Correlator Configurations

The correlator is composed of four independent boards, each a full correlator in its own right. One board can observe both polarizations, or just one of the two available from the chosen receiver, at a bandwidth and resolution that is set independently for each board. The signal may be three-level, three-level interleaved, or nine-level sampled. At bandwidths of 12.5 MHz or less, the signal to noise from three-level spectra can be improved by oversampling (i.e. using double-Nyquist sampling). The choice of any of these configurations is set by the ``lagsam'' variable in Table 1 , which is specified for each correlator board as an argument to the cor_lagc command.

Table: Correlator Configurations for the 4 Digital Boards

``lagsam''	Configuration	MaxBW	sbc/Pol	Boards	Lags/sbc
		per sbc		used	best resn at 1420 MHz
0	9-level, A pol.	25 MHz	2	4	2048/(2.6 km/s)
1	9-level, B pol.	25 MHz	2	4	2048/(2.6 km/s)
2	DON'T USE	-	-	-	-
3	DON'T USE	-	-	-	-
4	DON'T USE	-	-	-	-
5	3-level A & B pol.	25 MHz	4	4	2048/(2.6 km/s)
6	3-level interleav. A	50 MHz	2	4	4096/(2.6 km/s)
7	3-level interleav. B	50 MHz	2	4	4096/(2.6 km/s)
8	3-level interl. A & B	50 MHz	4	4	2048/(5.2 km/s)
9	9-level A & B pol.	25 MHz	4	4	1024/(5.2 km/s)
10	3-level polarization	25 MHz	4	4	2048/(2.6 km/s)

Double-Nyquist sampling can be used with all configurations, except interleaved, provided the bandwidth is less than or equal to 12.5 MHz. However, its use decreases the maximum bandwidth by a factor of two. 3-level, double-Nyquist, 12.5 MHz bw and below give 4 sub-bands with better resolution than the corresponding 9-level configuration.

9-level operation achieves 98% of the signal-to-noise of analog correlation whereas 3-level sampling achieves 81%. One advantage of 9-level sampling is to limit the effects of RFI (usually) to just a few channels.