SARAS 3 CD/EoR辐射计：接收机的设计和性能
SARAS 3 CD/EoR Radiometer: Design and Performance of the Receiver
摘要：
SARAS是一个正在进行的实验，旨在探测宇宙黎明（CD）和再电离时代（EoR）预期的红移全球21厘米信号。标准的宇宙学模型预测该信号在红移范围$z \sim $ 6- 35，对应于频率范围40- 200 MHz，在3 K宇宙微波背景中是一个振幅为20- 200 mK的谱畸变。由于该信号可能跨越多个倍频程的频率，并且该频率范围由强的地面射频干扰（RFI）和亮温高出几个数量级的银河系和河外起源的天体物理前景主导，因此设计用于测量该微弱信号的辐射计是一项具有挑战性的任务。至关重要的是，仪器系统学不会导致在测量的天空光谱中出现加法或乘法混淆的光谱结构，从而排除对微弱的21厘米信号的检测。本文介绍了SARAS~3型接收机的系统设计。改进设计中的新功能包括Dicke开关、双差分和光学隔离，以提高校准精度，并拒绝加法和乘法系统学。我们推导并给出了SARAS~3接收机配置和校准方案的测量方程，并提供了使用各种精密终端进行的实验室测试结果，这些终端使辐射计接收机的性能符合科学目标。
英文摘要：
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.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
