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.

SARAS是一项正在进行的实验，旨在检测宇宙黎明（CD）和电离时代（EoR）预期产生的红移全局21厘米信号。标准宇宙学模型预测信号将出现在红移范围\（z \ sim \）中6–35，对应于40–200 MHz的频率范围，在3 K宇宙微波背景下幅度为20–200 mK的频谱失真。由于信号的频率可能跨越多个八度，并且该频率范围主要受到强烈的地面射频干扰（RFI）以及银河系和银河系起源的天体物理前景的影响，这些前景的亮度温度要高几个数量级，因此需要设计用于测量的辐射计这个微弱的信号是一项具有挑战性的任务。至关重要的是，仪器系统不要在测量的天空光谱中导致加法或乘法混淆光谱结构，因此不能检测到21厘米的微弱信号。在这里，我们介绍了SARAS 3版本的接收器的系统设计。改进设计的新功能包括Dicke切换，双重差分和光学隔离，可提高加性和乘性系统的校准和拒绝精度。我们推导并提出了SARAS 3接收器配置和校准方案的测量方程式，并提供了使用各种精密端接进行的实验室测试的结果，这些端接使辐射计接收器的性能符合科学目标。