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. To preclude confusion in the following descriptions, please refer to these definitions of essential VLBA terminology:
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. As described in the section on Receivers, most receivers provide two IFs, in opposite circular polarizations. However, four IFs are available in certain specialized observing modes: 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 that generates the 1024-MHz sample clock. RDBEs accept two 512-1024 MHz IF inputs, and deliver packetized output via a 10G Ethernet interface. For observing semester 2013B, each VLBA station will be equipped with two RDBE units. When operating in dual-RDBE mode, their outputs will be sequenced by a software-based Ethernet switch before transmission to the Mark 5C recording system
Two separate "observing systems" are currently available within the VLBA's RDBE. Some suggestions for choosing between these two options, where both are possible, follow the functional outlines below.
PFB: The RDBE's initial observing system, in regular use for scientific observations since 2012 February 19, implements a polyphase filterbank (PFB) digital signal-processing algorithm. It produces sixteen fixed-bandwidth 32-MHz channels within a single RDBE unit, which 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 channels spaced every 64 MHz in each polarization; and (c) a single-polarization configuration of 16 channels, contiguous across the entire width of one IF. The selected channels are requantized at two bits per Nyquist 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: A newer observing system, available at a wide range of bandwidths in observing semester 2013B, implements a digital downconverter (DDC) algorithm in the RDBE's FPGA. A total of 1, 2, or 4 channels are supported within a single RDBE unit; 8 channels are available using both RDBEs. Available bandwidths range downward from 128 MHz to 1 MHz by factors of two; however, 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 transmitted to the recording system, at a total data rates ranging from 4 Mbps to 2 Gbps. The DDC also incorporates an advanced switched-noise detection methodology.
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. The primary instrumental differences are in the numbers and bandwidths of channels. The DDC's wide-bandwidth channels will be more subject to bandwidth smearing in wide-field observations, while the smaller number of channels it requires may be preferable for simplicity of data analysis.
Spectroscopic and other narrow-band observations will generally be best supported by the DDC system, which incorporates scientifically compatible counterparts for all modes of the VLBA legacy system. 16-channel legacy modes require using channels in both sidebands of the analog BBCs, and thus can be replaced with 8-channel equivalents at double the channel bandwidth. And while the DDC cannot currently match the narrowest legacy channel bandwidths due to formatting restrictions, nevertheless much narrower effective bandwidths can be achieved by exploiting the DiFX correlator's high spectral resolution and spectral zoom capabilities.
Observations using all four of the IFs available from any of the 4-IF capabilities require the dual-RDBE capability of the DDC.
