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VLBI Backend Systems

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The EHT data acquisition campaign in March 2015 will utilize new digital backends on many of the new and existing sites. We will briefly overview the state of the art for wideband digital backends for VLBI and discuss the several flavors that will be employed this coming year. Specifically, we will focus on details of the Roach2 Digital Backend (R2DBE), a new 16 Gbps wideband DBE unit that can be scaled to 64 Gbps, to be employed at single dish sites. The R2DBE digitizes 2.048 GHz of RF in each of two inputs and sends 8 Gbps single-channel data on each of two 10 GbE links to a Mark 6 data recorder with expansion chassis for a total of 16 Gbps. The system utilizes open source hardware, firmware, and software developed through the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER). Engineers at SAO have been members and developers for the CASPER group for several years, thus the challenge of rapidly prototyping a VLBI backend system on a limited budget and short time scale could be met by drawing on years of previous development and expertise. In four months the system was developed and demonstrated in 32 Gbps form for the South Pole Telescope, achieving fringes in a hybrid correlation between an R2DBE and an existing R1DBE. We present the tests and results achieved with the R2DBE 32 Gbps system for the South Pole, and upcoming developments in preparation for March 2015.