论文的前两章介绍了课题研究的背景，数字基带转换器的国内外研究情况以及VLBI新技术的进展。

The first two chapters of this thesis will introduce the background of this project, the research status in domestic country and abroad, and the new technology on VLBI.

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软件无线屯的设计思想之一是将A／D转换器尽可能靠近天线，即把A／D从基带移到中频甚至射频，把接收到的模拟信号尽早数字化。

One of the important conceptions of Software Radio is to make A/D converter close to antenna, we should convert IF analog signal even RF analog signal into digital signal.

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