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Back-end electronics for coherent receivers

In a coherent receiver the analysis of the radio signal (spectroscopy, spectropolarimetry, interferometry, temporal analysis, timing analysis of pulsars) is carried out on an electronic representation of the signal, accordingly converted in frequency. This analysis can be

carried out using acousto-optical systems, in which the radio signal is converted into an acoustic wave that modulates a laser beam, or digital systems, in which a numerical representation of the signal is processed. The current trend is to use digital systems with ever improving performance in terms of speed, complexity and miniaturisation, thanks to the increasing availability of fast, programmable digital circuits. Further, there exist, or are in development, generic hardware platforms based on FPGA (Field Programmable GateArray), for which libraries of application modules are being developed.
Especially in VLBI (Very Long Baseline Interferometry, the network of radio telescopes extending to all continents that carries out the most precise observations at radio frequencies), the tendency is to use a digital treatment of the signal starting from the mid-frequency stage. Responsibilities in this field are digital filters for ALMA, spectrometers for Medicina and Noto, digital receivers for Radio Science, the ItaSEL system, and DBBC, the Digital Baseband Converter for VLBI. In particular, the DBBC system has been adopted as a standard for the European VLBI network, and an industrial spin-off has emerged for its production. An advanced digital system for the analysis of fast pulsars is being studied for the SRT, that would allow this antenna to become a unique instrument in the northern hemisphere for the study of pulsars. These activities are inserted in a Seventh Framework Programme ("Digital Developments") with the aim of realising digital systems with instantaneous bandwidths of 4-8 GHz.

The possibility of connecting antennas via fast networks with bit rates of order Gbit/sec would allow VLBI observations in real time (e-VLBI), making the planning of observations much more flexible and dynamic, improving the quality of observations via constant feedback, a making the correlated data immediately available.

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