Context. The antennas of NASA's Madrid Deep Space Communications Complex (MDSCC) in Robledo de Chavela are available as single-dish radio astronomical facilities during a significant percentage of their operational time. Current instrumentation includes two antennas of 70 and 34 m in diameter, equipped with dual-polarization receivers in K (18-26 GHz) and Q (38-50 GHz) bands, respectively. Until mid-2011, the only backend available in MDSCC was a single spectral autocorrelator, which provides bandwidths from 2 to 16 MHz. The limited bandwidth available with this autocorrelator seriously limited the science one could carry out at Robledo. Aims. We have developed and built a new wideband backend for the Robledo antennas, with the objectives (1) to optimize the available time and enhance the efficiency of radio astronomy in MDSCC; and (2) to tackle new scientific cases that were impossible to investigate with the existing autocorrelator. Methods. The features required for the new backend include (1) a broad instantaneous bandwidth of at least 1.5 GHz; (2) high-quality and stable baselines, with small variations in frequency along the whole band; (3) easy upgradability; and (4) usability for at least the antennas that host the K-and Q-band receivers. Results. The backend consists of an intermediate frequency (IF) processor, a fast Fourier transform spectrometer (FFTS), and the software that interfaces and manages the events among the observing program, antenna control, the IF processor, the FFTS operation, and data recording. The whole system was end-to-end assembled in August 2011, at the start of commissioning activities, and the results are reported in this paper. Frequency tunings and line intensities are stable over hours, even when using different synthesizers and IF channels; no aliasing effects have been measured, and the rejection of the image sideband was characterized. Conclusions. The new wideband backend fulfills the requirements and makes better use of the available time for radio astronomy, which opens new possibilities to potential users. The first setup provides 1.5 GHz of instantaneous bandwidth in a single polarization, using 8192 channels and a frequency resolution of 212 kHz; upgrades under way include a second FFTS card, and two high-resolution cores providing 100 MHz and 500 MHz of bandwidth, and 16 384 channels. These upgrades will permit simultaneous observations of the two polarizations with instantaneous bandwidths from 100 MHz to 3 GHz, and spectral resolutions from 7 to 212 kHz.

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