The VLA correlator has enough chips to produce 16 complex frequency channels at the nominal ``50 MHz" (46 MHz) bandwidth. Halving the total bandwidth allows the chips to be ``time-shared" so that the number of channels is doubled. This total bandwidth halving and time-sharing can be continued to a maximum of 512 channels, at which point further halvings of the total bandwidth leave the number of channels at a maximum of 512. Table 7 summarizes the resulting channel number and channel width options for a single IF, with and without on-line Hanning smoothing. Hanning smoothing to reduce the Gibbs phenomenon is strongly recommended for all observations that require high spectral dynamic range; it may be applied either on-line or off-line.
| Without Hanning | With Hanning | |||
| Bandwidth | Channels | Separation | Channels | Separation |
| (MHz) | (kHz) | (kHz) | ||
| 0.195313 | 512 | 0.381 | 256 | 0.763 |
| 0.78125 | 512 | 1.526 | 256 | 3.052 |
| 1.5625 | 512 | 3.052 | 256 | 6.104 |
| 3.125 | 256 | 12.207 | 128 | 24.414 |
| 6.25 | 128 | 48.828 | 64 | 97.656 |
| 12.5 | 64 | 195.313 | 32 | 390.625 |
| 25 | 32 | 781.25 | 16 | 1562.5 |
| 46 | 16 | 3125 | 8 | 6250 |
The above total numbers of channels may also be split evenly among two, or four, IFs. For example, if an observation is set up at 12.5 MHz total bandwidth with 2 IFs and no on-line Hanning smoothing, there will be 32 channels in each of the two spectra (i.e., 32 per spectrum for a total of 64 as shown in the table). The splitting among the IF channels is selected on the //DS input ``card" by specifying the correlator ``mode". Note that the first channel (``channel 0") of each IF contains the complex average of the central 75% of the band that the correlator has available (even if not all of that band has been selected).
Table 8 summarizes the VLA spectral line correlator modes.
| Mode | Polarimetry? | Number | |
| Circ | Lin | and ID of IFs | |
| 1A | No | No | 1A |
| 1B | No | No | 1B |
| 1C | No | No | 1C |
| 1D | No | No | 1D |
| 2AB | No | No | 2A,B |
| 2AC | Yes | No | 2A,C |
| 2AD | No | No | 2A,D |
| 2BC | No | No | 2B,C |
| 2BD | Yes | No | 2B,D |
| 2CD | No | No | 2C,D |
| 4 | Yes | No | 4A,B,C,D |
| 4PA | Yes | Yes | 4A,C |
| 4PB | Yes | Yes | 4B,D |
Each mode in which the correlator can provide spectral line data is characterized by a number followed by zero, one or two letters. The number gives the number of spectra produced, the letters describe which IF channels (A,B,C,D) are in use. Recall that A and B are right circularly polarized, C and D are left circularly polarized, and that A and B are at one sky frequency, B and D another.
The single-channel modes are known as 1A, 1B, 1C and 1D. These produce only one spectrum but with the highest spectral resolution at any bandwidth.
The dual-channel modes are 2AB, 2AC, 2AD, 2BC, 2BD and 2CD. None of these will measure linear polarization, but 2AC and 2BD can determine circular polarization.
The four-channel modes are known as 4, 4PA and 4PB. The first provides spectra for all 4 IF channels and can measure circular, but not linear, polarization (no cross-correlations between the circular modes are produced). 4PA mode produces all four possible correlations from the A and C channels -- AA, AC, CA and CC - with a spectrum for each. 4PB provides the analogous output for the B and C channels. For these linear-polarization-capable modes, the descriptor 4 is often omitted.
Table 9 gives the averaging time restrictions that are imposed by the VLA correlator.
| Channels | Minimum (sec) |
| 1 | 3.33 |
| 8 | 3.33 |
| 16 | 3.33 |
| 32 | 3.33 |
| 64 | 3.33 |
| 128 | 6.66 |
| 256 | 10 |
| 512 | 20 |
The Spectral Line Users' Guide (available on the World-Wide-Web at http://www.nrao.edu/doc/vla/obstatus/splg1/splg1.html) has further information on the VLA spectral line system.