UArizona ARO Submillimeter Telescope

UArizona ARO Submillimeter Telescope (SMT) - Mt. Graham

Equipment Summary and Status - last updated July 30 2019

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SWRDS Digital Spectrometer

The Wideband Radio Digital Spectrometer (SWRDS) is an FPGA-based spectrometer for the SMT. Compared to the filterbank, it makes both larger bandwidths and much higher resolutions available to observers. Between the two SWRDS units available, a diverse set of configurations is possible. Please carefully read the following explanation with examples of basic configurations. Note that bands referred to as having a width of 2, 4, or 8 GHz have passbands that are actually wider to provide overlap, albeit with an increasingly reduced sensitivity the closer to their actual cutoff.

Main configuration considerations:

• There are two SWRDS units:
  • Each can take one frequency band, i.e., one 1.3mm and the other 0.8mm.
  • or both take the same frequency band, i.e. 1.3mm or 0.8mm
  • Each unit can be configured independently, with different band placements and/or different resolutions, etc. For instance:
    • The resolutions for 1.3mm and 0.8mm can be configured to more closely match in km/s.
    • The two units can be set up to complement one another to provide the widest possible bandwidth at a single frequency band.
    • The two units can be set up for a single frequency band, each with a different bandwidth and resolution.
• Each SWRDS unit can support only one resolution for all its windows (discussed below).
• Each SWRDS will always, with an identical setup, process both polarizations associated with each signal chain in parallel.
• Optionally, a Hanning taper can be applied as part of the digital processing by a SWRDS unit. This reduces the frequency resolution from 1.2 to 2.0 times the channel spacing, but results in less aliasing and channels that are statistically more independent.

SWRDS bands:

Since both polarizations are always processed with the same setup and result in complementary spectra, the subsequent discussion will focus on a single polarization only. Click on the figures for a full-size version.

Basic configuration for SWRDS that places two 4-GHz SWRDS bands within the 8-GHz signal bands (LSB, USB) from a receiver. The placement of the centers of the SWRDS bands within the signal band is set by the value selected for the IF ([6,10] GHz). Each SWRDS band consists of two (overlapping) 2-GHz sections. The 4-GHz SWRDS bands define the frequency ranges where spectral windows can be configured. The text in red illustrates an example configuration for a setup with 230 GHz (source velocity of 0 km/s, i.e., no Doppler correction) centered in the LSB SWRDS band, and with an IF of 9.2 GHz, which centers the SWRDS band in USB at 248.4 GHz).

• The 'logical' bandwidth unit of SWRDS is 4 GHz: SWRDS selects a 4-GHz band within the 8-GHz wide intermediate frequency (IF) signal band coming from the receiver in both lower (LSB) and upper (USB) sidebands. In hardware, this 4-GHz band is actually split into two overlapping 2-GHz bands, as illustrated. The sensitivity in the ~220 MHz overlap region is up to 10% lower than in the main part of the band.
• The 4-GHz SWRDS band can be flexibly placed within the 4–12 GHz IF from the receiver with its center anywhere from 6–10 GHz.
• To access the full 8 GHz at 1.3mm or 0.8mm, both SWRDS units will need to be used for the same frequency band, 1.3mm or 0.8mm, with one configured for the 4–8 GHz and the other for the 8–12 GHz part of the receiver's IF band (see next section). The overlap region between the two SWRDS units does not exhibit a significant level of decreased sensitivity.
• The SWRDS bands in LSB and USB from one unit can not be independently placed: their displacement from the receiver IF's 8-GHz center frequency is equal and opposite. I.e., if the SWRDS band in the LSB is set to the lowest sky frequency range, the SWRDS band in USB will correspond to the highest sky frequencies.
• The SWRDS bands determine the available frequency ranges for spectral processing. The actual frequency range of individual spectra is determined by the selected observing 'window' (see next section).

Observing windows:

Per SWRDS, up to four identical-bandwidth and resolution observing windows can be defined, combined across the LSB and USB SWRDS bands.

'Wideband' 4-GHz configuration with two 2 GHz windows in each SWRDS band.	'Wideband' 8-GHz configuration using both SWRDS units for 1.3mm or 0.8mm.

Example 'multiwindow' configuration with three high-resolution spectral windows in LSB and one in USB.

Windows that extend over the limit of a 2-GHz hardware section will show degraded performance where their passband drops in what would have been an overlap region or is blanked beyond its cutoff.

• For 'wideband' SWRDS observations, two 2-GHz windows are placed in both LSB and USB for 2x4 GHz coverage. For each sideband (and polarization), this will result in two overlapping spectra that can be 'stitched' together, using e.g. CLASS, as one contiguous 4-GHz spectrum for LSB and USB, respectively.
• To access the full 8 GHz at 1.3mm or 0.8mm, both SWRDS units will need to be used at the same frequency band, with one configured for the 4–8 GHz and the other for the 8–12 GHz part of the receiver's IF band.
• In addition, it is possible to use a (steerable) single 2-GHz window in both LSB and USB in case stitching may be a concern, giving 2x2 GHz coverage.
• For a SWRDS in high-resolution 'multiwindow' mode, up to four identical-resolution bands can be placed arbitrarily within the combined LSB and USB ranges (e.g., three can be in LSB and one in USB).
• Warning: a caveat for the placement of spectral windows is the hardware split of each SWRDS band into two 2-GHz bands. A high-resolution window that spans across the center of the SWRDS band will actually get its data from the edge of only one of the 2-GHz hardware bands and thus may show degraded performance where its passband drops in what would have been an overlap region or be blanked beyond its cutoff.

SWRDS bandwidth and resolution modes:

The table below gives bandwidths and velocities in the observatory frame (no Doppler correction).

Wideband 4-GHz and 8-GHz configuration details:

Configuration details for the wideband 4000 and 8000 setups showing the window bandwidth at the 1-dB and 3-dB passband roll-off, as well as the ~220 MHz region with lower sensitivity between the two 2-GHz hardware bands.

UArizona SMT banks (legacy filterbank)

The legacy filterbank presently operates < u> concurrently and in parallel with the new SWRDS spectrometers and is still available to observers.

• The IF center frequency is tunable from 4.5-7.5 GHz. The sideband separating receivers (2SB) output four IFs: horizontal (H) and vertical (V) polarizations for both the lower (LSB) and upper (USB) sideband. Dual sideband receivers (DSB) output two IFs: the H and V polarizations, respectively, for the combined LSB+USB sideband. For processing, the filterbank can select 1, 2, or 4 IFs of the maximum four IFs available by reducing the total bandwidth per IF.

Observing Restrictions

Sun Avoidance: The UArizona SMT requires a 45° Sun avoidance zone.

Observing: None.

Recently Fixed and Current Issues

Filters 1.0MHz: There are periods of instability in the IF down conversion stages that manifests itself in platforming. Problems appear worse in position-switching and on-the-fly observing modes.

Filters 250kHz: The instability noted in the 1.0 MHz filters also appears in these filters. This platforming step is located at the IF center frequency and can be avoided by offsetting the IF.

Position - switching mode: There is a standing wave present when using observing in position-switching mode, especially at the upper end of 1.3mm receiver observing band. This standing wave is not atmospherically induced but instead is related to the receiver/hardware.

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