VLA Samplers

The VLA is equipped with two different types of samplers, 8-bit, 1GHz bandwidth, and 3-bit, 2GHz bandwidth. The choice depends on your science goals, and on technicalities described below.

A) The 8-bit 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. Some restrictions apply, depending on band, as described in the section on Frequency Bands and Tunability

B) The 3-bit 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. The tuning restrictions are described in the section on Frequency Bands and Tunability.

C) Which set to use?

S, L, and 4/P-band observations, whether line or continuum, should use the 8-bit sampler set.

C and X-band receivers deliver upto 4 GHz bandwidth, so potentially the 3-bit system has ~40% better continuum sensitivity compared to 8-bit. In fact, the following trade-offs need to be considered: (see EVLA memo 166 ).

Reduction in sensitivity of about 15%, described further below.
Increased setup and overhead time needed for 3-bit.
Interference, especially at C-band, which can be better tuned out by the 8-bit system. (RFI page here)

Ku, K, Ka, and Q band continuum observations should use the 3-bit samplers for maximum bandwidth.

Wide-band spectral line searches requiring more than 2 GHz span should use the 3-bit samplers.

Spectral-line observations which fit within two (possibly disjoint) 1 GHz bands should use the 8-bit set. Tuning multiple sub-bands to match a desired set of lines is done in the General Observing Setup Tool, GOST. (requires java 7)

D) Major Characteristics of each Set
The 8-bit samplers have been in use for many years, and are well understood. This sampler set is warranted for 4/P-band, L-band, and S-band, for observations. The full analog bandwidth from the receivers fits within the 2048 MHz span covered by the samplers.

The 3-bit samplers are relatively new, and users need to be aware of the following issues:

Sensitivity: compared to the 8-bit system, the sensitivity of the 3-bit samplers is worse by ~15% (at equal bandwidth). Alternatively, a given continuum noise level, requiring on-source integration time T with the 8-bit (two bands of 1GHz), requires 0.33T with the 3-bit (4 bands of 2GHz, assuming the bandwidth is available from the front end.)
Resonances: each of the eight 3-bit samplers on an antenna has a resonance about 3 MHz wide. Each resonance is independent of all others, so there is no correlated signal between antennas. The resonance degrades the spectrum in its narrow frequency range, but has little effect on continuum observing. Bandpass solutions will be affected, but can be interpolated over. Spectral-line calibration and images at the affected frequencies will show significant loss in sensitivity. The resonances are easily seen in autocorrelation spectra, and it is recommended that users, especially spectral-line users, utilize these to locate the compromised frequencies.
Amplitude Calibration: The traditional method for both 8- and 3-bit systems is to observe a flux-density calibrator, use self-cal to determine the antenna amplitude calibration factors (gains), and transfer the gains to the phase calibrator and target. For 3-bit samplers this procedure gives results good to 5%, between elevations of 20-70degrees. (Expect worse at the upper edge of Q-band and/or during bad weather). The SY (switched power) data can be used to correct for system gain variations and works well for the 8-bit samplers. For 3-bit samplers, the Pdif depends on the Psum, i.e. Pdif is non-linear, and its application will bias the resulting visibilities by 5 to 10%. The origin of this effect is understood, but we have not yet determined how best to compensate for it. Because of this, we do not recommend use of the 'Psum' and 'Pdif' data in the SY table, to calibrate visibilities from the 3-bit samplers. We do, however, recommend that the 'Requantizer Gains' in the SY table be applied to remove gain changes.

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

Either set requires an initial scan for each individual LO (frequency) tuning, during which power levels are optimized.

For the 8-bit system, a 'dummy scan' of 1 minute duration is sufficient for each tuning. This is usually done while the antennas are slewing at the start of an observing file, as the pointing direction of the antennas is not critical.

For the 3-bit system, the requirements are more demanding, see the example observing script. The minimum setup time is 3 minutes for each tuning, to adjust the power levels and bandpass slopes across the 2GHz samplers. These values are retained and applied if the tuning is re-encountered in the same observation. In addition, every time the LO setup is changed, whether or not it is new (e.g. changing from 8-bit X-band reference pointing back to target), a scan of 90 seconds is needed to re-set the subband gains (requantizers) in the correlator. For better amplitude calibration at high frequencies, the 3-bit initial setup should be near the elevation of the target, so do it after the first 8-bit setup described above. For 3-bit observing without 8-bit (e.g. C or X-band, without reference pointing), the power variation with elevation is small so the 3-bit setup can be done at any elevation.

F) Other issues.
The 8-bit system provides much better immunity in the presence of strong RFI, which may significantly impact the 3-bit system. This is most likely to be an issue at C-band, where strong RFI exists from broadcast satellites and local microwave links. Our current understanding of the RFI spectrum at higher frequencies suggests there will be few problems with the 3-bit system, except possibly when pointing near geostationary satellites. See the example observing script for tips and tricks.

The overhead for setup of 3-bit samplers can eat into observing time, especially for projects with many different LO settings, and/or sources all over the sky, accompanied by a band change, reference pointing, and requantizer reset for each direction. The impact is most severe for short scheduling blocks.

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