Research Of Digital Down Conversion With FPGA

Digital frequency conversion is the core technology of Software-defined Radio. The function of digital down conversion is to move the intermediate frequency which is provided by A / D sampling to the baseband. The ultimate goal is to achieve the extraction of signal. Compared to common analog converter, the main advantage of digital converter is the ideal performance of frequency step and frequency interval. Also, the digital converter is easier to control, modify and achieve. The efficient algorithm structure and the programmable property of FPGA extend the performance of digital down conversion tremendously. Meanwhile, the problems in analog converter are solved properly in digital down conversion, such as the nonlinear of mixer, frequency stability in the analog local oscillator, sideband, phase noise, temperature drift and conversion rate.Digital down conversion system is composed of numerical controlled oscillator (NCO), mixer and low pass filter. In this study, the research is focused on digital down conversion, and the design program of general system is proposed. Firstly, NCO is designed using the idea of direct digital synthesizer (DDS). NCO is achieved by using the IP CORE provided by Xilinx, which is easy to achieve and has high frequency resolution. Mixer is a multiplier in fact and is easily realized by hardware. Secondly, the digital filter is designed using Matlab FDATool digital filter design toolbox and the call of ISE's IP CORE. Digital filter extractor is achieved using integrator comb filter (CIC) and the half-band filter (HB) in the cascade form. Finally, FIR shaping filter is achieved using distributed algorithms. The programming of the modules above is completed through hardware description language (Verilog) in the ISE software, and the digital down conversion system is ultimately composed.Each module is tested with simulation tool XST provided by ISE after the completion. The tests show that each module can work normally to meet the design requirements.