The ``WAPP'' Correlator

The WAPP (Wideband Arecibo Pulsar Processor, as this device first reached astronomers as a pulsar processor, with standard spectral-line operation appearing later), is the replacement for the ``interim'' correlator. The WAPP provides eight sub-correlators, each of these sub-correlators having sixteen 1024-lag chips. Each of 4 pairs of sub-correlators can be set up with their own independent bandwidths and configurations. Table 5 specifies the available configurations. The maximum bandwidth per sub-correlator is 100 MHz, with 9 other alternative bandwidths being available in decreasing octave steps. The WAPP offers both 3- and 9-level operation, both for standard autocorrelation (total-power) and auto+cross correlation (all four Stokes parameters) modes. The WAPP also offers a direct-sampling option. Details of the many WAPP capabilities are to be found at
http://alfa.naic.edu/hardware/backend/wapp_fact_sheet.html, with the WAPP 8-board mode being described at http://www.naic.edu/~cima/cima_dual_board.html.

Table: WAPP spectral-line configurations with all 4 digital filter boards

Config	Max Bw	Pol/Sbc$^{\ast}$	Lags/Sbc$^{\ast}$ & Resn -
	per Sbc$^{\ast}$ (MHz)		(kms$^{-1}$ @ 1420 MHz)
Single-Pixel 100-MHz Bandwidth
9-level	100	1	2048 (2.6)
9-level	100	2	1024 (5.2)
3-level	100	1	8192 (2.6)
3-level	100	2	4096 (2.6)
3-level Stokes	100	Full Stokes	2048 (2.6)
Single-Pixel 195-kHz - 50-MHz Bandwidth
9-level	50	1	4096 (2.6)
9-level	50	2	2048 (5.2)
3-level	50	1	16384 (2.6)
3-level	50	2	8192 (2.6)
9-level Stokes	50	Full Stokes	2048 (2.6)
3-level Stokes	50	Full Stokes	8192 (2.6)
ALFA 100-MHz Bandwidth
3-level	100	2	4096 (2.6)
3-level Stokes	100	Full Stokes	2048 (2.6)
ALFA 50-MHz Bandwidth
9-level	50	2	2048 (5.2)
3-level	50	2	8192 (2.6)
9-level Stokes	50	Full Stokes	1024 (2.6)
3-level Stokes	50	Full Stokes	4096 (2.6)

$^{\ast}$ Sbc = Subcorrelator

Notes:
1) The WAPP has its available bandwidths defined by the following filters: 100, 50, 25, 12.5, 6.25, 3.125, 1.563, 0.781, 0.391, 0.195 MHz
2) 9-level operation achieves 96% of the signal-to-noise of analog correlation, whereas 3-level achieves 81%.