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Additional Resource Features and Restrictions

32 Subbands per (8-bit) Baseband

Normally, a baseband contains eight 128 MHz continuum subbands per GHz (i.e., Fill subbands will create 8 subbands in an 8-bit baseband and 16 subbands in a 3-bit baseband). When observing in 8-bit spectral line mode, additional subbands may be added depending on the remaining number of available baseline board pairs. However, configuring more than 16 subbands in an 8-bit baseband will trigger additional hardware to configure the correlator in a non-standard way, allowing more flexibility when dealing with complex spectral line setups. When using (more than 16 and) up to 32 subbands per 8-bit baseband, the ultimate check is to configure and validate the resource in the RCT. Note that using more than 16 subbands in a baseband is not available in 3-bit setups.

The recipe to trigger this additional hardware is as follows:

  1. In the RCT in a new 8-bit resource, create all the desired line subbands in the "Line Placement" tab if appropriate. Then, in the "Subbands" tab click on Fill subbands. This creates the first eight subbands in the baseband, e.g., in the A0/C0 baseband pair.
  2. Add eight more subbands by clicking Add subband eight times. Note that the subband-number background color in the leftmost column is the same for all 16 subbands (numbered 0 through 15) as seen in Figure 3.22.
  3. Add another subband in the same way (i.e., click Add subband, up to 16 more times). The change in background color for the subband number between subband numbers 15 and 16 indicates the use of the additional hardware.
    However, if the setup already uses 16 or more baseline board pairs in the baseband (summed over all the line subbands generated in Line Placement) before adding any new subbands to the ones generated with Line Placement, the use of the additional hardware may not be automatically invoked. In such a case, each additionally added subband in the baseband needs to be manually switched to use the additional hardware by clicking on the colored subband number in the leftmost column of the row; this should change the background color of the particular subband number, and typically start with number 16. This manual switching can be done after adding each individual subband or after adding a bunch of subbands first and then changing them all. Note that this toggle must be done before validating the resource. If not taken care of at this stage, the validation of the resource will throw an error in the "Validation" tab.
  4. Change subband frequencies, bandwidths, recirculation etc., or even remove subbands from the original 16 (or fewer) in the baseband, to configure the correlator to match your anticipated observations for this baseband. The same recipe applies to the other IF pair (e.g., B0/D0). Finally, adhere to the good practice of designating some subbands "Desired" instead of "Essential", especially if the total number of baseline board pairs used approaches the maximum of 64.

Figure 3.22: The change in color indicates which subband uses the additional hardware (in 8-bit).


Ka-band Restrictions

There is an issue with specifying the frequency of IF pair AC at Ka-band. That is, tuning any part of the AC IF pair band below 32.24 GHz will not result in valid data, regardless whether this is A0/C0 or any of A1/C1 or A2/C2. Only the BD IF pair can be tuned to frequencies below 32.24 GHz; use the BD IF pair instead of AC IF pair when you only need one IF pair for your resource with a frequency tuning below 32.24 GHz. If the RCT validation detects that any part of the bandwidth of IF pair AC is tuned below this 32.24 GHz it will try to swap the AC IF pair with the BD IF pair. If this is not possible, it will issue an error (in red font) in the interface feedback strip if this frequency is specified as a fixed sky frequency. It will issue a warning (blue font) for rest frequencies, as the particular tuning depends on the details of observing date, telescope pointing direction and source velocity definitions. Note that a rest frequency above 32.24 GHz may shift to below 32.24 GHz once it is assigned to a scan in the OPT. This should give you an error in the OPT; you should be aware of this possibility and pay attention to this. However, it is better to assign IF pair BD to the resource if you anticipate this might happen, if you still have this freedom in your resource of course.

When observing very close to 32 GHz with both IF pairs, some combinations where frequency coverage of AC and BD are overlapping are not possible. Consult the NRAO Science Helpdesk for options, preferably before submitting the proposal.

The very wide bandwidth of the Ka-band receiver, from 26.5 to 40 GHz, would suggest that IF pair separations of up to ~13 GHz are possible. Restrictions in the signal path, however, limit this separation to 10 GHz. The RCT validation will issue an error if the separation between IF pairs AC and BD is more than 10.5 GHz in sky frequency (with IF pair AC tuned to have the higher frequency centers). A separation of more than 10.5 GHz in rest frequency will result in a warning as, e.g., highly red-shifted lines may end up with less separation when the actual sky frequencies are calculated.

K and Q-band Restrictions

If you choose the RF signals in the different IF paths to be separated by a large amount, it is possible that the RCT will only let you create a resource where the baseband frequency center(s) in IF pair AC is higher than the baseband frequency center(s) in IF pair BD, similar, but the reverse of the Ka-band restriction above.