Study And Design Of Wideband Digital Down Converter

With the continuous development of radio technology, from the original analog wireless communication to digital wireless communication, and to the current popular software radio, the evolution of communication technology has made a higher requirement of the communication devices. As a bridge between the antenna and the signal demodulation circuit, the downconverter's performance directly influences the sensitivity of the device and a number of other important indicators. The traditional downconverters usually adopt analog structure which has a better performance. However, it brings much inconvenience to design, make and debugging because of its bulky size, system redundancy and especially the complexity of the filter design. This problem can be easily solved by using digital circuit to make the downconverter. The whole system, which quantizes the signal by an analog to digital converter (ADC) and then do the digital signal processing such as down-conversion and filtration by a high-speed digital signal processer, will be simpler because almost all of the signal processing is done by software with high flexibility. But the bandwidth of the sampled signal cannot be very wide, because it is limited by the ADC sampling rate. Therefore, the design of the wideband digital downconverter becomes the focus of current research.In this paper, a new solution of wideband digital downconverter has been proposed via the study of quadrature down-conversion and Direct Digital Synthesis(DDS) technology, and it's a quadrature digital downconverter that uses DDS output stray frequency spectrum for mixing. The output frequency spectrum of the previous DDS is relatively low, but through the analysis of the output frequency spectrum, many higher mirror frequency spectrums are found besides the main fundamental frequency spectrum. The mirror frequency spectrum are provided to the wideband digital downconverter as the quadrature local oscillator (LO) for mixing and the purpose of wideband digital down-conversion is achieved.Firstly, this paper makes a specific comparison between various types of receivers and deeply analyzes the theory of signal sampling, multirate signal processing and quadrature frequency conversion which is needed by digital downconverter. Secondly, it comes up with the mirror frequency spectrum mixing scheme according to the analysis of the DDS output frequency spectrum, and based on this scheme it has designed the wideband digital down-conversion system and emulated each part of it. At last, the experimental test shows that the performance of the wideband digital downconverter has achieved the expected goals, and such design which is on the basis of DDS output mirror frequency spectrum for mixing is feasible.