The Digital Down Converters for a Radio Astronomy Data Acquisition Systems

Abstract

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 AF = 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.