Design And Realization Of A DDS Radar Signal Generator For Microwave Remote Sensing

Microwave Remote Sensing is a remote detecting technique developed early in 1960s. Its active form, known as the microwave Radar remote sensing, has a lot of applications in agriculture, mapping, coast and ocean, forest, hydrology, usage of land and research of ocean ice. In recent years, it shows great potential in vital signal detection and early tumor detection. For different applications, different requirements are set for the Radar source, especially the waveform, working frequency and bandwidth, has big influence on the detecting result. So it is of great importance to design and realize a Radar signal generator for microwave remote sensing.The DDS technique has many advantages, such as accurate frequency and phase resolution, wide relative bandwidth, short frequency shifting time, continuous phase at frequency shifting, arbitrary waveform output and digital modulation to the signal. It fits the requirements of the microwave Radar source.In this paper, the DDS technique is used to generate Baseband signal, which is modulated with a local oscillator to get a radio frequency output. The Baseband signal generator uses FPGA as a controller, SDRAM as high speed waveform data memory, and high performance DAC as digital to analog converter. It can generate Baseband signals with frequency up to 60 MHZ and chirp signal with bandwidth up to 120 MHZ.As the digital structure is used, DDS technique has its inherent disadvantage in noise and side band suppression. We adopt two methods to improve it, firstly a low pass filter is designed to improve the SNR, secondly a pre-distortion method is used to make spectrum within band much flatter. At last, we get the output signal with the best SNR of 45～50 dB, and the flatness of in-band spectrum within 0.2 dB.