The RDBE is a primary component of the VLBA Sensitivity Upgrade, introduced in this document to support proposals for its use in semester 2011-B, with observations beginning in mid-2011. It replaces much of the VLBA's original analog signal processing, principally the analog baseband converters. Instead, the RDBE samples the station's received signals directly from the 512-1024 MHz IF bands, with all subsequent processing 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 Center for Astronomy Signal Processing and Electronic 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.
Initial FPGA firmware for the ROACH supports a polyphase filterbank (PFB) personality that forms thirty-two 32-MHz sub-bands, sixteen per IF input. Any subset of 16 of these sub-bands, quantized at 2 bits, will be selectable for output to the Mark 5C recorder at a 2 Gbps data rate. This FPGA personality was developed mainly by Haystack Observatory, with some assistance by NRAO at the end of the development process. Important auxiliary functions including detection of the switched broadband noise (Section 6.3) and pulsed (Section 13.2) calibration signals will be supported within the PFB personality.
A separate digital downconverter (DDC) personality, under development by NRAO, will provide multiple, independently fine-tuned sub-bands. In combination with the DiFX correlator's spectral-zoom mode (Section 7.1) the DDC will support spectroscopic and other observations with bandwidths as narrow as 62.5 kHz, and probably narrower. The VLBA's original analog signal-processing subsystems remain available to support such observations as previously.
Two RDBE units are required at each VLBA station to provide adequate FPGA processing capacity for all anticipated applications, and to VLBA observing modes involving four IF signals. The first set or RDBEs is currently installed across the VLBA; fabrication of the second is under way.
Further information on the RDBE is available in the Sensitivity Upgrade memo series, at http://www.vlba.nrao.edu/memos/sensi/ .