VLA Samplers

The VLA is now equipped with two different types of samplers. Which set you should use depends primarily on your science goals.

A) 8-bit Set

This set consists of four 8-bit samplers running at 2.048 GSamp/sec. The four samplers are arranged in two pairs, each pair providing 1024 MHz bandwidth in both polarizations. The two pairs are denoted A0/C0 and B0/D0. Taken together, the four samplers offer a maximum of 2048 MHz coverage with full polarization. The frequency spans sampled by the two pairs need not be adjacent, but some restrictions apply, depending on band. Tuning restrictions for these two pairs are described in the Frequency Bands and Tunability section of this document.

B) 3-bit Set

This set consists of eight 3-bit samplers running at 4.096 GSamp/sec. The eight samplers are arranged as four pairs, each pair providing 2048 MHz bandwidth in both polarizations. Two of these pairs, denoted A1/C1 and A2/C2 cannot span more than 5000 MHz (lower edge of one to the higher edge of the other). The same limitation applies to the second pair, denoted B1/D1 and B2/D2. A more complete description of the tuning restrictions is given in the VLA Frequency Bands and Tunability section of this document.

C) Major Characteristics of each Set

The 8-bit samplers have been in use for many years, and their characteristics are well understood. Use of this sampler set is warranted for low-band, L-band, and S-band observations, where the full analog bandwidth provided by the receivers is less than or equal to the 2048 MHz span covered by the samplers.

The 3-bit samplers, at the time of writing, are still being implemented on the array. Their characteristics are currently being explored, so users should refer to other sections of this document for updated and current instructions for optimum use. Major issues users need to be aware of include:

The sensitivity of the array is reduced by 10 to 15% with the 3-bit system, compared to the ideal analog system. For comparison, note that the loss is less than 3% for the 8-bit system.
Each of the eight 3-bit samplers has a resonance of approximate width 3 MHz. Each sampler's resonance is independent of, and different from, all others, so there is no correlated signal between antennas. The resonance, in effect, greatly reduces the SNR within the narrow frequency span of that resonance. This will degrade bandpass solutions within the narrow resonance window, and images made at frequencies within any one of the resonances will show significant loss in sensitivity. The resonance effect is easily seen in autocorrelation spectra, and it is recommended that users utilize these to inform themselves of the compromised frequencies. As not all samplers are (as of this writing) outfitted on the array, we can not yet provide a table of the compromised frequencies.
The SY (switched power) values utilized to correct for system gain variations are sensitive to the total power from each antenna. Application of the switched power values will bias the resulting visibilities by a value of 5 to 10%. This origin of this effect is well understood, but we have not yet determined how best to compensate for it. Because of this, we do not recommend use of the SY table data for data taken with the 3-bit samplers.

D) Setting up the 3-bit or 8-bit Samplers

Setting up either set of samplers requires a short initial observation for each individual LO (frequency) tuning. For the 8-bit system, a scan of 1 minute duration is sufficient for each tuning. The pointing direction of the antennas is not critical.

For the 3-bit system, the requirements are more demanding. The recommended duration is two minutes per frequency tuning. In addition, we recommend that the antennas be pointed at an elevation near to 30 degrees. A slew time of up to three minutes will likely be required to position the antennas at this elevation.

E) Which set to use?

Observations at S, L, and low bands should use the 8-bit sampler set in all cases. There is no benefit from use of the 3-bit samplers at these bands.

Continuum-application observations at Ku, K, Ka, and Q bands, where the best sensitivity is critical, should use the 3-bit sampler sets. Similarly, wide-band spectral line searches requiring more than the 2 GHz span of the 8-bit set should use the 3-bit sampler sets.

Spectral-line application observations whose frequency span lies within the two 1024 MHz-wide spans offered by the 8-bit set should use the 8-bit set.

It is not known at this time if full-band continuum observations at C and X bands, where the maximum bandwidth is 4 GHz, should use the 8-bit or 3-bit sets. In principle, the 3-bit system should offer ~40% better continuum sensitivity, but this will be eroded by:

the reduction of 10 to 15% noted above,
the increased setup time needed for 3-bit observations,
the extra RFI present at these bands which can be avoided by tuning the 8-bit system. This is especially notable at C-band.

F) Other issues to be aware of.

The 8-bit system provides much better protection against strong RFI. However, we do not yet know if the 3-bit system will be
significantly compromised by RFI at any band where its use is warranted. This is most likely to be an issue at C-band, where
strong RFI from broadcast satellites and local microwave links are known to be present. Our current understanding of the RFI spectrum at higher frequency bands suggests there will be no problems with the 3-bit system, except possibly when pointing near geostationary satellites.

Polarization testing conducted so far indicates no degradation of performance by using the 3-bit samplers.