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For MWAX, an array of 24 (+2 spare) identically configured MWAX Servers are connected to the core Cisco 9504 switch (on site at the MRO) via 40Gb Ethernet. Each MWAX Server is responsible for one coarse channel (of bandwidth 1.28 MHz) of bandwidth. For a 256T array, the input data volume is approximately 11 Gbps per coarse channel.
Packets from the multicast stream are assembled in shared memory (RAM) into 8 second blocks of high time resolution voltage data based on their time and source (known as a “sub-observation”). At the completion of each 8 second block, the RAM file is closed and made available to another process. Depending on the current observing mode, the block may be:
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The FX Engine treats individual 8 second sub-observations as independent work units. Most of its mode settings are able to change on-the-fly from one sub-observation to the next. It operates on Each 8 second sub-observation file contains 160 blocks of 50 ms units of input data , i.e. a total of 160 blocks over each 8 second sub-observation. An An additional block of metadata (of the same size as a 50 ms data block) is prepended to the data blocks, making a total of 161 blocks per sub-observation file. At the start of processing each new sub-observation file, the metadata block is parsed to configure the operating parameters for the following 160 data blocks.
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The design supports a future option to apply delays to each signal path to point the telescope to a specified correlation pointing centre. When that mode becomes available, it will involve the integer sample component of the required delays being applied using whole-sample shifts as the sub-observation data file is assembled, with the residual fractional delays being applied within the FX engineEngine. The required fractional delay values for each signal path are passed to the FX Engine via the prepended metadata block written to the input ring buffer. Delays are applied by multiplying the frequency-domain samples of each sub-block by a phase gradient, whose complex gain values are taken from a pre-computed look-up table to increase speed. Delay corrections can be static over the entire sub-observation (e.g. to eliminate fixed cable delays) or dynamic over the sub-obervation (with a 5 ms resolution) to implement fringe stopping, i.e. to keep the correlation pointing centre on a fixed RA/Dec.
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