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With the successful completion of the SKA_LOW's element-level Critical Design Review (CDR) in 2018, the project now has sufficient momentum to carry it through the all-important transition from design to delivery.
This transition is taking place with the bridging phase, an on-site roll-out of 256 SKALA-4.1 antennas in a field node placement similar to AAVS1. This new field node is denoted AAVS1.5, and incorporates technology that is more easily compared to the MWA/EDA than to AAVS1.
These SKALA-4.1 antennas no longer use a hybrid copper/fibre cable for data transportation and power delivery, but instead a simple coaxial cable like those in the MWA. Data from a group of 16 antennas (much like an MWA tile or EDA cluster) is then combined in a SMART box ('Small Modular Aggregation & RFoF Trunk'); this box contains front-end modules that convert the coaxial cable RF signal to fibre, suitable for transport. The SMART boxes sit on the field node with the SKALA antennas and have the same physical chassis as an MWA beamformer. These SMART boxes have also been deployed for the same purpose in EDA-2.
Signal aggregation no longer happens in a central APIU either; a large shielded container now sits on the north side of the node for easy access. This container, called the Field Node Distribution Hub (FNDH), has a unit to deliver power to the node and one that combines and sends the 16 SMART box signals to the control building. One more piece of receiving equipment in the correlator room converts these signals back to copper, and then they can be passed into Tile Processing Modules (TPMs) where data computational tasks begin.
AAVS1.5 from above, populated with 48 antennas.
SKALA-4.1 antennas on the AAVS1.5 station. Image credit: ICRAR/Curtin
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Once AAVS1 was completed, it became clear that there were many disadvantages to having a signal aggregation unit (the APIU) in the middle of the antenna field, and that their hybrid power and signal cables were needlessly complex. However, the early signal conversion to fibre remained a necessary part of the SKA-LOW system, so a new component was required to bridge this gap in functionality. Engineers from INAF and CIRA worked in collaboration to design what is known as the SMART boxes.
These 'Small Modular Aggregation and RFoF Trunk' devices convert coaxial cable RF signal to RF over fibre. With the SMART box, RF signal from each individual antenna is directly transported via fibre optic cable to a Tile Processing Module (TPM) inside the MRO control building, located approximately 5 km away from the field node where it resides. The main component of a SMART box is the front-end modules, handling the copper-fibre conversion, but they also supply each antenna amplifier with 5V power. The principal components associated with the SMART box are shown in the schematic in Figure 3.
SMART boxes have been deployed on both AAVS1.5 and EDA2.
Figure 1: Inside a SMART box, showing the power transformer, the front-end modules for coax-to-fibre conversion, and the output fibre fed through a waveguide
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