Specs

Specification for PDFB2

Existing backend systems used for observing pulsars do not have the flexibility to provide data for a range of projects including high precision timing of millisecond pulsars, studying the polarization characteristics of pulsars and searching for previously unknown pulsars. The ATNF pulsar digital filterbank (PDFB) is being designed mainly for the Parkes pulsar timing array project which aims to achieve better than sub-microsecond timing precision for many millisecond pulsars. The PDFB will give 9-bit Nyquist-sampled data over bandwidths of up to 1 GHz, synchronous folding at the pulsar period, and the ability to provide real-time interference-mitigation. It also provides a flexible system for searching for pulsars and studies of individual pulse emission: the number of stored bits, bandwidth, polarisation products and sampling time can all be set by the user. Data in folding and search modes will be output as FITS files. The DFB also has a mode that provides digitized baseband output suitable for the next generation of real-time coherent dedispersion systems.

1.0 Modes

The DFB will be able to run in three modes:

Folded data mode. In this mode the data will be folded on-line by the DFB system. The output file will be a standard, pulsar folded profile with full polarisation and a given number of phase bins, frequency channels and subintegrations.
Search mode. This mode will provide streamed data for pulsar searches or individual pulse studies. The data will be recorded using 2, 4 or 8 bit resolution, will be integrated in memory before output into a standard FITS file.
APSR baseband mode. Required for the new generation of baseband recorders. This mode will provide multiple streams of baseband data with a total bandwidth of 1 GHz for input into APSR.

Two other modes exist (raw data capture and spectrometer which are currently not included in these specifcations). The APSR mode and the folded data mode should be able to be run in parallel.

In the following sections we provide the specifications that are common to all modes and then give the specifications specific to each mode.

2.0 Common specifications

Specification	Quantity	Comments
IF inputs	4	IF inputs (typically these will be used for two polarisations from the main antenna and two RFI inputs
Sample rate	2GHz	2GSPS of real sampled data
Sample resolution	9 bits
Bandwidth	1GHz	2GSPS of real sampled data provides a maximum of 1GHz of bandwidth. The bandwidth should be selectable by the user from 8, 16, 32, 64, 128, 256, 512 and 1024 MHz. The user must supply band-limited signals.
Correlations	4	Direct and cross products: XX, YY, real(XY), imag(XY)
Pulsar Timing Resolution	+/-7.5ns	Existing pulsar timing unit (PTU) specifications apply here.
Control and output		Control via ethernet
RFI spike zapping
RFI Processing		RFI processing is possible. See below (to write)
Digitiser statistics		Mean, maximum, rms digitiser counts are output once per cycle and count histograms are also available.

2.1 Specifications for the folded data mode

The DFB is specified to allow 2048 frequency channels and 2048 phase bins for a 4-ms pulsar recorded with 8-bit sampling. It should be possible to increase the number of frequency channels or phase bins whilst decreasing some other parameter. For instance, observing a longer period pulsar, with a reduced bandwidth or with smaller numbers of bits. A maximum required number of frequency channels is defined to be 8192 and a minimum of 16. The number of phase bins and frequency channels can vary by factors of two (e.g. 8k x 8k, 2k x 4k, 1k x 512 etc).

Specification	Quantity	Comments
DFB Frequency Channels	2048	See above notes on the possibility of larger numbers of frequency channels
Pulsar Bins	2048	See above notes on the possibility of larger numbers of phase bins
Output	PSRFITS	Standard PSRFITS format data files
Fractional folding period precision	< 10^-12
Pulsar period range	< 20 s	The standard mode of 2048 frequency channels and 2048 phase bins will work for pulsar periods > 4ms. Shorter periods (to 0.5 ms) are possible with reduced phase bins and/or frequency channels
Simultaneous Pulsars?	1-4	Simultaneous PPU Parallel Hardware for monitoring multiple pulsar periods (depending on hardware resources)

2.2 Specifications for the search mode

A running mean of user-definable length between 1s and 10s may be subtracted before re-digitisation. The output digitiser threshold can be set at user-defined intervals. The number of products, output bits, frequency channels and integration time can be selected, but must not exceed a data rate of 66 MBps. For data checking in this mode a "tag" should be placed every "N" samples through the data. The contents of this tag and the value for "N" are currently being defined.

Specification	Quantity	Comments
Frequency channels	up to 8192
Integration	N > 4	Onboard integration for "N" Nyquist samples
Output	PSRFITS	Search mode PSRFITS format data files (the definition of these files is currently being finalised)

2.3 Specifications for the APSR-baseband mode

Specification	Quantity	Comments
Output	4, 8 or 16	Number of bands of Nyquist-sampled baseband data
Channel widths	16, 32 or 64 MHZ	bandwidths of baseband channels, contiguous across the input bandwidth
Maximum data rate	64 MB/s	Maximum data rate per baseband channel