A Radio Astronomy Correlator Optimized for the Xilinx Virtex-4 SX FPGA

This paper describes a correlator that is optimized for the Xilinx Virtex-4 SX FPGA, and its application in the SKAMP radio telescope at the Molonglo Radio Observatory. The digital backend of the SKAMP telescope consists of more than 800 Virtex-4 FPGAs. Correlation is performed between each and every pairing of antenna inputs, so the SKAMP telescope, with its 384 inputs, has approximately 74,000 antenna correlations; with 100 MHz of input bandwidth from each antenna this requires real-time processing of more than 7 tera complex multiply-accumulates per second. The correlation cell described takes advantage of the hard IP blocks found within the Virtex-4 FPGA to perform one 4+4-bit complex correlation per cycle at a clock rate exceeding 256 MHz. At the core of each cell is an efficient 4-bit signed complex multiplier, implemented using the 18-bit signed multiplier of the Virtex-4 DSP slice, and a short term accumulator, implemented using the adjacent Block RAM. Nearly 30,000 correlation cells are instantiated across 192 Virtex-4 SX35 devices in order to process all the data from the SKAMP telescope.