Performance evaluation of 8-channel ADC ATCA card for direct sampling of 1.3 GHz

Nowadays 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. Increasing requirements for field detection precision at most advanced machines reveal limitations of classical LLRF system receivers. Recently developed technology made it possible to design data acquisition cards allowing for direct sampling of cavity field without a need for down-converters. This paper describes the design and measurements of an eight-channel ATCA card developed for the evaluation of direct sampling techniques for 1.3 GHz signals at the FLASH and European XFEL accelerators. Two versions of the board were tested, each holding a different set of analog-to-digital converters. One was equipped with 400 MSPS, 14-bit ADCs with an analog bandwidth of 1.4 GHz while the other held 500 MSPS, 12-bit ADC's with a bandwidth of 2.3 GHz. The boards were tested in the laboratory and with “accelerator-like” signals and revealed very good results. The paper shows results of the measured sampling parameters, as well as results of different non-IQ sampling schemes with various bandwidths and “reaction times” 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 ILC accelerators.