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Mathematisch-Naturwissenschaftliche Fakultät

I. Physikalisches Institut

Sonderforschungsbereich 494

Project Section D7 (Proposal)


Digital wide-band spectrometers

We propose to expand our development of digital backends based on the newest generation of FPGA microchips and fast analog-to-digital converters. We aim to build a digital spectrometer with a monolithic bandwidth of 2 to 4 GHz, and thereby provide the basis to assemble a cost-efficient digital backend which allows for a total bandwidth coverage of more than 36 GHz at high resolution, filling a need for the present and future array receivers operated or build by the partners in this SFB. The performance of the digital spectrometers will be directly compared with the acousto-optical spectrometers designed within this SFB.

The rapid increase in the sampling rate of commercially available analog-to-digital converters (ADCs) and the increasing power of field programmable gate array (FPGA) processors has led to the technical possibility to directly digitize the down-converted intermediate-frequency (IF) signals of coherent radio receivers, and to transform the digital signals into a power spectrum in real-time. With simultaneous bandwidths of the order of 1-2 GHz and very high spectral resolution, they are an important complement to the acousto-optical Spectrometer technology (see SFB TP D6), which is limited to a resolution of about 1 MHz, but can cover many GHz of instantaneous bandwidth within a compact, low power configuration. A first generation of prototype digital fourier transform spectrometers (FFTS, more general: digital spectrometer) has been assembled and is being commissioned at the Effelsberg 100m telescope (50 MHz and 1 GHz bandwidths), at KOSMA (in collaboration with A. Benz, ETH Zürich), at APEX (2× 500 MHz), and Arecibo (300 MHz). The efforts to build such FTS are currently concentrated at U.C. Berkeley (D.Werthimer), Zürich (A.Benz/ETH & Acqiris), and Bonn (RAIUB, MPIfR).

The currently most powerful card is commercially available from Acqiris; it employs two 1 GS/s ADCs which feed a XILINX Virtex 2 Pro70 FPGA chip. A software core that turns this card into an FFT spectrometer is now either available commercially from RF Engines Limited (500 MHz bandwidth, 16,384 channels), or from the ETH Zürich research group of A. Benz (1 GHz bandwidth, 16,384 channels). A 50 MHz bandwidth, 1024 channel core running on a PCIbus board (ICS-554C) from ICS was built through a RAIUB/MPIfR collaboration in 2004 as a prototype development for a 21cm multi-feed receiver for the Effelsberg 100m telescope.

The FFTSs now in use or being tested demonstrate that this novel technology is superior in sensitivity as well as frequency resolution, simplicity, and cost compared with autocorrelators and analogue filterbanks. Motivated by our immediate need for wide-band, high spectral resolution spectrometers for the submillimeter array receivers at the new telescopes such as APEX and NANTEN2, but also for future large HEMT/MMIC focal plane arrays with hundreds of channels, and by the high cost of commercial digital spectrometer cards and cores, we propose to aggressively expand our own development of digital backends. Using the newest generation of XILINX Virtex 4 FPGAs – as they become available – and fast (at least 2 GS/s) ADCs we propose to build digital spectrometers with a monolithic bandwidth of 2 to 4 GHz, well matched to the wide bandwidths with high velocity resolution and/or wide velocity coverage, which are crucial for galactic spectral line surveys and extragalactic line observations. Our central goal is to 1) thouroughly test these spectrometers in direct performance comparison with the well established AOS and ACS, 2) build a cost-efficient digital backend which allows coverage of a total bandwidth of ca. 36 GHz (composed of 2-4 GHz monolithic bandwidth spectrometer cards) at highest resolution for the present and future array receivers operated and being constructed by the partners in the SFB 494.

 

See the home page of Prof. Bertoldi for more information.