Development and Application of Real-Time Baseband Spectral Line Digital Backend

The Qitai 110 radio telescope plans multiple research fields of radio spectral lines science based on its large aperture and advanced receivers. Multi-scientific planning and the staged construction of UWB and PAF receivers present significant challenges for developing spectral line digital backends. These include addressing the challenges of digitizing, processing, and transmitting massive observational data, building an easily scalable and iterative architecture to minimize time-to-science, and providing highly potential data foundations. As the QTT precursor instrument verification, we proposes the baseband-based spectral line digital backend and implements it via SNAP+GPU in this article. The SNAP-based RF digital baseband frontend constructs two digitization links(1000 MHz, 8-bit), two quad-channel parallel digital signal preprocessing modules, a quantization module, and a finite state automaton packaging module. It completely achieves a 100 MHz bandwidth RF link from the analog RF signal to high-speed Ethernet transmission. The GPU node receives preprocessed baseband packets, constructs the PASDADA-based ring buffer for lossless unpacking and distributing to the GPU, and implements real-time reception and caching of high-speed network data streams. It achieves real-time spectrum analysis with 3.051 kHz frequency resolution for 100 MHz baseband data. We verified the system performance via experimental testing and observation of spectral lines in NSRT. For the QTT's universal baseband backend, the SNAP frontend can be seamlessly migrated to RFSoC, enabling a 5x instantaneous bandwidth and a 10x data throughput. The low-coupling frontend and backend submodules can be easily extended to multiple nodes.