ASAC Guidelines for the ALMA Enhanced/Future Correlator

The following specifications and goals should be taken into account 
by the European, Japanese and North American teams working in the design 
of an Enhanced/Future Correlator for ALMA.

In general terms, the ASAC stresses that the Enhanced Correlator 
developments should be guided by the goals of achieving 
          (a) high number of channels in wide band modes
          (b) high configuration flexibility
          (c) high sensitivity
          (d) high spectral resolution, and 
          (e) power consumption as low as possible.

As in previous occasions, the ASAC strongly encourages a tight collaboration
of the different teams to optimize the design and to select the best possible
architecture and manufacturing method within the budget limits.

What follows is a summary of the requirements and goals which were 
put forward by the ASAC at recent meetings. 

1.- In addition to the final total number of 12m antennas, the Enhanced/Future
Correlator must accommodate the ALMA Compact Array.

2.- A total number of about 8000 channels is the minimum required.
This seems sufficient for most astronomical observations. 
Observations using multiple sub-bands and polarizations would accordingly 
have less channels available per product (sub-band and/or polarization). 
A good, more ambitious, goal would be to obtain 4000 to 8000 channels 
per product. Nevertheless, if the total number of channels were significantly 
larger than 8000, there should be ways of selecting or compressing them for
further processing.  The baseline correlator provides 4096 channels in most
modes. When used at the maximum bandwidth, full polarization, 256 channels
cover 8 GHz, corresponding to a resolution of 31.25 MHz. With one polarization,
1024 channels give a resolution of 7.8125 MHz. 

3.- Three-bit digitizing format and three-bit (or even four-bit) correlation 
format are recommended to obtain high sensitivity by diminishing quantization 
losses.  In widest bandwidth, the baseline correlator provides a two-bit 
digitizing format.  In narrower modes, three and four bit correlation are
available (though three bit quantization at the digitizers and FIR filter
limit usefulness of the latter).

4.- A highest spectral resolution of 5 kHz is required.
This corresponds to 0.05 km/s at 30 GHz, which is necessary, e.g., 
for the observation of lines in cold dark molecular clouds.  The baseline
correlator can provide 1.9 kHz single baseband single polarization; it
is 15.3 kHz for full polarization single sub-band.  As in example D4 lf Table
1 of Memo 194, resolution of 1 Hz is possible.

5.- A reasonable goal for the Enhanced/Future Correlator is to provide 
16 sub-bands (in total, not per polarization). Note that the equivalent 
number of sub-bands in the Baseline Correlator is 8. 

6.- ALMA will have the ability to be split in different logically-independent
sub-arrays, and to observe at a maximum of 4 different frequencies. 
The Enhanced/Future Correlator should be able to accommodate at least 
16 independent sub-arrays, which is the number provided by the Baseline 
Correlator.

7.- For solar (continuum) observations, minimum integration times of 1 msec 
are required. For spectral line observations, the fastest integration period 
should be made compatible with the minimum dump time. In side band separation 
mode, integration times of the order of 120 msec should be possible.