VLBI offers the highest angular resolution of all astronomical observing techniques and is therefore an essential tool for research in several highly important astrophysical areas, e.g., formation and propagation of powerful plasma jets, fundamental physics near the event horizon of supermassive black holes, environment and surface of nearby radio-stars, astrometry with highest precision possible which could e.g. lead to the detection of extrasolar planets. The technical developments, which are described in this JRA, would serve as a cornerstone for accomplishing key elements of EVN2015 and at the same time reach beyond the objectives formulated in the VLBI vision paper. In this vision document it was stated that: "Technology will be available to extend the IF bandwidth of the EVN stations to 1 GHz in the L-band, and to 2-4 GHz in the C-band and higher. Full digital sampling of these IFs will be possible soon (DBBC project)." This has been accepted by the VLBI community in Europe and was formulated as a recommendation for further developments to the EVN Board of Directors.

In DIVA key technology building blocks will be developed to consolidate the role of European VLBI and European radio astronomy in general as a leading competitor with respect to developments in the USA and Asia.

VLBI is facing new challenges in the coming years. Single pixel wideband feeds have become available and are for instance an essential part of the global geodetic VLBI efforts (VLBI2010 roadmap). New wide-band samplers, and bigger, as well as faster FPGAs have been announced. Astronomers on the other hand demand significantly more sensitive VLBI observations and better UV coverage to enlarge the parameter space that can be probed with VLBI observations. To address the demand for and options of wider bandwidths of feeds, receivers and IF systems at telescopes worldwide, including EVN and ALMA, VLBI has to adapt its observing bandwidths and strategies to the needs for increased sensitivity. This JRA will develop

(a) wide-band dm/cm-receivers with at least 4:1 frequency bandwidth and
(b) wide bandwidth / high-bitrate VLBI backends which will allow to utilise the full potential of the above mentioned wide-band dm/cm-receivers and of planned or existing receivers in the cm/mm/submm-range. Such a backend can also be adapted to the requirements of VLBI with ALMA, which was proposed as an ESO project.

(a) While the initial low noise performance achieved with off-the-shelf components is promising, progress has been relatively slow towards dedicated and optimal low power/low noise receiver IC's (MMIC's) essential for optimized integrated receivers in wide-band single pixel feeds such as the wide-band 11-feed developed in Europe through OSO/Chalmers. As a consequence, present low-noise receiving systems operate over at most a 2:1 frequency bandwidth as proven by the new e-VLA receivers. The 11-feed has, unlike the log-periodic antenna developed for the Allen Telescope Array ("ATA"), a fixed phase centre over a similar wide frequency range of order 10:1. This in combination with its wide frequency performance makes it an extremely interesting concept for VLBI provided that combined with suitable LNA's, it could be proven to advance the state-of-the-art in combination with reflectors as primary sensors. One of our aims is to address these through a three-year dedicated R&D activity towards a prototype LNA-wideband feed in the attractive 1-4 GHz frequency range. Although more limited than the intrinsic bandwidth the 11-feed offers, the proposed reduced range makes our approach more realistic for the purpose of high bandwidth, low frequency VLBI with high potential also for other areas of radio astronomy.

The key elements here are new transistor technologies that promise excellent results for ambient or modest cryo-cooled low noise amplifiers. These processes, like 70nm mHEMTs from OMMIC, 50nm mHEMTs from IAF (Fraunhofer), have been tested in lab conditions with good results and are now advanced enough for use in astronomical receivers intended for VLBI. For this to happen, DIVA will research, design and evaluate wide bandwidth and very low noise temperatures with a focus on low power.

(b) In the last few years VLBI backends were developed both in Europe as an EVN project - the Digital Base-Band Converter Vers. 2; DBBC2) and later in the US by NRAO/Haystack/Casper (Roach Digital Backend; RDBE) with bandwidths of 2 times 512 MHz. Both systems will eventually offer similar performance at comparable cost. The DBBC2 development has a few months head start and it is the first time for 40 years that the dependency of European VLBI on US developed hardware could be broken.

In this JRA the next generation samplers and FPGAs, which will become available soon, will be used to develop the next generation digital backend: the DBBC3. At least 8 GHz samplers and a Virtex 7 FPGA will be used. This new dedicated, straightforward VLBI backend with the new state of the art hardware will defend the present independent status of VLBI backend developments in Europe and will be able to compete on the market against the US competitors. A particular strength of our approach is that the existing DBBC2 systems could easily be upgraded in a cost-effective way.

The DBBC3 is one component that (together with state of the art receivers as developed in this activity) could enable "first class" VLBI with "new" dishes from dm to mm and sub-mm wavelengths.

The leader of this activity is Walter Alef.

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