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.




Specifications and goals
------------------------
In the following, a "baseband" denotes an individual input band of 2 GHz width 
which is analyzed by a single A/D converter.
A "sub-band" denotes a continuous frequency chunk to be  analyzed 
spectroscopically (i.e. a sub-band is a sub-element of a "baseband").
A "sub-array" denotes a sub-set of 12m antennas which can operate as a 
logically independent interferometer (i.e. a sub-array can work at a 
frequency different from the rest of the ALMA antennas, and can receive 
specific control commands: start, stop, integration times, etc).

1.- In addition to the 64 ALMA antennas of 12m, the Enhanced/Future
Correlator must accommodate the ALMA Compact Array (ACA).
The ACA specifications are not established yet. Current ACA simulations 
assume 12 antennas of 7m diameter. For calibration purposes, the ACA
will be correlated jointly with about 4 antennas of 12m.

2.- A total number of 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 spectral product (per sub-band and/or 
polarization).
 
      A more ambitious goal would be to obtain 4000 to 8000 channels 
per spectral product (sub-band and/or polarization). This goal should
be fixed by considerations of technical feasibility and cost.  
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 banwidth, full polarization, 256 channels cover
8 GHz, corresponding to a 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 bandwith, 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 bandwidth obtained at this highest resolution will be determined
by the maximum number of channels provided by the correlator (see item 2).

     The Baseline Correlator can provide a resolution of 1.9 kHz single 
baseband single polarization ; it is 15.3 kHz for full polarization
single sub-band. As in example D4 of Table 1 of Memo 194, resolution of 
1 kHz 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. 
Thus the Enhanced/Future Correlator should be able to accommodate a minimum of
4 independent sub-arrays and the ACA (see item 1).
The Baseline Correlator has the capability of accomodating 16 sub-arrays.

7.- For solar observations, the required minimum integration times 
are 1 msec for the continuum, and about 0.1 sec for spectral lines.