Back-End Signal Processors

The Observatory provides a range of signal processing equipment, including a pair of correlation spectrometers, high-speed direct sampling, a variety of pulsar processors, and VLBI equipment. It also supports the use of visitor-supplied equipment, and in some cases this equipment can be made available to other users,
http://www.naic.edu/~astro/general_info/backends.shtml).

For spectral-line observing, the original post-upgrade ``interim'' correlation spectrometer provides four independent sub-correlators, each having eight chips with 1024 lags per chip. Each sub-correlator can be set up with its own independent bandwidth and configuration. Table  specifies the available configurations. The maximum bandwidth per sub-correlator is 50 MHz, with 8 other alternative bandwidths being available in decreasing octave steps. Popular configurations combine chips in groups of four to provide 9-level sampling (96% efficiency), interleaved operation (50-MHz bandwidth) or auto+cross correlation (all four Stokes parameters).

Table: ``Interim'' correlator configurations with all 4 digital filter boards

Config	Max Bw	Pol/Sbc$^{\ast}$	Boards	Lags/Sbc$^{\ast}$ & Resn -
	per Sbc$^{\ast}$ (MHz)		Used	(kms$^{-1}$ @ 1420 MHz)
9-level	25	1	4	2048 (2.6)
9-level	25	2	4	1024 (5.2)
3-level	25	2	4	2048 (2.6)
3-level interleaved	50	1	4	4096 (2.6)
3-level interleaved	50	2	4	2048 (5.2)
3-level Stokes	25	Full Stokes	4	2048 (2.6)

$^{\ast}$ Sbc = Subcorrelator

Notes:
1) The spectrometer has its available bandwidths defined by the following filters: a) Analog filter - 50 MHz, b) Digital filters - 25, 12.5, 6.25, 3.125, 1.563, 0.781, 0.391, 0.195 MHz
2) Double Nyquist sampling can be used with all configurations except interleaved, but decreases the maximum bandwidth by a factor of two.
3) 3-level, double Nyquist, 12.5-MHz bw and below will give 4 sub-bands with better resolution than the corresponding 9-level configuration.
4) 9-level operation achieves 96% of the signal-to-noise of analog correlation, whereas 3-level achieves 81%.
5) Different correlator boards can operate with different configurations to each other.
6) The minimum dump rate for spectral-line usage is about 10 Hz.
7) The number of lags used can be reduced by a factor of 2$^{n}$ down to 16 lags. The size of the output file will be proportional to the number of lags.

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 also provides four independent sub-correlators, but each of these sub-correlators has sixteen 1024-lag chips. Again, each sub-correlator can be set up with its own independent bandwidth and configuration. Table  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 current WAPP capabilities are to be found at http://alfa.naic.edu/hardware/backend/wapp_fact_sheet.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%.

Continuum observing at Arecibo can be made in two ways. Firstly, via detecting the signal by square-law detectors, passing the output of these through integrators, and then recording the signal as a time series via the Radar Interface (RI). The RI consists of $4 \times 12$-bit Analog-Digital converters, a fifo buffer memory, packer, multiplexer, and control system. This can take dual-polarization data at a 10-MHz rate with quantization at 2 bits, or at slower rates with higher level quantization. To use this option, the user needs to connect the last part of the signal path manually via cables. The detailed procedure for this can be found in Astronomy User's Manual and is available via the link at http://www.naic.edu/~astro/continuum.shtml. Secondly, the data can be recorded via the spectral-line correlator, and the measurements treated as spectral-line observations in which the spectral channels are collapsed across frequency during analysis to give a broad-band continuum signal. This has the advantage that radio interference can be edited out before the broad-band signal is derived.

A number of pulsar back-ends are available for user experiments at Arecibo. These consist of the ``facility'' instruments built and maintained by NAIC - the WAPP and AOFTM - while some ``user-owned public-access instruments'' have been made available to the community by their owners. The pulsar backends currently accessible by general users are summarized in Table .

Table: Publically Available Pulsar Back-ends at AO

Machine	Max BW	Max Chan	Min Samp	Usage
	(MHz)		($\mu$s)
WAPP$^{a}$	100	1024	25	Search, Timing, Poln, Single Pulse
AOFTM	10	1024	100	Search
PSPM	8	128	12	Search,
''	8	128	80	Timing
Mk-III	40	32	10	Timing
ABPP	112	32	varies	Timing
''	28	32	varies	Poln
ASP	56	?	?	Poln
FPDAS	2000	-	0.0005	Single Pulse

Notes:
a) For the WAPP, limits on the combination of number of channels and sampling rate used is set by the output rate of the machine.

Machine	Design	Owner	Remarks
WAPP	Correlator	Facility	Presently being replicated
AOFTM	FT Spectrometer	Facility	Designed by Cordes, Cornell
PSPM	Filter Bank	Wolszczan, Penn State
Mk-III	Post-Filter Bank	Taylor, Princeton	Needs AO filter banks
ABPP	Coherent Dedispersion	Backer, Berkeley
ASP	Coherent Dedispersion	Backer, Berkeley	PC Cluster
FPDAS	Coherent Dedispersion	Hankins, NMT	digital oscilloscope