Station Signal Processing

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 observing wavelengths shorter than 30 cm, while the prime focus is used at longer wavelengths. The antenna features a wheel-and-track mount, with an advanced-design reflector support structure. Antenna motions are relatively rapid for an antenna of this size, to facilitate rapid source changes: 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 wavelengths shorter than 30 cm 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 frequency-selective dichroic system enables simultaneous 13/4-cm observations. The 90- and 50-cm feeds are crossed dipoles mounted on the subreflector near prime focus; simultaneous 90/50-cm observations are possible.

The polarizer extracts orthogonal circularly-polarized signals, which are routed separately to dual receivers. In receiver wavelength bands shorter than 30 cm, the polarizer is cooled to cryogenic temperatures.

The receiver amplifies the signal. Most VLBA receivers use HFET (Heterostructure Field Effect Transistor) amplifiers at a physical temperature of 15 K, but the 90- and 50-cm receivers use GaAsFETs (Gallium Arsenide FETs) at room temperature. All receivers produce dual-polarization outputs, in opposite hands of circular polarization.

The IF converter mixes the receiver output signals with the first LO generated by a front end synthesizer. Two IF bands between 512 and 1024 MHz are output by each converter, one in 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, D, carry the IF signals from the antenna vertex room to the station control building. Normally only two IFs 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, several 4-IF configurations are available for special cases. These include dual-polarization observations at two arbitrary frequencies anywhere within the 4-8 GHz range of the new 6-cm receiver, and combinations of dual IF outputs from both the 13- and 4-cm receivers (using the dichroic system described in the paragraph on feeds above). Either of these configurations can be activated simply by specifying the desired frequencies and polarizations. The 4-cm receiver also supports a 3-IF configuration, with a second first LO available for RCP signals; this mode requires that the SCHED setup include the 'dualx' parameter.

Essential auxiliary instrumentation, required to make simultaneous observations feasible at VLBA stations separated by 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 reference signals to convert the receiver output from RF to IF, with lock points at (n× 500) ± 100 MHz (for n an integer). Output frequencies range 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, and detected elsewhere in the VLBA system, with derived corrections applied in data analysis:

The switched-noise system injects well calibrated, broadband noise, switched on-off in a 50% duty cycle at 80 Hz. 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, spaced at multiples of 1 MHz or 5 MHz. The tones are detected in the VLBA DiFX correlator. 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 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''). Information about delay and phase changes in PFB data between individual observations is available in VLBA Sensitivity Upgrade Memo #48.

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. With the new Mark 6 recording systems, a full eight channels at 128-MH bandwidth can be accommodated. 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 4096 Mbps (subsequently "4 Gbps"). Important information about delay and phase "jumps" near the beginning of DDC observations is available in VLBA Sensitivity Upgrade Memo #47.

Suggestions for Observing System Selection: Wideband science will be possible using either the PFB observing system, at its fixed 2 Gbps data rate, or the DDC system at 4 Gbps 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- to 21-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.  While the DDC mode provides wider bandwidth (4096 Mbps recording) and tuning flexibility, the PFB mode (2048 Mbps recording) provides more accurate amplitude calibration and should be used if <10% flux density accuracy is required.

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, described in the Spectral Resolution (7.2) of the Correlator section of the VLBA OSS.

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: Hopefully no conversions from the legacy schedule system are required at this stage. Should you be considering doing so we suggest looking at the SCHED user manual and examples. If further help is required please don't hesitate to contact the NRAO helpdesk.

A software-based network switch, purchased from XCube Research and Development, is an essential element of the data path at each VLBA station. Its primary functions are to merge the packet streams from each of the two RDBE units into a single stream that is sent to the Mark 6 recorder, and to regulate the timing of these packets so as not to overflow the Mark 6's input buffer.

Other switching and real-time data analysis functions may be added as part of future developments. A phase-cal detection capability is operational and used for diagnostic purposes. Real-time data transmission capabilities are currently being tested on the XCube system.

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.

A transition of the VLBA to the Mark 6 recording/playback system, designed by Haystack Observatory and Conduant Corporation, is now complete at the VLBA stations and the correlator in Socorro. Current documentation on the Mark 6 system is available at Haystack's Mark 6 website.

Although Mark 6 is specified to record data at up to 16 Gbps, inputs available from the current RDBE systems limit its application to 4 Gbps.  There is, thus, substantial headroom to support recording of data from newer, higher-capacity digital signal processing units that may be developed to replace the RDBE.

The new 4-Gbps limit makes available additional observational modes within the limits of the RDBE.  The VLBA's current media can support a relatively high duty cycle at 4 Gbps.  The fraction of observing time that is expected to be available for VLBA observations is specified in each Call for Proposals for upcoming cycles.