A new generation of spectrometers for radio astronomy: fast Fourier transform spectrometer

We present a new generation of very flexible and sensitive spectrometers for radio astronomical applications: Fast Fourier Transform Spectrometer (FFTS). 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) chips has led to the technical possibility to directly digitize the down-converted intermediate-frequency signal of coherent radio receivers and to Fourier transform the digital data stream into a power spectrum in continuous real-time with no gaps in the data. In the last years FPGAs have become very popular for building fast and reconfigurable hardware. State-of-the-art chips include several hundred dedicated 18 bit×18 bit multipliers, which allow up to 80 billion multiplication and nearly 500 billion 36-bit additions per second. This extremely high computing power makes it now possible to implement real-time FFTs to decompose a 1GHz frequency band into 16384 spectral channels. In this paper we present the technological concept and results of our novel broad-band 1GHz FFTS with 16k frequency channels, which is installed at the APEX telescope. This backend can be considered prototypical for spectrometer developments for future radio astronomical applications.