Roach Digital Backend (RDBE)
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 directly from the IF outputs of each station's receivers, with 8-bit precision. All subsequent processing is performed digitally. For clarity in the following descriptions, two items of essential VLBA terminology are defined here:
An "IF" refers to one of a maximum of four 512-MHz wide intermediate-frequency analog signals transmitted from the receiver(s) to the RDBE. Most receivers provide two IFs, in opposite circular polarizations. However, four IFs are available to support specialized observing modes at some wavelengths: two dual-polarization pairs, at arbitrary frequencies within the full range of the new 6-cm receiver; or from different receivers in 13/4-cm or 90/50-cm dual-receiver operation.
A "channel" refers to a single contiguous frequency range (of any bandwidth), observed in a single polarization, that is sampled, filtered, and recorded as a separate entity. This approach is essential for the VLBA, where capabilities are fundamentally limited by the overall data-transmission bandwidth.
'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 which generates the 1024-MHz sample clock. RDBEs accept two 512-1024 MHz IF inputs, and deliver packetized output via a 10G Ethernet interface. Each VLBA station is equipped with two RDBE units.
Currently, two separate "observing systems" are available within the VLBA's RDBE. Inputs to either data system can come from any of the four VLBA IFs. Some suggestions for choosing between the observing systems follow the functional outlines below.
PFB: The polyphase filterbank digital signal-processing algorithm produces sixteen fixed-bandwidth 32-MHz channels within a single RDBE unit. Channels can be selected flexibly between two input IFs, and can be placed 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 channels at matching frequencies in each polarization; [b] a spanned-band dual-polarization configuration, with eight 32-MHz channel pairs spaced every 64 MHz; and [c] a single-polarization configuration of 16 channels, contiguous across the entire width of one IF. (In case [c], one end channel will not lie within the IF band, and does not produce usable data.) The selected channels are requantized at two bits per Nyquist sample and output in a packetized stream at a total data rate of 2048 Mbps (referred to subsequently as "2 Gbps'').
DDC: The digital downconverter algorithm supports a wide range of bandwidths. A total of 1, 2, or 4 channels can be processed within a single RDBE unit; 4 or 8 channels are available using both RDBEs. Available bandwidths range downward from 128 MHz to 1 MHz in binary steps; recording rate limitations restrict the 128-MHz bandwidth to a maximum of 4 channels. All channels must use the same bandwidth within an observing scan. Channels can be selected flexibly among up to four input IFs, and in either sideband. Tuning of individual channels can be set in steps of 15.625 kHz, although 250-kHz steps are recommended when compatibility with legacy systems is required. Channels may not cross IF zone boundaries at 640 and 896 MHz. Each channel is requantized at two bits per Nyquist sample and output in a packetized stream, at a total data rate ranging from 4 to 2048 Mbps (subsequently "2 Gbps").
Suggestions for Observing System Selection: Wideband science will be possible using either the PFB observing system, at its fixed 2048 Mbps data rate, or the DDC system at 2048 Mbps or lower rates. Both systems provide output at two bits per Nyquist sample. The primary instrumental differences are in the numbers and bandwidths of channels, and in the channel passbands. The PFB's many narrower channels may be advantageous in avoiding spectral ranges impacted by interference, particularly in the 18-cm band. On the other hand, the smaller number of wider-band channels available in the DDC may simplify data analysis in some cases. Digital logic capacity of the RDBE limits the PFB's signal processing to fewer filter taps for each of its 16 channels than for the 4-channel DDC system, so that the DDC's passbands cut off significantly more sharply.
Spectroscopic and other narrow-band observations will generally be best supported by the DDC system, which incorporates scientifically equivalent counterparts for all modes of the VLBA legacy system, and extends these to wider bandwidths. Even extremely narrow bands can be accommodated by observing at 1 MHz bandwidth and selecting a narrower range using the DiFX correlator's spectral zoom mode.
Most VLBA receivers produce only two IFs, in opposite polarizations, but some receivers support four-IF modes, such as dual-polarization dual-frequency. The four-IF capability of the DDC allows these modes to be exploited.
Conversion of Legacy Schedules to RDBE/DDC: A separate web page describes the relatively straightforward conversion of SCHED “keyin” files applicable to the VLBA's legacy data system, to use the DDC system instead. Explanations of the "scientifically equivalent" modes referred to above are included. It is designed primarily for users with some VLBA experience.
