High-Speed Data Acquisition System


64-m ATNF Parkes Telescope, Australia

Datatape LP-400 digital cassette recorder

The 512-node Intel Paragon XPS computer

The High-Speed Data Acquisition System is used for the study of radio pulsars. It is based around a high-speed instrumentation tape recorder manufactured by Datatape Inc. of Monrovia, CA, which is capable of writing at a sustained rate of 400 Mbit/s using D1-format cassettes, and a signal digitizer based on a custom VLSI chip incorporating both digital and analog components.

Pulsar instrumentation

Pulsar observing requires the use of large radio telescopes (although the strongest pulsars are easily detected with modest equipment). The two key features for pulsar instrumentation are: These requirements are most stringent for pulsars with high DM / P, where DM is the dispersion measure (column density of electrons along the line of sight to the pulsar) and P is the pulsar period. Distant millisecond pulsars represent the biggest observational challenge.

High Speed Data Acquisition System

The High Speed Data Acquisition System differs from conventional pulsar system by taking a problem normally addressed in custom telescope hardware and making it primarily a software task. It satisfies the above requirements by storing a quantized representation of the received voltage signal; it comprises the following elements: Note that we list a supercomputer as part of the observing system. This is necessary because we cannot average the voltage signals and hence must record very large data volumes. A typical workstation can only store and analyze a few seconds of voltage data. We analyze the `raw' data on a supercomputer in a number of ways: The High Speed Data Acquisition System is set by the available RF bandwidth, which in turn is limited by the recorder data rate. Previous systems to record RF data directly have been extremely limited in bandwidth. At low frequency, our system can take advantage the entire interference-free bands (~25 - 50 MHz) typically available at observatories.

The system is very flexible compared with the conventional hardware approach. For instance, the number of filterbank channels is simply a parameter, and can be set to the optimum value. The desired sample rate is obtained by averaging the `software filterbank' output to the desired value.

Observations

A prototype system was tested at Caltech's Owens Valley Radio Observatory, in October 1994 - only about 9 months after the first conceptual design. A paper describing the instrument design and first observations is almost complete. We observed a number of known strong pulsars and some globular clusters.

The first full scientific use system was at the 64-m Telescope of Parkes Observatory, Australia Telescope National Facility, for several days in July 1995 at 436, 665, and 1380 MHz, to search for fast pulsars in southern globular clusters and the profile shapes of selected millisecond pulsars. We recorded a total 10 Tbyte of data, equivalent to the contents of the Library of Congress collection!! These data are presently being analyzed on the Paragon. A paper describing initial results on the millisecond pulsar PSR 0437-4715 has been published.

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Updated 21 Sep 1996
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