The compatibility of Data Acquisition Systems of RT-32 and RT-13 Radio Telescopes

L. V. Fedotov

Transactions of IAA RAS, issue 57, 34–40 (2021)

DOI: 10.32876/ApplAstron.57.34-40

Keywords: radio telescope, VLBI, data acquisition system, digital signal processing, separation of narrow-band channels

Abstract

The RT-32 radio telescopes of the Quasar VLBI Network are equipped with narrow-band (up to 32 MHz) multichannel data acquisition systems, that were developed in the early 2000s and ensured compatibility with similar foreign systems. The systems installed on the new RT-13 radio telescopes differ significantly in the wide (512 MHz) bandwidth of digital channels and make it possible to compensate for the loss of radio interferometer sensitivity due to the reduction in the size of the antennas. The modern concept of VLBI development (VGOS) provides for broadband conversion and signal recording, but assumes compatibility with previously created narrowband systems. To unify the RT-13 equipment within the framework of this concept, the compatibility of the data acquisition systems on the RT-13 and RT-32 radio telescopes is required. For a number of years, IAA RAS has been conducting research and development in the direction of ensuring the compatibility of domestic data acquisition systems. The paper is devoted to the generalization of the results of these works, the analysis of the problems that arose in this case, the possibilities and the results of their solution. For the compatibility of broadband and narrowband data acquisition systems, it is necessary to arbitrarily select narrowband channels in each broadband channel, similar to those of a narrowband system. However, in this case, the sensitivity of the radio interferometer naturally decreases due to the smaller size of the RT-13 antenna, which must be taken into account when planning observations. Allocation of narrow-band channels directly on the RT-13 radio telescope makes it possible to 4 times reduce the data flow transmitted from the radio telescope to the correlator, and can be carried out using a special digital converter developed at the IAA RAS. When this converter was created, the problems of eliminating signal distortions and possible synchronization violations associated with digital processing of wideband signals in FPGAs were successfully solved. It is necessary to take into account the possibility of phase jumps during multiphase filtering of the phase calibration signal and the need to correctly select the repetition frequency of the pulses of this signal if digital separation of narrowband channels from broadband channels is used. The results of studies on the RT-13 radio telescope of the indicated digital converter of data streams were used in the development of a new unified Multifunctional Digital Backend system for radio telescopes (MDBE). It is compatible with other systems of RT-13 and RT-32 radio telescopes and provides VLBI observations with both broadband and narrowband signal recording. Equipping all radio telescopes of the Quasar VLBI Network with such systems will make it possible to maximize the compatibility of signal conversion equipment on them.

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