorithms introduced are general for RF signal detection in particle accelerators.