Roach Digital Backend (RDBE)

by Gustaaf Van Moorsel last modified Jul 08, 2012

The RDBE replaces much of the VLBA's original analog signal processing in the station control building.  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 via a 10G Ethernet interface.

Two separate FPGA "personalities" are currently available within the VLBA's RDBE:

PFB:  The RDBE's initial personality, in regular use for scientific observations since 2012 February 19, implements a polyphase filterbank (PFB) signal-processing algorithm.  It produces sixteen fixed-bandwidth 32-MHz sub-bands, which can be selected flexibly between two input 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 at matching frequencies 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, is also supported by the PFB.

DDC: Newer FPGA firmware, still under development, implements a digital downconverter (DDC) algorithm.  The current version, available for scientific use on a shared-risk basis, supports 1, 2, or 4 sub-bands with equal bandwidths of 64 or 128 MHz.  Sub-bands can be selected flexibly between two input IFs (typically equivalent to polarizations), and in either sideband.  Tuning of individual sub-bands within the input IFs can be set in steps of 15.625 kHz, although 250-kHz steps are recommended when compatibility with legacy systems is required.  Sub-bands may not cross IF zone boundaries at 640 and 896 MHz.  Each sub-band is requantized at two bits per sample and transmitted to the recording system, at a total data rates ranging from 256 Mbps to 2 Gbps.  The DDC also incorporates an advanced switched-noise detection methodology.

Development of the DDC firmware continues, toward its primary goal of supporting narrowband spectroscopic observations.  Current specifications limit the narrowest bandwidth to 1 MHz due to formatting restrictions.  However, with the DiFX correlator's high spectral resolution and spectral zoom capabilities, much narrower effective bandwidths can be achieved.

Two RDBE units are required at each VLBA station to provide adequate signal processing capacity for all anticipated applications, including four-IF and/or 8-sub-band observing modes.  The first set of RDBEs has been in regular use on the VLBA since mid-2011; fabrication of the second set is nearing completion.

Further information on the RDBE is available in the Sensitivity Upgrade memo series.