  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.
