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Description
A readout system based on analog down-conversion electronics and a synchronized local oscillator (LO) was developed and tested with a cavity beam position monitor (cBPM) installed at the end of the Accelerator Test Facility (ATF) linac at KEK, Japan. The cBPM operates with dipole- and monopole-mode resonances at 1.725 GHz and 1.25 GHz, respectively, requiring frequency down-conversion to an intermediate frequency (IF) suitable for digital acquisition and processing using a digital down-conversion (DDC) algorithm. The broadband response of the cBPM allows the monopole and dipole components to be separated, enabling the extraction of X, Y, and reference signals from a single BPM. Initial beam tests were performed by recording the down-converted signals with 2 GS/s and 20 GS/s oscilloscopes. The LO was synthesized from the 714 MHz ATF RF reference using a fractional-N PLL, providing synchronization with the ATF RF system and preserving the phase coherence required for accurate digital signal processing and beam position reconstruction.
The complete cBPM and readout chain was installed at ATF and characterized during beam test campaigns in 2025 and 2026, including the evaluation of dynamic range, linearity, and spatial resolution. Different LO synchronization schemes and analog down-conversion architectures were investigated, resulting in IF frequencies ranging from 417 MHz to 60 MHz. The impact of the implemented improvements in synchronization, filtering, and hardware integration on the overall cavity BPM performance is evaluated in terms of dynamic range, linearity, and spatial resolution.