At any given time, only one frequency band is used, but with the two polarizations available. Each polarization delivers a 4GHz bandwidth (from IF=4 to 8GHz). The current correlator accepts as input two signals of 1GHz bandwidth, that must be selected within the 4GHz delivered by the receiver. In practice, the new IF processor splits the two input 4-8GHz bands in four 1GHz ``quarters'', labeled Q1...Q4. The system allows the following choices:
where HOR and VER refers to the two polarizations:
| Quarter | Q1 | Q2 | Q3 | Q4 |
| IF1 [GHz] | 4.2 - 5.2 | 5 - 6 | 6 - 7 | 6.8 - 7.8 |
| input 1 | H | H | V | V |
| input 2 | V | V | H | H |
How to observe two polarizations? To observe simultaneously two
polarizations at the same sky frequency, one must select the same quarter
(Q1 or Q2 or Q3 or Q4)
for the two correlator entries. This will necessarily result in each
entry seeing a different polarization. The system thus gives access
to 1GHz x 2 polarizations.
How to use the full 2GHz bandwidth? If two different quarters
are selected (any combination is possible), a bandwidth of 2GHz can
be analyzed by the correlator. But only one polarization per quarter
is available in that case; this may or may not be the same
polarization for the two chunks of 1GHz.
Is there any overlap between the four quarters? In fact, the
four available quarters are 1GHz wide each, but with a small overlap
between some of them: Q1 is 4.2 to 5.2GHz, Q2 is 5 to 6GHz, Q3 is
6 to 7GHz, and Q4 is 6.8 to 7.8GHz. This results from the combination
of filters and LOs used in the IF processor.
Is the 2GHz bandwidth necessarily contiguous? No: any combination
of two quarters can be selected. Adjacent quarters will result in a
continuous 2GHz band. Non-adjacent quarters will result in two
independent 1GHz bands. Note that in any case, the two correlator
inputs are analyzed independently.
Where is the selected sky frequency in the IF band? It would be
natural to tune the receivers so that the selected sky frequency
corresponds to the middle of the IF bandwidth, i.e. 6.0GHz. However,
this corresponds to the limit between Q2 and Q3. It is therefore
highly recommended to center a line at the center of a quarter (see
Section ``ASTRO'' below). At 3mm, the receivers offer best
performance in terms of receiver noise and sideband rejection in Q3
(i.e. the line should be centered at an IF1 frequency of 6500 MHz)
whereas at 1mm best performance is obtained in Q2 (i.e. the line
should be centered at 5500MHz).