It is possible to have more than one spectral window per baseband and this is known as multi-region mode. In the correlator, this works by splitting a single FDM spectral window into multiple sections (regions), each of which can be placed independently within the baseband. Notice that this does not change the spectral resolution - the bandwidth per channel stays the same. Although the correlator could create more, users are currently restricted to a maximum of four spectral windows per baseband. TDM spectral windows cannot be split in this way.
Setting this up in the OT is slightly different to that described above as one doesn't explicitly choose a specific correlator mode and then split that into smaller sections. Instead, one just creates the spectral windows that would result from following such a procedure, using the fraction parameter to change the available spectral window bandwidths in the drop-down menu. For example, using the default fraction of unity, there are six available FDM spectral windows. If the fraction is then changed to , the same spws are displayed, but their bandwidths are one half of their previous value. Also, there is one less spw available as it is not possible to split the narrowest spw. This is illustrated in Table 5.1.
As usual, the OT will warn if anything illegal is attempted. The main things to be aware of are that the total fraction cannot exceed 1 and that each spectral window, before spectral averaging, must have the same spectral resolution. If required, each spectral window can subsequently be spectrally averaged, but the ``native'' (unaveraged) spectral resolution must be the same for each spectral window.
Fraction = 1 | Fraction = ![]() |
Fraction = ![]() |
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Bandwidth (MHz) | # channels | Bandwidth (MHz) | # channels | Bandwidth (MHz) | # channels |
1875 | 4096 | 937.5 | 2048 | 468.75 | 1024 |
937.5 | 4096 | 468.75 | 2048 | 234.375 | 1024 |
468.75 | 4096 | 234.375 | 2048 | 117.118 | 1024 |
234.375 | 4096 | 117.118 | 2048 | 58.594 | 1024 |
117.118 | 4096 | 58.594 | 2048 | not available | |
58.594 | 4096 | not available | not available |
An example of a multi-region mode is shown in Fig. 5.1 where a 1875 MHz-wide mode has been split up into three spectral windows. In addition to the restrictions on spectral window placement mentioned in Section 5.3.3.3, in multi-region mode the windows must all lie within the 2-GHz baseband width. Technically speaking it is not necessary to use the whole of the original spectral window i.e. the sum of the fractions can be less than one.