High-Speed Data Acquisition System
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:
- Rapid time sampling of the received signal. Detection of millisecond
pulsars requires sample rates of several kHz or faster.
- A system to divide the radio spectrum into narrow channels. Pulse
arrival times are `dispersed' as a function of frequency,
due to propagation of the signals through ionized plasma in the Galaxy.
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:
- Analog
downconverter - converts the RF signals to baseband (video);
this is the only part of the system which is observatory-specific
- Digitizer board - performs 2-bit
digitization of two polarization data streams up to 25 MHz bandwidth
- The Datatape LP-400 digital cassette recorder
- store the data streams
at up to 50 Mbyte/s, using D1-format cassettes (capacity 96 Gbyte)
- The 512-node
Intel Paragon XPS
- a massively parallel computer, for
playback and analysis of the recorded data
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:
-
Software simulation of a filterbank or autocorrelator - replicating in
software the functions of dedicated hardware systems
- Coherent dedispersion - applying a linear filter to the voltage
data to `undo' the dispersive effects of the interstellar medium.
This process preserves the full time resolution of the data.
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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