Research On The Key Techniques For The Wideband Real-Time Pulsar Digital Receiver

Pulsars are rapidly spinning neutron stars whose discovery opens up a new research field for the study of radio astronomy. Pulsar digital backend is an important instrument for pulsar observation and search. It also provides a powerful tool for celestial spectral observations and the search for extrasolar planets, and so on. With the deepening of the research on pulsar radio astronomy, higher requirements of the pulsar observation equipment are also required. It requires high-speed real-time processing and continuous recording for pulsar signals under the conditions of wider bandwidth, higher time resolution and higher frequency resolution. The key techniques for the pulsar digital backend are studied within this thesis, and the pulsar digital backends with different bandwidths based on the software defined radio are developed according to the actual observation. The main research contents of this thesis are as follows:(1) Such sub-band decomposition algorithms for wideband signals as Polyphase Discrete Fourier Transform (PDFT) and Weighted Overlap-Add (WOLA) filter banks are studied, and a channelization processing algorithm based on an improved WOLA (TWOLA) filter bank is then presented according to the characteristics of the PDFT and WOLA filter banks. It solves the problem of insufficient hardware resources for a Field Programmable Gate Array (FPGA) chip in system design and makes the subsequent processing units identical for each sub-band. The channel characteristics of the IWOLA and WOLA filter banks are analyzed by MATLAB simulations. An application of the proposed filter bank algorithm in radio astronomy observation is discussed. The detailed analyses and comparisons of the IWOLA and WOLA regimes and the combined complex FFT algorithm on the FPGA hardware resource consumption are conducted. The IWOLA regime saves more on-chip RAM than the WOLA regime and the combined complex FFT algorithm.(2) An algorithm of a multilevel sub-band incoherent dedispersion is presented based on the studies of pulsar’s direct, tree and sub-band incoherent dedispersion algorithms, and its performance is analyzed by means of MATLAB simulations. The multilevel sub-band incoherent dedispersion algorithm does not consume any extra on-chip RAM resources and alleviates the pressure of data output interface, which improves the time resolution of the system under the premise of ensuring the frequency resolution. A system based on the proposed incoherent dedispersion is designed by using a hardware platform of a Xilinx Virtex-6 architecture pulsar digital backend. It realizes partly dedispersed function by hardware, and reduces the data output rate which lowers the amount of data processing for the dedispersion by software.(3) The system requirements for the pulsar digital backend are analyzed in detail, and a design scheme for the Xilinx Virtex-5 architecture pulsar digital backend is then presented and implemented. Accordingly, a design scheme for the Xilinx Virtex-6 architecture-based counterpart is then presented, and the Xilinx Virtex-6 architecture-based system has the advantages of strong processing power, small size, low power consumption and better extensibility. The Xilinx Virtex-6 architecture pulsar digital backend has better performance and more abundant hardware resources than the Xilinx Virtex-5 architecture counterpart. A design scheme for the pulsar digital backend with a bandwidth of 4 GHz is proposed based on the Xilinx Virtex-6 architecture hardware platform.(4) The FPGA programs and the corresponding control software of the incoherently dedispersed pulsar systems with different bandwidth ranging from 140 MHz to 1.2 GHz and different time and frequency resolutions are designed and implemented based on the Xilinx Virtex-5 architecture hardware platform. It includes the hi-speed DDC-based systems with the bandwidths of 300 MHz and 140 MHz, the WOLA filter bank-based systems with the bandwidth of 150 MHz and the time resolutions of 0.2 ms and 0.1 ms, and the combined complex FFT-based systems with the bandwidths of 600 MHz and 1.2 GHz. The saving amount of the FPGA hardware resources for the pulsar digital backend based on the WOLA filter bank is then analyzed by comparing the one of based on the hi-speed DDC.(5) The single channel wideband spectrometer with high frequency resolution, which is based on the Xilinx Virtex-6 architecture-based hardware platform, is optimized, resulting in stronger processing power of the hardware platform. Its performance is verified.In this paper, the research on the key techniques for the wideband digital backend is done well. The Xilinx Virtex-5 architecture-based pulsar digital backend is completed. A large number of pulsar observations are conducted by means of the Xilinx Virtex-5 architecture-based pulsar digital backends with different bandwidths from 140 MHz to 1.2 GHz at an observatory to verify its correctness and performance. The total number of the pulsars observed is more than 100 so far. The observable pulsar period can be several milliseconds and the pulsar radiation flux as low as 3 mJy. The proposed system has a maximum time resolution of 0.1 ms. The error of the pulse TOA is about 1μs within one observation. In this paper, the direct experience has been accumulated and technical support provided by designing and implementating the proposed backend for pulsar observations by means of the pulsar radio astronomy equipment developed independently in China.