A high-speed 4-Gsample/s 2-bit ADC system for radio astronomy

Abstract

In recent years, the use of very long baseline interferometer (VLBI) devices has increased in the geodesy and radio astronomy communities. To enable these devices to pick out very small variations, it is desirable to increase the sensitivity of the detection of the VLBI. For observing quasars, it is beneficial to increase the observation bandwidth and improve the sensitivity of detection. However, for such wide bandwidth systems, gigahertz-order sampling high-speed A/D converters are necessary and the technology for achieving these devices is not yet developed. In this work, we developed, using Si-bipolar LSI processes (SST-1B), a 4-Gsample/s 2-bit ADC LSI and created a high-speed ADC system. We evaluated the loss in this system due to phase stability, sampling jitter, and the uncertainty region of the comparator of the LSI to evaluate the coherence of the VLBI. We used a frequency transformer device and a picosecond phase comparator as the measurement device for measuring the phase stability of the gigahertz-order ADC system. From the measurement results, we determined that the Allen standard deviation over 1-s of integration time was 3.6 × 10−13, the sampling jitter was 13.4 ps, and the comparator uncertainty was 20 mV. For a wide bandwidth burst mode VLBI observation system with an observation frequency of 230 GHz, a coherence loss of 7.6 percent was observed. We then demonstrated the operation of the ADC system by detecting real radio astronomical observation spectra using this device.