Vector modulator card for MTCA-based LLRF control system for linear accelerators

Modern Low Level Radio Frequency (LLRF) control systems of linear accelerators are designed to achieve extremely precise field amplitude and phase regulation inside superconducting cavities. One of the crucial components of the feedback loop is a vector modulator used to drive the high power RF chain supplying accelerating cavities. The LLRF control systems for the Free Electron Laser in Hamburg (FLASH) and European X-ray Free Electron Laser (XFEL) are based on emerging Micro-Telecommunications Computing Architecture (MTCA, μTCA) platform offering numerous advantages for high performance control systems. This paper describes the concept, design and performance evaluation of world's first Vector Modulator (uVM) module dedicated for LLRF systems compatible with MTCA specification. The module has been designed as a double-width, mid-size AMC form factor Rear Transition Module (RTM) according to developed by PCI Industrial Computer Manufacturers Group (PICMG), MTCA.4 specification. The uVM board incorporates digital, analog and diagnostic subsystems. The digital part is based on Xilinx Spartan 6 family FPGA, with several fast gigalink connections to control module. The uVM module is equipped with an Intelligent Platform Management Interface (IPMI) circuit required by MTCA.4 standard. The FPGA controls the analog part, which includes fast, high-precision DACs, IQ modulator chips, programmable attenuators, power amplifier and fast RF gates for external interlock system. The RF chain can be adopted to different carrier frequencies covering frequency range from 50 MHz to 6 GHz. The design has been carefully optimized for high linearity and low output signal phase noise. The diagnostic system of RF chain allows to monitor input and output power levels and detect failure in RF part. Low noise and high performance clocking system makes the uVM an universal device for applications exceeding the LLRF control system. Extensive tests of the board were performed and measurement results are presented and discussed in this paper.