High-resolution wide-band Fast Fourier Transform spectrometers

TL;DR

This paper presents the design, implementation, and performance of high-resolution, wide-band digital FFT spectrometers optimized for radio astronomy, demonstrating their reliability and advanced signal processing capabilities in airborne observatory conditions.

Contribution

The paper introduces a novel wide-band FFT spectrometer with high spectral resolution and an optimized FPGA processing pipeline, suitable for demanding radio astronomical applications.

Findings

Achieved 2.5 GHz bandwidth with 88.5 kHz resolution

Demonstrated reliable operation in airborne environment for extended periods

Enhanced spectrometer will double spectral channels for improved resolution

Abstract

We describe the performance of our latest generations of sensitive wide-band high-resolution digital Fast Fourier Transform Spectrometer (FFTS). Their design, optimized for a wide range of radio astronomical applications, is presented. Developed for operation with the GREAT far infrared heterodyne spectrometer on-board SOFIA, the eXtended bandwidth FFTS (XFFTS) offers a high instantaneous bandwidth of 2.5 GHz with 88.5 kHz spectral resolution and has been in routine operation during SOFIA's Basic Science since July 2011. We discuss the advanced field programmable gate array (FPGA) signal processing pipeline, with an optimized multi-tap polyphase filter bank algorithm that provides a nearly loss-less time-to-frequency data conversion with significantly reduced frequency scallop and fast sidelobe fall-off. Our digital spectrometers have been proven to be extremely reliable and robust, even under the harsh environmental conditions of an airborne observatory, with Allan-variance stability times of several 1000 seconds. An enhancement of the present 2.5 GHz XFFTS will duplicate the number of spectral channels (64k), offering spectroscopy with even better resolution during Cycle 1 observations.