SARAS CD/EoR Radiometer: Design and Performance of the Digital Correlation Spectrometer.

Girish, B. S.; Srivani, K. S.; Subrahmanyan, Ravi; Udaya Shankar, N.; Singh, Saurabh; Jishnu Nambissan, T.; Sathyanarayana Rao, Mayuri; Somashekar, R.; Raghunathan, A.

In the currently accepted model for cosmic baryon evolution, Cosmic Dawn (CD) and the Epoch of Reionization (EoR) are significant times when first light from the first luminous objects emerged, transformed and subsequently ionized the primordial gas. The 21 cm (1420 MHz) hyperfine transition of neutral hydrogen, redshifted from these cosmic times to a frequency range of 40 MHz to 200 MHz, has been recognized as an important probe of the physics of CD/EoR. The global 21 cm signal is predicted to be a spectral distortion of a few 10's to a few 100's of mK, which is expected to be present in the cosmic radio background as a trace additive component. Shaped Antenna measurement of the background RAdio Spectrum (SARAS) is a spectral radiometer purpose designed to detect the weak 21 cm signal from CD/EoR. An important subsystem of the radiometer, the digital correlation spectrometer, is developed around a high-speed digital signal processing platform called pSPEC. pSPEC is built around two quad 10-bit analog-to-digital converters (EV10AQ190) and a Virtex 6 (XC6VLX240T) field programmable gate array, with provision for multiple Gigabit Ethernet and 4.5 Gbps fiber-optic interfaces. Here, we describe the system design of the digital spectrometer, the pSPEC board, and the adaptation of pSPEC to implement a high spectral resolution (61 kHz), high dynamic range (1 0 5 :1) correlation spectrometer covering the entire CD/EoR band. As the SARAS radiometer is required to be deployed in remote locations where terrestrial radio frequency interference (RFI) is a minimum, the spectrometer is designed to be compact, portable and operating off internal batteries. The paper includes an evaluation of the spectrometer's susceptibility to RFI and capability to detect signals from CD/EoR.

RADIO interference; BARYONS; ANALOG-to-digital converters; FIBER optics; SPECTROMETERS

Journal of Astronomical Instrumentation, 2020, Vol 9, Issue 2, pN.PAG

2251-1717

Academic Journal

10.1142/S2251171720500063