The RDBE replaces much of the VLBA's original analog signal processing. The baseband converters, in particular, are eliminated by sampling the station's received signals from the 512-1024 MHz IFs, with 8-bit precision. All subsequent processing is performed digitally.
`RDBE' is an acronym for ``ROACH Digital Backend''. ROACH, in turn, refers to the FPGA-based central signal processing board (``Reconfigurable Open Architecture Computing Hardware'') that was developed in a collaboration among NRAO, the South African KAT project, and the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) at UC Berkeley. In addition to the ROACH, the RDBE includes an input analog level control module, a sampler developed by CASPER, and a synthesizer board that generates the 1024-MHz sample clock. Each RDBE accepts two 512-1024 MHz inputs, and delivers packetized output to the Mark 5C recording system (Section 6.10) via a 10G Ethernet interface.
The RDBE's initial FPGA personality provides sixteen fixed-bandwidth 32-MHz sub-bands, which can be selected flexibly between IFs (typically equivalent to polarizations), and at 32-MHz steps along the entire IF frequency range. Some typical selection modes include (a) a compact dual-polarization configuration of eight contiguous 32-MHz sub-bands in each polarization; (b) a spanned-band dual-polarization configuration, with eight 32-MHz sub-bands spaced every 64 MHz in each polarization; and (c) a single-polarization configuration of 16 sub-bands, contiguous across the entire width of one IF. The selected sub-bands are requantized at two bits per sample and transmitted to the recording system, at a total data rate of 2.048 Gbps (referred to subsequently as ``2 Gbps'').
An important auxiliary function, detection of the switched broadband noise calibration signal (Sections 6.3 and 12), is now supported by the most recent firmware version. This version was incorporated into the ongoing commissioning tests in October 2011, and has been performing correctly and reliably.
A separate FPGA personality, supporting much greater tuning flexibility and non-identical sub-band bandwidths, is under development. In combination with the DiFX correlator's spectral-zoom mode (Section 7.1), bandwidths as narrow as 62.5 kHz, and probably narrower, will be available for spectroscopic and other observations requiring high spectral resolution. Specifications for these capabilities will be added to this document when they become available.
Two RDBE units are required at each VLBA station to provide adequate FPGA processing capacity for all anticipated applications, and to support VLBA observing modes involving four IF signals. The first set of RDBEs is currently operational across the VLBA; fabrication of the second is nearing completion.
Further information on the RDBE is available in the Sensitivity Upgrade memo series, at http://www.vlba.nrao.edu/memos/sensi/ .