Development and application of a real-time baseband spectral line digital backend

Owing to its large aperture and advanced receivers, research plans for the Qitai 110 m radio telescope (QTT) include a variety of spectral line scientific studies. Sequential construction of receiver systems and multidisciplinary planning require overcoming serious challenges to spectral line digital backend development, notably to digitize, process, and transmit considerable quantities of observational data, to minimize time-to-science with an easily scalable architecture, and to provide robust, high-quality data. As a proof-of-concept for the QTT backend, here we implement a baseband spectral line digital backend with a SNAP+GPU architecture. The SNAP-based digital frontend comprises two digitization links (1000 MHz, 8-bit), two parallel quad-channel preprocessing modules, a quantization module, and a finite-state packaging module, generating a 100-MHz bandwidth digital link from the original analog signal through high-speed Ethernet transmission. The GPU node receives preprocessed baseband packets, constructs a ring buffer for lossless unpacking and distributing, with real-time data reception and caching, and conducts real-time spectral analysis (frequency resolution: 3.051 kHz) of the 100 MHz baseband data. We evaluated system performance experimentally using spectral line observations with the Nanshan 26-m radio telescope ((NSRT)). For the QTT, the SNAP digital frontend will be seamlessly migrated to a radio frequency system-on-chip (RFSoC) architecture, resulting in five- and tenfold increases in instantaneous bandwidth and data throughput, respectively. The low-coupling digital frontend and GPU node can be easily extended to multiple nodes.