VLBA Station Data Path

This sub-section describes the instrumentation that collects and amplifies radio-frequency (RF) radiation from a source, converts, and transmits it to the station control building.  Napier et al. (1994) includes further information on most of the following aspects.

The antenna brings the RF signals to a focus at one of ten feeds.  The main reflector is a 25-m diameter shaped figure of revolution with a focal-length-to-diameter ratio of 0.354.   A 3.5-m diameter Cassegrain subreflector with a shaped asymmetric figure is used at all frequencies above 1 GHz, while the prime focus is used at lower frequencies.  The antenna features a wheel-and-track mount, with an advanced-design reflector support structure.  Antenna motions, designed to facilitate rapid source changes, are at 30° per minute in elevation and 90° per minute in azimuth.

The feed couples free-space electromagnetic waves into waveguides for transmission to the receiver system. Feeds at observing bands above 1 GHz are located on a ring at the offset Cassegrain focus, and are selected by rotation of the subreflector with a maximum transition time of about 20 seconds. A dichroic system enables simultaneous 2.3/8.4 GHz observations. The 330 and 610 MHz feeds are crossed dipoles mounted on the subreflector near prime focus; simultaneous 330/610 MHz observations are possible.

The polarizer extracts orthogonal circularly-polarized signals, which are routed separately to dual receiver channels. For receivers above 1 GHz, the polarizer is cooled to cryogenic temperatures.

The receiver amplifies the signal. Most VLBA receivers are HFETs (Heterostructure Field Effect Transistors) at a physical temperature of 15 K, but the 90 cm and 50 cm receivers are GaAsFETs (Gallium Arsenide FETs) at room temperature. Each receiver has 2 channels, in opposite circular polarizations. The 1 cm, 7 mm, and 3 mm receivers also perform an initial frequency down conversion.

The IF converter mixes the receiver output signals with the first LO generated by a front end synthesizer. Two signals between 512 and 1024 MHz are output by each IF converter, one for each sense of circular polarization. The same LO signal is used for mixing with both polarizations in most cases. However, the 4 cm IF converter has a special mode that allows both output signals to be connected to the RCP output of the receiver and to use separate LO signals, thereby allowing the use of spanned bandwidths exceeding 512 MHz. Also, the 90 cm and 50 cm signals are combined and transmitted on the same IFs. The 50 cm signals are not frequency converted, while the 90 cm signals are upconverted to 827 MHz before output.

Four IF cables, designated A, B, C, and D, carry the IF signals from the antenna vertex room to the station control building. Normally only two cables are in use at a time, with the signals from each IF converter transmitted via A and C, or B and D; by convention, RCP is normally carried by IFs A and B, and LCP by C and D. However, switching is available to support other configurations needed for special cases. These include dual-band dual-polarization modes that use all four IFs, and in particular, the capability of the new C-band receiver for dual-polarization observations at two frequencies anywhere within its 4-8 GHz range.

Essential auxiliary instrumentation, required to make simultaneous observations feasible at VLBA stations separated by, typically, thousands of kilometers, is described in this sub-section.

A hydrogen maser provides an ultra-stable frequency reference at each VLBA station.  Its standard signals, at 100 MHz and 5 MHz, and multiplied versions thereof, are used throughout the station electronics, both in the antenna and in the station building.

The front end synthesizer generates the reference signals used to convert the receiver output from RF to IF. The lock points are at (n× 500) ± 100 MHz, where n is an integer. The synthesizer output frequency is between 2.1 and 15.9 GHz. There are 3 such synthesizers, each of which is locked to the maser. One synthesizer is used for most frequency bands, but two are used at 1 cm, at 7 mm, 3 mm, and for the 4 cm wideband mode.

VLBA stations support several different types of calibration measurements.

Two calibration signals are injected near the beginning of the primary data path, detected elsewhere in the VLBA system, and applied subsequently:

The switched-noise system injects well calibrated, broadband noise, switched at 80 Hz in a 50% duty-cycle square wave.  This noise signal is synchronously detected in the RDBE, to provide a time-tagged system-temperature table that is delivered with the primary fringe visibility data.  Application of these measurements for amplitude calibration is discussed separately.

The pulse-cal system injects a series of pulses at intervals of 1.0 or 0.2 microseconds, to generate monochromatic, phase-stable tones at frequency intervals of 1 MHz or 5 MHz.  The tones are detected currently in the VLBA DiFX correlator, and eventually will be available from the RDBE. Application of these measurements for phase calibration is discussed separately.

There is also a round trip cable calibration scheme that monitors the length of the signal cables, to enable corrections for temperature and pointing induced variations.

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.

The VLBA's data transmission system comprises the recorder units at the stations, playback units at the correlator, and the magnetic disk modules that are shipped between those units.  The new Mark 5C system was developed jointly by NRAO, Haystack Observatory, and Conduant Corporation.  It closely resembles the Mark 5A version used previously by the VLBA, and the Mark 5B used at some other observatories.  In particular, identical disk modules are used.  However, Mark 5C is functionally more straightforward than its predecessors.   It simply records the payload of each 10G Ethernet packet received from the RDBE without imposing any special recording format.  All formatting of the observed data -- most essentially, the precision time tags -- is internal to the packet payloads, which are transmitted directly from recorder to playback by the Mark 5C system.  Initially, Mark 5B format is being used internally, for compatibility with some existing correlators.  An eventual transition is planned to the VLBI Data Interchange Format (VDIF).

Each Mark 5C unit accommodates two removable modules, each in turn comprising eight commercial disk drives.  As used on the VLBA, these modules are recorded sequentially at a maximum rate of 2 Gbps, matching the current maximum RDBE output rate.  Modules of 16-TB capacity, intended to suffice for recording a majority of VLBA observations at the 2-Gbps data rate, were procured with funding awarded through NSF's MRI-R² program.  Unfortunately, commissioning tests of the 2-Gbps capability encountered unacceptably high failure rates in these modules, which limit the throughput currently achievable in wideband observations.

Further information on the Mark 5C system is available in the Sensitivity Upgrade memo series, and in the Haystack Mark 5 series.