Performance Evaluation of 8-Channel Card for Direct Sampling of 1.3 GHz Signals

Nowadays Low-Level Radio Frequency (LLRF) control systems for linear accelerators incorporate complex and expensive high-precision field detection receivers with multichannel down-converters and low noise LO generation systems. Recently developed technologies have made it possible to design data acquisition cards allowing for the direct sampling of cavity field without need for down-converters. This paper describes the design and measurements of an eight-channel Advanced Telecommunications Computing Architecture card developed for the evaluation of the use of direct sampling techniques when digitizing 1.3 GHz signals at the FLASH and European XFEL accelerators. Two versions of the board were tested, each holding a different set of state-of-the-art high speed analog-to-digital converters. The paper shows results of the measured sampling parameters like the Signal-to-Noise Ratio and Effective Number Of Bits, as well as results of different non-IQ sampling schemes with various bandwidths and latencies for acquiring the amplitude and phase of the analyzed signals and determining the precision of the analysis. Drift measurements for determining the long-term stability are also presented. Achieved results satisfy precision requirements for machines like The European XFEL main LINAC and the ILC.