A Design Of Radar Signal Source Based On DDS

As an important part of the radar system, the radar signal source is playing an increasingly important role in modern radar system. The quality of the radar signal source plays a decisive role in improving the performance of the radar system. Compared with the traditional methods of frequency synthesis, the direct digital frequency synthesis(DDS) technology as a new frequency synthesis technology has found more and more applications in the field of radar system design, due to its high frequency resolution, fast frequency agility, wideband property, high frequency stability, continuous phase variation and etc.In the first chapter, the direct frequency synthesis technology, the indirect frequency synthesis technology and the direct digital frequency synthesis technology are introduced at first, and then the advantages and disadvantages of these three kinds of frequency synthesis technology are discussed. Finally, the development and application of the direct digital frequency synthesis technology at home and abroad are introduced.The chapter two describes the realization of the DDS technology mathematically and introduces its four components, including the phase accumulator, the sine query table, the digital to analog converter and the low-pass filter. The spectrum characteristics of the DDS technology are discussed in ideal and non-ideal circumstances as well as the forming reasons of the spur waves. According to the forming reasons and characteristics of the phase truncation error and amplitude quantizing error, several methods for suppressing spur in the DDS system are presented.Based on the principle and the structure features of the DDS technology, a radar signal source system is designed which can produce four channels of sine signals and linear frequency modulated signal, a PCB board is developed with FPGA+DAC as the main core. Each major module of radar signal source is discussed, including the FPGA module, the DA module, the USB control module, the DDS control module, the low-pass filter module, the time control module and the power supply module. Besides, how to choose the chip of each module and how to connect each module in hardware is introduced as well.In the fourth chapter, the function of each major module of the system is realized in logic. To begin with, the SPI protocol which is usually used in configuration of chip’s register is introduced. The configuration of the clock chip is realized through the protocol. Secondly, the communications between the computer and the FPGA are realized using a USB chip. And then, a sine signal which can change frequency, phase and amplitude as needed is produced. Next, the major modules of the DDS technology, the phase accumulator and the sine query table, are realized in the FPGA, producing two commonly used signals in radar systems: the sine signals and the LFM signals. Finally, aiming at reducing the problem of spur in DDS system, a method by increasing the address’ s number of the sine query and compressing the waveform’s storage is proposed. The two methods reduce the phase truncation error and improve the quality of the output spectrum. The process of the DDS technology is improved and optimized as well.The main work in this thesis and what remained to be improved are summarized in the last chapter.