An Integrated Circuit for Radio Astronomy Correlators Supporting Large Arrays of Antennas

Radio telescopes that employ arrays of many antennas are in operation, and ever larger ones are being designed and proposed. Signals from the antennas are combined by cross-correlation. While the cost of roost components of the telescope is proportional to the number of antennas N, the cost and power consumption of cross-correlation are proportional to N-2 and dominate at sufficiently large N Here, we report the design of an integrated circuit (IC) that performs digital cross-correlations for arbitrarily many antennas in a power-efficient way. It uses an intrinsically low-power architecture in which the movement of data between devices is minimized In a large system, each IC performs correlations for all pairs of antennas but for a portion of the telescope's bandwidth (the so-called "FX" structure). In our design, the correlations are performed in an array of 4096 complex multiply-accumulate (CMAC) units. This is sufficient to perform all correlations in parallel for 61 signals (N=32 antennas with two opposite-polarization signals per antenna). When N is larger, the input data are buffered in an on-chip memory and the CMACs are reused as many times as needed to compute all correlations. The design has been synthesized and simulated so as to obtain accurate estimates of the ICs size and power consumption. It is intended for fabrication in a 32 mn silicon on.-insulator process, where it will require less than 12 mm(2) of silicon area and achieve an energy efficiency of 1.76-3.3 pJ per CMAC operation, depending on the number of antennas. Operation has been analyzed in detail up to N = 4096. The system-level energy efficiency, including board-level I/O, power supplies, and controls, is expected to be 5-7 pJ per CMAC operation. Existing correlators for the JVIA (N = 32) and ALMA (N 64) telescopes achieve about 5000 pJ and 1000 pJ, respectively using application-specific ICs (ASICs) in older technologies. To our knowledge, the largest-N existing correlator is LEDA at N = 256; it uses CPUs built in 28 rtru technology and achieves about 1000 pJ. Correlators being designed for the SKA telescopes (N = 128 and N = 512) rising FPGAs in 16 um technology are predicted to achieve about 100 pJ.