Phase Drift Compensating RF Link for Femtosecond Synchronization of E-XFEL

Modern high-energy particle accelerators and free-electron lasers incorporate large quantities of sensitive RF and microwave devices distributed over several kilometers. Such devices require extremely stable phase and time synchronization by means of high-frequency signals distributed along the accelerator facility. Coaxial cables are frequently used to distribute the reference signal over the large machine to synchronize electronic systems. These cables are the main source of undesirable phase drifts in the synchronization system. Signal phase drifts in cables are mainly caused by temperature and humidity variations and their values usually exceed required phase synchronization accuracy by more than an order of magnitude. Air pressure can also influence on the signal phase drifts but with a much smaller impact than from temperature and humidity changes. There are several approaches to reduce signal phase drifts in coaxial cables. This article describes the realization of an active phase stabilization system based on the interference phenomenon. A phase-locked signal from the transmitter is reflected at the end of a coaxial cable link. Directional couplers placed along the cable pick up the forward and reflected signals and interfere them to cancel out the cable phase drifts. Distributed hardware, including interferometer controller/transmitter and receiver modules, was built to demonstrate the system concept and performance. FPGA I/O boards with an Ethernet interface are used to control the system operation. Specialized firmware and software were developed to calibrate and control the system. This article describes the concept of the interferometer link, the designed hardware, basic control algorithms, and performance evaluation results. A prototype of the link was built to distribute a 1.3-GHz signal through a coaxial cable. Measured values of the phase drift suppression factor exceeded the level of 100.