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    ETU “LETI” Researchers Prove the Effectiveness of Digital Signal Converter Systems in Radio Telescopes

     ETU “LETI” scientists in cooperation with colleagues from the Institute of Applied Astronomy of the Russian Academy of Sciences have developed an effective way to digitize narrowband signals, which will improve the reliability and performance of radio telescopes.

    Radio telescopes of VLBI complexes usually record several signals with relatively narrow (up to 32 MHz) bands, which are isolated by video converters from an analog intermediate frequency noise signal with bands up to 1 GHz. Band synthesis is used in the data processing. New small radio telescopes (e.g. RT-13) digitize broadband intermediate frequency signals.

    To integrate radio telescopes with broadband channels into existing VLBI networks where narrowband video frequency signals are recorded and processed, it is necessary to isolate signals with relatively narrow bands at given frequencies from a high-speed digital intermediate frequency signal and convert them to video frequencies.

    The module for digital isolation of narrowband signals, developed at the Institute of Applied Astronomy in 2019, solves this problem. It allows connecting the radio telescope RT-13 to the VLBI complex called Kvazar-KVO and international VLBI networks.

    “It is necessary to find out to what extent the replacement of the analog signal converter system by the module of digital signal isolation can affect the sensitivity of the radio interferometer and the accuracy of measurements of interference group delays of received radio emissions,” Nikolai Koltsov, Professor of the Department Branch of Radio Astronomy of ETU “LETI,” pointed out the problem.

    “This information is needed for rational planning of VLBI-observations using different types of signal converter system and for selection of reference sources of radio emission. It is also useful for the development of a multifunctional digital signal converter system to upgrade the radio telescopes RT-32 and equip new small-size radio telescopes.”

    The research team under the guidance of Professor Koltsovhas set a goal to determine the accuracy of measurement of interferometric group delay of a radio interferometer signal with a digital module for the isolation of recorded signals and to compare the sensitivity of interferometers with analog and digital signal isolation systems.

    For this purpose, researchers calculated the sensitivity losses of interferometers with different systems of detection of recorded signals. On the example of analog-to-digital converter R-1002M installed on radio telescopes RT-32 VLBI-complex Kvazar-KVO, it was found out that this index is about 3% in aggregate. The total loss of sensitivity, introduced by the digital module of narrowband signal isolation, does not exceed 0.5 %.

    The developers also compared the accuracy of a multichannel interferometer with the synthesis of bandwidth and an interferometer that records digital broadband IF signals without band synthesis. Studies have shown that the accuracy of the interference delay signal detection by a five-channel interferometer with 8 MHz bands when synthesizing the 512 MHz frequency band will be 2.5 times worse than the accuracy of a single-channel interferometer with the 512 MHz band.

    The accuracy of the five-channel 16 MHz interferometer for the 1 GHz frequency band will be 30% lower than that of the two broadband interferometers that cut off the radio receiver band.

    The efficiency of using a digital signal converter system on radio telescopes was confirmed during tests carried out at Zelenchukskaya and Badary observation points, which are part of the Kvazar-KVO network.

    “When replacing conventional signal converter systems with digital ones, the accuracy of measuring interference group delays in the received signals is virtually unchanged, and the sensitivity of the interferometer even slightly (by about 4%) improves. Complex amplification and transmission channels for broadband analog IF signals are replaced by fiber-optic digital signal transmission lines,” Nikolai Koltsov, Professor of the Department Branch of Radio Astronomy of ETU “LETI,” says.

    “When using the developed digital system, it becomes possible for radio telescopes to operate in the mode of recording IF broadband signals, which will significantly increase the sensitivity of the interferometer and expand the list of available reference sources used in VLBI observations.”

    The method of digital isolation of narrowband signals from the intermediate frequency band, developed by scientists of the Department Branch of Radio Astronomy of ETU “LETI,” will be implemented in a new multifunctional system of signal conversion and registration, created at the Institute of Applied Astronomy of the Russian Academy of Sciences.

    It is planned to use this system to modernize existing radio telescopes of the Kvazar-KVO complex. It will also be used in the design of new generation small-size radio telescopes. Advanced antenna irradiators and ultra-wideband receivers will improve the accuracy of radio astronomical measurements and open up new opportunities for space explorers.

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