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In the currently accepted model for cosmic baryon evolution, Cosmic Dawn andthe Epoch of Reionization are significant times when first light from the firstluminous objects emerged, transformed and subsequently ionized the primordialgas. The 21 cm hyperfine transition of neutral hydrogen, redshifted from thesecosmic times to a frequency range of 40 to 200 MHz, has been recognized as animportant probe of the physics of CD/EoR. The global 21-cm signal is predictedto be a spectral distortion of a few 10's to a few 100's of mK, which isexpected to be present in the cosmic radio background as a trace additivecomponent. SARAS, Shaped Antenna measurement of the background RAdio Spectrum,is a spectral radiometer purpose designed to detect the weak 21-cm signal fromCD/EoR. An important subsystem of the radiometer, the digital correlationspectrometer, is developed around a high speed digital signal processingplatform called pSPEC. pSPEC is built around two quad 10 bit analog-to-digitalconverters and a Virtex 6 field programmable gate array, with provision formultiple Gigabit Ethernet and 4.5 Gbps fibre optic interfaces. Here we describethe system design of the digital spectrometer, the pSPEC board, and theadaptation of pSPEC to implement a high spectral resolution of about 61 kHz,high dynamic range correlation spectrometer covering the entire CD/EoR band. Asthe SARAS radiometer is required to be deployed in remote locations whereterrestrial radio frequency interference is a minimum, the spectrometer isdesigned to be compact, portable and operating off internal batteries. Thepaper includes an evaluation of the spectrometer's susceptibility to radiofrequency interference and capability to detect signals from CD/EoR.

SARAS CD/EoR Radiometer: Design and performance of the Digital Correlation Spectrometer