Arcminute MicroKelvin成像仪数字关联函数的改进
A Digital Correlator Upgrade for the Arcminute MicroKelvin Imager
摘要：
位于剑桥附近的Mullard射电天文台的角分微开尔文成像仪（AMI）望远镜由于安装了一个新的数字相关器而得到了显著的增强，该数字相关器具有1.2 MHz的光谱分辨率。该系统取代了750兆赫分辨率的模拟滞后关联函数，旨在减轻射频干扰的影响，特别是地球静止卫星对低磁偏角观测造成的干扰。升级后的仪器由18个ROACH 2现场可编程门阵列平台组成，用于实现一对实时FX解调器-AMI的两个阵列中的每一个。新系统将来自每个AMI接收器的下变频RF基带信号分离为两个2.3 GHz宽的子带，每个子带以5 Gsps的8位精度进行数字化。这些数字数据流被过滤到2048个频率通道中，并使用FPGA硬件进行交叉相关，商用10 Gb以太网交换机提供高速数据互连。使用来自新的数字关联函数的数据形成的图像显示出比以前的系统在动态范围上的一个数量级的改进。在低赤纬下的观测能力也得到了显著提高。
英文摘要：
The Arcminute Microkelvin Imager (AMI) telescopes located at the Mullard Radio Astronomy Observatory near Cambridge have been significantly enhanced by the implementation of a new digital correlator with 1.2 MHz spectral resolution. This system has replaced a 750-MHz resolution analogue lag-based correlator, and was designed to mitigate the effects of radio frequency interference, particularly from geostationary satellites that contaminate observations at low declinations. The upgraded instrument consists of 18 ROACH2 Field Programmable Gate Array platforms used to implement a pair of real-time FX correlators -- one for each of AMI's two arrays. The new system separates the down-converted RF baseband signal from each AMI receiver into two 2.3 GHz-wide sub-bands which are each digitized at 5-Gsps with 8 bits of precision. These digital data streams are filtered into 2048 frequency channels and cross-correlated using FPGA hardware, with a commercial 10 Gb Ethernet switch providing high-speed data interconnect. Images formed using data from the new digital correlator show over an order of magnitude improvement in dynamic range over the previous system. The ability to observe at low declinations has also been significantly improved.
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