SARAS 3 CD/EoR Radiometer: Design and Performance of the Receiver

TL;DR

SARAS 3 is a specialized radiometer designed to detect the faint 21-cm signals from Cosmic Dawn and Epoch of Reionization, featuring advanced calibration and systematic rejection techniques to overcome terrestrial and astrophysical interference.

Contribution

The paper introduces the SARAS 3 receiver design with novel features like Dicke switching, double differencing, and optical isolation to enhance calibration accuracy and systematic rejection.

Findings

Laboratory tests demonstrate the radiometer's performance meets science requirements.

Design improvements effectively reduce systematic errors and interference.

Calibration scheme ensures reliable measurement of weak 21-cm signals.

Abstract

SARAS is an ongoing experiment aiming to detect the redshifted global 21-cm signal expected from Cosmic Dawn (CD) and the Epoch of Reionization (EoR). Standard cosmological models predict the signal to be present in the redshift range $z \sim $6--35, corresponding to a frequency range 40--200~MHz, as a spectral distortion of amplitude 20--200~mK in the 3~K cosmic microwave background. Since the signal might span multiple octaves in frequency, and this frequency range is dominated by strong terrestrial Radio Frequency Interference (RFI) and astrophysical foregrounds of Galactic and Extragalactic origin that are several orders of magnitude greater in brightness temperature, design of a radiometer for measurement of this faint signal is a challenging task. It is critical that the instrumental systematics do not result in additive or multiplicative confusing spectral structures in the measured sky spectrum and thus preclude detection of the weak 21-cm signal. Here we present the system design of the SARAS~3 version of the receiver. New features in the evolved design include Dicke switching, double differencing and optical isolation for improved accuracy in calibration and rejection of additive and multiplicative systematics. We derive and present the measurement equations for the SARAS~3 receiver configuration and calibration scheme, and provide results of laboratory tests performed using various precision terminations that qualify the performance of the radiometer receiver for the science goal.