R2DBE: A Wideband Digital Backend for the Event Horizon Telescope

The Event Horizon Telescope (EHT) is an earth-size aperture synthesis radio astronomy array capable of making high-resolution measurements of submillimeter emission near the event horizon of supermassive black holes. The EHT uses existing standalone submillimeter radio telescopes which are retrofitted to serve as VLBI stations. Current instrument development goals include increasing the number of stations in the array and increasing their sensitivity. We have developed a 4 GHz bandwidth digital backend (DBE) unit, based on the CASPER (Collaboration for Astronomy Signal Processing and Electronics Research) open source ROACH2 (Reconfigurable Open Architecture Computing Hardware) platform. The ROACH2 digital backend, which we call the R2DBE, has dual channels each sampling at a rate of 4096 MSps (megasamples-per-second), a factor of 4 improvement over the previous generation system. Recording 2-bits per sample, the bandwidth is equivalently stated as 16 gigabits-per-second (Gbps). This paper includes system design of the R2DBE, discusses laboratory test results of the system using correlated noise input, and presents field test results. The R2DBE was distributed to seven sites in early 2015, enabling the EHT campaign in 2015 March to collect data with 2 GHz bandwidth in each polarization. The 16 gigabit-per-second (Gbps) R2DBE can be scaled to create a 64 Gbps system using four R2DBEs in parallel. Thus, it enables a clear path to the EHT's goal of 4 GHz dual-polarization and dual-sideband across the array.