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The Digital Down Converters for a Radio Astronomy Data Acquisition Systems

N. Koltsov, S. Grenkov

Известия вузов России. Радиоэлектроника, №5, 19–27 (2017)

Keywords: Radio telescope, digital signal converter, down converter

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The digital down converters can be used on the radio telescopes with small antennas which are equipped with digital data acquisition systems with bands Bs = 512 MHz. The down converters provide extracts a number of signals with bands dF = 16 or 8 MHz at specified frequencies from the digital sequence of samples of a wideband signal with the frequency fd = 1024 MHz, 4-level quantization and formatting of the extracted signals according to the international standard VDIF that used in radiointerferometry. This is necessary to connect a radio telescope with broadband data acquisition systems (for example, RT-13) to international radio interferometry networks (VLBI), which use systems with narrowband (up to 16 MHz) signals. The developed module is made on the basis of the programmable logic integrated circuit (FPGA) XC7K325T and provide extracts and formats up to 16 narrowband (16 or 8 MHz) signals from three digital broadband signals that received from the broadband channels of the radio telescope via fiber optic lines by the 10 gigabit ethernet. The resulting data stream is sent to the buffering device for subsequent transmission to the VLBI correlator. The digital down converters operating at a clock frequency fm = 128 MHz, allow to extract signals in band Bc = 64 MHz and are tuned with 10 kHz steps by digital oscillator synthesizers. The Bs band of input signal are splitted by 8 channels using 8-channel polyphase filters, which reduce the clock frequencies by eight times. The module provides the same mode of signal extraction and formatting, which is widely used in VLBI networks with astrometric and geodetic observations in two wave bands.