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The global 21 cm signal from Cosmic Dawn (CD) and the Epoch of Reionization(EoR), at redshifts $z \sim 6-30$, probes the nature of first sources ofradiation as well as physics of the Inter-Galactic Medium (IGM). Given that thesignal is predicted to be extremely weak, of wide fractional bandwidth, andlies in a frequency range that is dominated by Galactic and Extragalacticforegrounds as well as Radio Frequency Interference, detection of the signal isa daunting task. Critical to the experiment is the manner in which the skysignal is represented through the instrument. It is of utmost importance todesign a system whose spectral bandpass and additive spurious can be wellcalibrated and any calibration residual does not mimic the signal. SARAS is anongoing experiment that aims to detect the global 21 cm signal. Here we presentthe design philosophy of the SARAS 2 system and discuss its performance andlimitations based on laboratory and field measurements. Laboratory tests withthe antenna replaced with a variety of terminations, including a network modelfor the antenna impedance, show that the gain calibration and modeling ofinternal additives leave no residuals with Fourier amplitudes exceeding 2~mK,or residual Gaussians of 25 MHz width with amplitudes exceeding 2~mK. Thus,even accounting for reflection and radiation efficiency losses in the antenna,the SARAS~2 system is capable of detection of complex 21-cm profiles at thelevel predicted by currently favoured models for thermal baryon evolution.

SARAS 2: A Spectral Radiometer for probing Cosmic Dawn and the Epoch of Reionization through detection of the global 21 cm signal