Overview of Modern Digital Data Acquisition and Recording Systems for VLBI Radio Telescopes

L. V. Fedotov

Transactions of IAA RAS, issue 55, 24–35 (2020)

DOI: 10.32876/ApplAstron.55.24-35

Keywords: Very Long Base Interferometry, Data Acquisition System, recording system, digital signal processing, Field Programmed Gate Array

Abstract

On modern radio telescopes, digital systems have practically supplanted analog equipment for converting and recording signals during radiointerferometric observations. However, there are very few overview papers about such systems both in our country and abroad. Often, scarce information about the existing and promising digital radio astronomical systems is scattered among various publications devoted to radio astronomy observations. The purpose of this paper is to analyze the current state and prospects for the development of domestic and foreign digital systems for VLBI radio telescopes. It is necessary to assess the state of things in this field and the most promising directions for improving these systems. Digital systems that have been developed in different countries in recent years were investigated by comparative analysis. The history of the development, structural diagrams, parameters and peculiarities of the following systems are considered: R2DBE (USA), DBBC3 (European Union), K6/GALAS (Japan), CDAS2 (China), KVN DAS (South Korea), BRAS and MDBE (Russia). The number of channels for digital processing of intermediate frequency signals in such systems can vary from 2 to 12. In each channel, digital streams are formed after the analog-to-digital conversion of signals, usually in VDIF format. Not only 2-bit quantization of signals is provided, but also the possibility of increasing the number of bits per each signal sample to 4, 8, 10 or 16. For digital signal processing, all systems use Field-Programmed Gate Arrays, since only they are able to provide the above processing in real time. It is important to use a standard Ethernet interface and high-speed optical lines to transmit digital streams to data recording equipment. With modern radio telescopes, this equipment is based on high-performance commercial off-the-shelf (COTS) hardware and open-source software. The main trend in the development of digital data acquisition systems is an increase in the total speed of the output information stream, which can reach 96 Gbps. Recording systems are characterized by the rejection of the development of specialized devices and the layout of the system based on the network storage technologies of a big data. Other trends in the development of modern digital systems on radio telescopes include: minimizing the use of analog methods and devices for signal processing; using not unique, but widely available and standardized digital devices; the placement of the digital system directly on the antenna of the radio telescope with transmission of digital streams to the recording system located in the equipment room via fiber-optic lines. An important direction of the development of digital data acquisition systems is the expansion of their functional capabilities and the creation of unified systems capable of providing both radiointerferometric and radiometric or spectral observations without replacement of equipment. It has been shown that Russian-made digital systems in terms of their parameters are not inferior to the best foreign samples, and in some characteristics they are superior to them.

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