An FPGA based Phased Array Processor for the Sub-Millimeter Array

Abstract

It has been widely acknowledged that Very Long Baseline Interferometry (VLBI) in the submillimeter wavelengths can make imaging observations of super massive black holes possible. The Sub-Millimeter Array (SMA) along with the James Clerk Maxwell Telescope (JCMT) and Caltech Submillimeter Observatory (CSO) on the Mauna Kea summit in Hawaii can together provide a large collecting area as one or more stations for VLBI observations aimed at studying an event horizon. To work as a VLBI station with full collecting area the SMA (or a combination SMA, JCMT, CSO antennas) would need a processor to enable phased array operation. This masters project focusses on building such a processor. Back end processing for high bandwidth radio telescopes has traditionally been done using custom designed application specific integrated circuits (ASIC). Recent advances in Field Programmable Gate Array (FPGA) technology have made FPGAs both powerful and economically viable for radio astronomy back ends. We have attempted to take advantage of these advances and built a proof-of-concept 500 MHz phased array processor for the SMA using FPGAs. The phased array processing is done in the time domain using high speed sampling and digital delay lines. The design is capable of spooling the phased sum to a Mark 5b VLBI data recorder. It is based on hardware built by the Berkeley Wireless Research Center and the Berkeley Space Science Laboratory. We digitize signals after the 1st SMA downconvertor using 1024 MHz sampling and have demonstrated the capability to sum signals from 8 antennas through programmable digital delay lines up to a precision of (approx) 1/10 the sampling rate i.e. 0.1 ns. To calibrate geometric, atmospheric and instrument delays for accurate phasing, a single baseline 512 MHz 32 channel FX correlator has also been designed to fit on a single FPGA chip.