Progress of Testing of a VLBI Digital Baseband Converter

With the development of A/D converters,it becomes practical to sample the radio signals at frequencies of 1G~2GHz.Furthermore, the development of digital signal processing circuits such as FPGA makes it possible to achieve with digital circuits many applications at high frequencies that tradionally require analog circuits.BBCs (Base Band Converters)are important in VLBI systems.A BBC converts signals in a given band down to the base band.Currently used BBCs are mostly based on analog circuits.It is difficult to maintain these analog circuits since many parts are no longer produced.It has become a trend to develop digital circuits BBCs (DBBCs).
Research on digital filter and DBBC in the SHAO (Shanghai Astronomical Observatory)started in 2002.The SHAO research program on VLBI DBBCs was launched in 2007.The VLBI DBBC system developed by the SHAO consists of an analog part and a digital part.The paper describes the structure and performance of such a DBBC.
An experiment on a VLBI was conducted in July,2009 with the help of the CVN (Chinese VLBI Network)to analyze the performance of the VLBI DBBC.In the experiment radio sources were observed as the signal sources.Sources with high flux densities and sources with low flux densities were both included in the observation.Data were recorded using suits of VLBI DBBCs installed in the Yunnan Observatory and the Urumuqi Station.The ABBCs (Analog BBCs)at the two locations were also used in the experiment for comparison.The recorded data were correlated using a CVN correlator.The fringe SNRs of the Kunming-Urumuqi baseline were calculated from the output of the correlator.We conclude from the experiment that the VLBI DBBC of the SHAO has better bandpass characteristics than a traditional ABBC and the VLBI DBBC reaches higher fringe SNR than an ABBC.