KEYWORDS: Antennas, Design, Signal processing, Clocks, Phased arrays, Analog to digital converters, Digital signal processing, Electromagnetic coupling, Field programmable gate arrays
The Low-Frequency Array, or LOFAR, is the world's largest low-frequency radio telescope consisting of over 100 000 antenna elements spread across more than 50 stations throughout Europe. LOFAR2.0 is an upgrade of LOFAR which will significantly improve its sensitivity and overcome several limitations encountered during the last 10 years of operation. The digital beamformers form the core of each LOFAR station. They are called antenna processing sub-racks (APS), where all antenna signals are digitized and digitally processed to form beams on the sky. These beamformers have stringent performance requirements, such as high linearity due to strong radio-frequency interference, good timing for high beamformer efficiency, and very low common-noise and cross-talk to be sky-noise limited over long integration times. The designs of the new LOFAR2.0 beamformer are presented, showing how a balance was struck between performance and cost enabling the production of high volumes, easy installation and maintenance in the field. The antenna processing subrack consists of low-noise receiver units (RCU) which digitise about a hundred RF signals, a clock and control board (APSCT) to distribute the sampling clock and control the digitisers, a power generation board (APSPU), UniBoard2s where FPGAs perform the beamforming, and a midplane that connects all the boards together while also shielding the sensitive receivers. The APS therefore has boards ranging from high-speed, high-density digital processing devices and high-current power converters to low-noise RF electronics. It has hundreds of devices to power, cool, control and monitor and hundreds of gigabits of data which need to be transferred between boards. The first LOFAR station has been upgraded with new beamformers and the first results will be presented. This demonstrates the new capabilities LOFAR2.0 will have with the new beamformers.
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