Argus: A Scalable W-band 16-pixel focal plane array for the Green Bank Telescope

1. M. Sieth, K. Devaraj, P. Voll, S. Church – Stanford University, Kavli Institute for Particle Astrophysics and Cosmology R. Gawande, K. Cleary, R. Reeves, A. C. S. Readhead – California Institute of Technology A. Harris – University of Maryland L. Samoska, P. Kangaslahti – Jet Propulsion Laboratory, California Institute of Technology J. Gundersen – University of Miami D. Frayer – National Radio Astronomy Observatory Argus: A Scalable W-band 16-pixel focal plane array for the Green Bank Telescope Objective: We are building Argus, a 16-pixel W-band focal plane array with a fully integrated heterodyne receiver module, based on state-of-the-art MMIC technology, that will be deployed at the Green Bank Telescope. Argus will be used for 85-115.5 GHz spectroscopy, with 75-85 GHz possible. Each pixel comprises a feed that couples the incoming radiation to a miniaturized radio receiver that we refer to as a ‘MMIC module’. The array architecture is designed to be both scalable and sufficiently modular such that broken or poorly performing elements can be repaired or replaced. MMIC Modules: The module amplifies the incoming radio frequency (RF) signal using a chain of InP MMIC HEMT low noise amplifiers (LNAs) and then down-converts the signal to the required intermediate frequency (IF) band. The latest state-of-the-art module tested at Caltech has a minimum receiver noise temperature of 27 K, with less than 40 K noise in the range of 75-107 GHz [1]. The band-averaged noise temperature is 33 K. Argus Array and Projected Performance: A concept drawing of the 16-pixel Argus array is shown below. The projected performance of the receiver based on prototype measurements is tabulated below. The MMIC module contribution is based on band-averaged measurements. Science Motivation: Our prime science interest in Argus is to study the star-formation processes within the Galaxy as well as other nearby and high redshift galaxies by mapping abundant molecular tracers of material from which stars form. In particular, CO is key for detecting gas in faint sources; all CO isotopologues (110–115.3 GHz) are needed to study the overall structure and dynamics of molecular clouds. The frequency range of 86–98 GHz is dominated by the bright lines of SiO, HCN, HCO+, HNC, N2H+, and CS. Compared to CO, these transitions require higher densities for excitation, and therefore better trace the compact, dense condensations of molecular gas out of which stars are actively forming. Argus can be used to study high redshift sources, for example, transitions of CO and its isotopologues involving redshift range of 1-3. View of the front of the array with some preliminary dimensions, in inches. Measured noise temperature and gain for the module for the I IF output, when operated at T=25 K [1]. Photograph showing the interior of a module. The input RF signal is via WR10 waveguide on the left, the LO input signal and the two IF output signals from the I-Q mixer are routed through miniature GPPO connectors on top. Cryogenic Signal Routing: The IF signals are routed from the modules to the GBT backend via multilayer printed circuit boards and flexible boards, rather than to individual coax and waveguide connections. A board at 20 K mates to the module via miniature push-on GPPO connectors. The board also splits the LO signal and routes the LO into each module, and routes the I and Q IF outputs to a second board at 77 K. The 77 K board routes the signals and provides IF amplification. Microstrip transmission lines patterned onto flexible circuit boards termed “IF flex cables” provide thermal breaks and are used to connect the 20 K board to the 77 K board and then ultimately to route the signals to and from the room temperature electronics. The lines are implemented on 0.005” polyimide substrate with copper cladding. The thermal conductivity per signal is comparable to that of stainless steel semi-rigid coaxial cables. The line spacing is 2.54 mm, which yields better than 15 dB cross talk between adjacent lines across the DC-20 GHz band [2]. The flexible circuits are connected to the PCBs by soldering the ground plane and wirebonding the microstrip traces. DC flexible lines route the bias signals for the modules and the IF amplifiers that are housed on the 77 K board. Receiver Schematic The figure below shows a schematic of the receiver. Each pixel in the spectrometer comprises a feedhorn that couples the incoming radiation to a miniaturized radio receiver, which is a MMIC amplifier-based multi-chip module. All of the receiver components are integrated into a single module, which amplify, filter, and down-convert the signal to the intermediate frequency (IF) band. The local oscillator (LO) signal is between 43-57 GHz, and the double sideband (DSB) IF has a tunable bandwidth of 1.5 GHz, made available in both In-phase (I) and Quadrature (Q) outputs. The 20K board that can accommodate four modules via push-on GPPO connectors. The prototype LO distribution rat-race splitter along with the microstrip and stripline transmission lines are visible. Photograph of parallel line test structure, with a twist to show both the transmission line layout (left side) and the ground plane patterning (right side). • Timeline • Argus project commenced in July 2012. • Prototype four-pixel array has been built to test various subcomponents and interfaces. Testing is ongoing at Stanford. • The full 16-pixel instrument is scheduled to deploy in November 2014. • References: • R. Gawande, et al. IEEE MTT-S IMS, Montreal, Canada, June 2012. • A. Harris et al., submitted to Rev. of Sci. Instr, arXiv:1206.1461v1, 2012 Acknowledgments: This research is funded by NSF ATI grant 1207825, and the preparatory work was funded by NSF ATI grant 0905855. Schematic for a single pixel for the 16-element Argus array.