IF processor and narrow-band correlator

**Figure:** Example of spectral coverage, as produced by the ASTRO `LINE` command. The image band is plotted for information only, the receivers are **single side band.** Possible contamination by instrumental interferences in the signal band are indicated
$\includegraphics[width=\textwidth]{pdbi-intro-if1}$

The narrow-band correlator accepts as input two signals of 1GHz bandwidth, that must be selected within the 3.6GHz delivered by the receiver. In practice, the IF processor splits the two input 4.2-7.8GHz bands in four 1GHz ``quarters'', labeled Q1...Q4 (see Fig.

). Two of these quarters must be selected as narrow-band correlator inputs. The system allows the following choices:

Quarter	Q1	Q2	Q3	Q4
IF1 [GHz]	4.2 - 5.2	5.0 - 6.0	6.0 - 7.0	6.8 - 7.8
input 1	HOR	HOR	VER	VER
input 2	VER	VER	HOR	HOR

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 narrow-band correlator entries. This will necessarily result in each entry seeing a different polarization. The system thus gives access to 1GHz $\times$ 2 polarizations.

How to use the full 2GHz bandwidth? If two different quarters are selected (any combination except VER VER is possible), a bandwidth of 2GHz can be analyzed by the narrow-band correlator. 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 5.0 to 6.0GHz, Q3 is 6.0 to 7.0GHz, 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 (except VER VER) of two quarters can be selected. Adjacent quarters will result in a (quasi) continuous 1.8-2GHz band. Non-adjacent quarters will result in two separate 1GHz bands.

Where is the selected sky frequency in the IF band? It would be natural to tune the receivers such 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. If your project depends on the narrow-band correlator, it is therefore highly recommended to center a line at the center of a quarter (see Section ``ASTRO'' below). In all bands, 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).