The RPG eXtended bandwidth Fast Fourier Transform Spectrometer (XFFTS) was optimised for a wide range of radio astronomical applications. The digitiser and analyser boards made use of the latest versions of GHz analogue-to-digital converters (ADC) and the most complex field programmable gate array (FPGA) chips commercially available at the time.
These state-of-the-art chips made it possible to build a digital spectrometer with instantaneous bandwidth of 2.5 GHz and 32768 (32K) spectral channels. The APEX facility XFFTS consisted of two units which had a fixed overlap region of 1.0 GHz, hence covering the entire IF bandwidth of SHeFI. The number of spectral channels was fixed at 32768 channels.
For more information you can check the spectrometer paper:
High-resolution wide-band fast Fourier transform spectrometersB. Klein, S. Hochgürtel, I. Krämer, A. Bell, K. Meyer, R. Güsten, Astronomy & Astrophysics, Volume 542, id. L3, 6 pp.
The XFFTS spectrometers were decommissioned in 2019, to be replaced by the more powerful and wider bandwidth dFFTS4G units, currently in place.
The facility Fast Fourier Transform Spectrometer consisted of two units with a bandwidth of 1 GHz each. Both units were configurable to provide up to 8192 spectral channels, thus providing a spectral resolution of up to 122 kHz. The two units could be connected to one receiver each, or the two units could be configured to produce a single backend with almost 2 GHz bandwidth. The placement of each unit is restricted by the IF range of the receiver.
The FFTS units were decommissioned in July 2011, to be replaced by the more powerful and higher bandwidth XFFTS units.