Mark5B格式器及10G网络传输系统在VLBI中的应用

由于遥远的天体发出的无线电信号极其微弱,信噪比极低,VLBI系统要想得到较高的测量精度,必须尽量加大测量带宽和提高采样位数,但是这样会导致VLBI终端(比如数字基带转换器)产生的观测数据激增。传统的VLBI终端数据传输系统采用VSI接口,数据最高传输速率限制在2 Gbps,且数据记录设备必须采用定制的Mark5B设备,极其不灵活,因此已经不能适应现在VLBI系统的观测需求。为了提高数据传输速率,增加数据记录设备的灵活性,上海天文台新研制的基于多相滤波器组和快速傅里叶变换方式的数字基带转换器的数据传输系统采用了高速灵活的10 G网络接口。10 G网络系统中数据传输采用报文交换方式,因此数据到达接收端的时间不是精确可靠的,这要求数据在进入10 G网络接口之前必须已经具有标准的VLBI数据格式,所以在10 G网络前端设计了Mark5B格式器。详细介绍了基于现场可编程门阵列的Mark5B格式器及10 G网络传输系统的设计原理,并在文章的最后通过三组实验验证了其功能的正确性和性能的稳定性。

A MarkSB Formatter and a 10G Network Transmission System Applied to VLBI

Radio signals from distant radio sources are extremely weak and have very low signal-to-noise ratios when observed with current radio telescopes. In order for VLBI data to reach high measurement accuracies, large measurement bandwidths and sampling bits need to be adopted, resulting in huge amounts of observation data produced by the VLBI terminals （such as Digital Base-Band Converters）. Traditional data transmission systems of VLBI terminals use VSI interfaces, which cannot meet the requirements of broadband VLBI observation. The limitation of VSI interfaces is due to two factors： First, their data transmission speeds do not exceed 2Gbps, and second, their data-recording equipments are conformed to the rather rigid MarkSB format. We have designed a 10G network system for a new Digital Base-Band Converter （which is developed by the Shanghai Astronomical Observatory based on polyphase-filter banks and the FFT） to enhance its flexibility and transmission speed. The data-transmission method of this system is the packet switching, so a data arrival time at the data receiver is not accurate and reliable. This requires that data must be of a standard VLBI format before being processed by the 10G system. To achieve this we have also designed a MarkSB data formatter to convert data before being sent to the 10G system. This paper describes the principles of the MarkSB formatter and the 10G network system, which are both based on FPGA. The final part of the paper describes three sets of experiments verifying the accurateness and stableness of our designs.