Experimental Studies of an Ultra-Wideband Analog-to-Digital Converter for Radio Astronomy Equipment

S. A. Grenkov, A. V. Krokhalev, L. V. Fedoto

Transactions of IAA RAS, issue 58, 17–23 (2021)

DOI: 10.32876/ApplAstron.58.17-23

Keywords: radio telescope, ADC, digital signal processing

Abstract

Modern systems for receiving and converting radio astronomical signals on radio telescopes are developing towards expanding the operating frequency band and maximizing the use of digital signal processing methods. The use of ultra-wideband analog-to-digital converters (ADCs) in such systems makes it possible to abandon most of the analog devices in the signal chain of the radio telescope and replace it with compact digital backend. Direct sampling of RF signals without mixers and local oscillators in most of the frequency ranges used in radio astronomy, as well as in ultra-wideband radio astronomy receiving systems in accordance with the VGOS concept, requires an ADC with an operating signal sampling frequency of about 20 GHz or higher. One of the commercially available ADCs that can work with such a clock frequency is the HMCAD5831 chip from Hittite Microwave. Search for ways to use such ADCs to create digital radio astronomy systems requires an experimental study of the characteristics of this chip, taking into account the specifics of converting broadband radio astronomy signals. The paper presents the parameters, structure and features of the operation of this chip. To study the ultra-wideband ADC, an experimental setup was developed and manufactured on the basis of the evaluation board with HMCAD5831 and the digital signal processing board on an XC7K325T FPGA. The description of the methodology for studying the main characteristics of an ultra-wideband ADC, namely, the characteristics of the signal conversion, the amplitude-frequency response and the differential nonlinearity of the conversion is given in the paper. The results of measuring these characteristics, as well as the harmonic spectra and broadband noise signals after their conversion in the ADC are presented. It is shown that the accuracy of the installation and stability of the reference voltages, as well as the mutual synchronization of the ports to which the ADC output data is received, are essential for ensuring the correct operation of an ultra-wideband ADC. The main difficulties of using the HMCAD5831 chip in radio astronomy equipment are related to these circumstances. Studies have shown that the HMCAD5831LP9BE ADC provides stable and accurate conversion of broadband signals into 3-bit codes with a sampling frequency of up to 16 GHz. Increasing the clock frequency to 20 GHz or more requires special design, as well as careful and high-precision manufacturing of the printed circuit board for the ADC chip and all related devices. This type of ADCs can be used in radio astronomy. Signals of both harmonic and noise types can be successfully sampled, entered into the FPGA and converted into the desired form using multi-stream data processing methods (the number of parallel streams is more than 50), which is quite possible on the basis of modern FPGA resources. The main difficulty in this case will be the insufficient availability of ultra-high-frequency ADC chips, even in commercial execution in the conditions of trade sanctions.

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