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 the 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 the emerging Micro-Telecommunications Computing Architecture (MTCA.4) platform offering numerous advantages for high-performance control systems. This paper describes the concept, design, and performance of the first vector modulator (uVM) module dedicated for RF controls compliant with the MTCA.4 specification developed by PCI Industrial Computer Manufacturers Group (PICMG). The uVM module has been built as a double-width, mid-size rear transition module (RTM) that is accessed by an advanced mezzanine card (AMC). The uVM module incorporates digital, analog, and diagnostic subsystems. The digital part is based on Xilinx Spartan 6 family field-programmable gate array (FPGA), with several fast gigalink connections to a control module. The uVM module is equipped with an intelligent platform management interface (IPMI) circuit required by the MTCA.4 standard. The FPGA controls the analog part, which includes fast, high-precision digital-to-analog converters (DACs), In-phase and quadrature modulator chips, programmable attenuators, power amplifiers, and fast RF gates for an external interlock system. The RF chain can be adopted to different carrier frequencies covering a 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 the RF chain allows monitoring of input and output power levels and failure detection in the RF circuits. A low-noise and high-performance clocking system makes the uVM a universal device, extending the scope of applications beyond the LLRF control systems. Extensive tests of the board were performed, and the measurement results are presented and discussed in this paper.