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DISPERSE: Sensor array technology applied to Radio Astronomy (in-depth version)


Present day radio-telescopes consist of large amounts of antennas. This display is an example of how a cluster of these antennas in the LOFAR telescope looks like. So, what you see here are a number of low-band antennas, distributed in an antenna field and also a number of high-band antennas, which are grouped in smaller clusters. The data of these antenna clusters is transferred in the analog electrical domain to a signal processing unit here in this street cabinet. Where the data from the fields are being handled and the data is prepared for being transferred to a central location via an optical fibre. This data processing is needed for pointing
the beam of an antenna station, but is also needed to reduce the amount of data, the amount of signal paths, such that the costs of such a system are in line with the amount of money
that is available for construction. As such, we lose quite a lot of data and this
data is not available at the central location, thereby limiting the capabilities of the astronomy
science. This display also shows how a high band antenna
in such a high-band antenna tile looks like. Again, with some electronics also for signal
processing and signal reduction. The goal in DISPERSE is to equip each antenna
with its own transmitter capable of transferring the data in a direct way to the central processor
as such we do not lose any data going from the antenna to that central processor and
in such a way we strongly increase the capabilities of the radio astronomy that can be done with
future radio telescopes. This tile is an example of ASTRON’s current
approach for the telescope architecture. What you see here is an antenna tile consisting
of 2D array of about 100 antenna elements. Instead of transferring the complete data
that has been received to a central location, we currently do a lot of signal processing
at the antenna. So, these signals are received and are
being processed here in the electronics at the other side of this tile. The advantage of the current approach is that
this is a low cost solution, because sending all the data to a central location is still
quite costly at this moment. The disadvantage is that we leave a lot of
the data here in the tile and do not have that data available at the central location. The big advantage of the DISPERSE technology
will be that all data will be available at the central location giving the astronomers,
our customers, most freedom in the signal processing and providing the biggest opportunities
for finding new sience and new events in space. So when the antenna receives a signal, it
gets transmitted to the LNA. The LNA amplifies this signal and its gets
transmitted to a our analog-digital conversion board. So before we started developing our AD-converter
we bought a commercial off-the-shelf unit. So we had to program this board and it supports RF, but also optical transmission and receiving of signals. With this board we can see if our link is
properly working and we can test stuff like bit-error rates and the quality of the link
itself. This board is a custom-designed with an commercial-off-the-shelf ADC module. It is used for the digitisation of the really
low frequency signals from our application. It has a bit depth of 9 to 12 bits in order
to receive a signal with sufficient sensitivity. The ADC-board supports two channels to support the two polarizations that we receive in to our systems. Once the signal has been digitized both signals are transferred to the optical transmitter module after which the signals are brought
to the central location, where the processing of the signals take place. This module is the heart of the astronomy
system in the DISPERSE project. It is an optical transmitter module, with
which the signals that are being received by the antenna are transferred to a central
location. The module consists of an electronic part
with electronic driver IC and an optical chip, which transfers the signals via a fibre to
the central location. There is a PCB in this module containing the
driver chips. The PCB and the drivers have been designed
and fabricated by ASTRON. The combining of the driver IC’s with the
optical chip has been done by the company Technobis, who has placed the optical chip
and the PCB’s in to this module. Technobis also has realised the electrical
interconnections via wire-bonding and has also realized the optical connection to the
chip via their pig-tailing technology. This module is capable of transferring the
digitized signals up to a date rate of 10 Gb/s to that central location. In DISPERSE we work on all technologies for the handling of the signals at the antenna side of the astronomy application, but ASTRON also works on technology for the processing of signals at the central location. This is one of the demonstrator boards that we have developed here at ASTRON, partly in European projects, and this signal processing system, called UniBoard, has a lot of optical inputs and is capable of handling data rates
up to a Tb/s. And for processing all these signals, we have
on this board several FPGAs. All the work in DISPERSE, so the transmission side,
close at the antennas, is aligned with this technology, such that in the future the DISPERSE
technology can work together with the technology at the central location for building up a
complete radio telescope.

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