Frequency Bands and Observing Systems
Frequency Bands
The VLBA antennas are equipped with 8 cryogenically cooled receivers and one UHF receiver. Together, they provide frequency coverage from 312 MHz to 90 GHz. The receiver bands and the System Equivalent Flux Density (SEFD) at a specified center frequency can be found in the VLBA Observational Status Summary (OSS).
The SEFD can be thought of as a sensitivity rating, where a lower SEFD means better sensitivity. Note that the lowest SEFD (best sensitivity) is in the 6 cm (C-band) receiver. The further the observing wavelength is from 6 cm, the worse the sensitivity gets. Consult the OSS for additional details about the receivers and their performance.
The VLBA also has a special 13/4-cm dichroic system that can observe at both 13 cm (S-band) and 4 cm (X-band) simultaneously. This mode is particularly useful for determining the impact of the ionosphere on observations, which can cause both Faraday rotation and dispersive delays in the data. If your science requires extremely well-calibrated phases at low frequencies or if you need the best possible precision in your polarimetry, you may want to supplement your observations with some 13/4-cm dichroic system observations on a bright calibrator to fully characterize the ionosphere.
Observing Systems
A Note on Terminology: The VLBA Observational Status Summary (OSS) now uses the term “data channel” to describe a subband with a single polarization. However, the Proposal Submission Tool (PST) still uses the term "baseband channels" and SCHED still uses the term “baseband converter” or “BBC”. To be consistent with the VLBA OSS, this document will use the newer terminology of “data channel”. Please keep in mind that “data channel” and “spectral channel” are NOT the same; each data channel is further divided into spectral channels.
The VLBA currently has two separate observing systems: The polyphase filterbank (PFB) and the digital downconverter (DDC).
PFB: The polyphase filterbank digital processing algorithm produces 16 fixed-bandwidth 32 MHz data channels within a single ROACH digital backend (RDBE). The total data rate is 2048 Mbps (2 Gbps).
DDC: the digital downconverter algorithm supports a wider range of bandwidths than the PFB system, with data channels as narrow as 1 MHz and as wide as 128 MHz. However, only 1, 2, 4, or 8 data channels are allowed. The total data rate can be as low as 4 Mbps or as high as 4096 Mbps (4 Gbps).
Choosing an observing system: The PFB has more data channels than the DDC, so it may be preferable when attempting to avoid specific narrow-band RFI. The ability to produce very narrow data channels with DDC make it better suited to spectroscopic observations. Users who require maximum sensitivity for continuum observations will also likely prefer the 4096 Mbps capability of the DDC system. Also, please note that while the DDC mode provides wider bandwidth and tuning flexibility, the PFB mode provides more accurate amplitude calibration and should be used if <10% flux density accuracy is required. You should carefully consider which system is the best match for your science goals.
Additional information on both observing systems can be found in the OSS in the Signal Processing chapter.
