Synchronization and Delay Stability of Signals in the Multifunctional Digital Backend of Radio Telescope

S. A. Grenkov, L. V. Fedotov

Transactions of IAA RAS, issue 67, 14–23 (2023)

DOI: 10.32876/ApplAstron.67.14-23

Keywords: radio telescope, digital Data Acquisition System, synchronization, instrumental delay of signals, measurements

Abstract

The instrumental delay in the signal path of the radio telescope must be taken into account when conducting radio interferometric observations. The stability of this delay, as well as possible signal synchronization errors, play an essential role in ensuring the accuracy of VLBI measurements. One of the main elements of the signal path of a radio telescope is a Data Acquisition System. Currently, all radio telescopes of the Quasar VLBI network are equipped with new digital Multifunctional Digital Backend (MDBE) systems, which differ in a wide operating frequency band of input signals (up to 2 GHz) and, accordingly, in the high speed of analog-to-digital conversion (4 Gbps). Delay and synchronization of signals in such a system require special research. The article discusses the effect of instrumental signal delay stability on the accuracy of radio interferometric measurements, substantiated the requirements for this stability. Signal synchronization in the Multifunctional Digital Backend system is discussed in detail. The corresponding time charts are given. The features and advantages of technical solutions for synchronizing signals embedded in the considered multifunctional digital system are shows. The procedure is described and the results of experimental studies of delay and phase stability of signals in a multifunctional digital conversion system are presented. At the same time, both the signal analysis capabilities provided in the system and external standard measuring devices are used. Studies have shown that, unlike the previously used Data Acquisition Systems in the new multifunctional system, the analog-to-digital conversion process is synchronized with time stamps and starts again every second. This eliminates errors in synchronizing the input samples within each second. The phase instability introduced by the signal path of the multifunctional system is an order of magnitude less than the calculated error in determining the signal delay during VLBI observations using broadband channels and practically cannot affect the results of observations. The amount of signal delay during its processing in the channel of the multifunctional system varies widely depending on the observation mode and the tasks solved by the system. However, it remains quite definite and unchanged during the observation session.

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