Frequency Synthesizer Direct Digital Fpga Design

Direct digital frequency synthesizer(DDS) is an all digital frequency synthesizer, which is based on digital sampling technology, which consists of the phase accumulator, ROM look-up table, the digital to analog converter and the low-pass filter. Direct digital frequency synthesis technology has a high frequency resolution, fast frequency switching speed and frequency switching, low phase noise phase continuous advantages, has become the best in the modern frequency synthesis technology. DDS technology is widely used in aerospace, radar, instrument and meter, electronic warfare, communications and other fields.DDS design is completed by using FPGA. DDS technology is a departure from the concept phase, using the sampling theorem, direct sampling of reference sine signal, get different phase. Then the voltage amplitude technique to generate corresponding through digital calculation, finally filtering desired output frequency.Through the spectrum analysis and the simulation of the DDS, DDS output signal that is mainly affected by the actual conversion error of phase truncation error, ROM amplitude quantization error and DAC and other factors.The structure of 32 bit accumulator is improved, the structure of pipeline, can improve the speed of operation. Based on the analysis of the shortcomings of the traditional DDS structure, the paper presents an improved structure of DDS. The cycle accumulator structure, with overflow signal of the phase accumulator address cycle signal generator as the clock signal, without high output as accumulator address. This can be avoided from the source and frequency error. Using the address signal generator, the addressing cycle accumulator overflow pulse signal counting to achieve the ROM lookup table.Only 8 sample points of waveform data need data in the ROM table, than all the other algorithms are good for the ROM table compression effect data, and no loss of signal information, a simple algorithm. As long as the change of ROM in the lookup table of data, you can easily generate other periodic waveform.The corresponding peripheral circuit that DAC conversion circuit and low pass filter is designed.Using Matlab software simulation analysis of the DDS spectrum, using the VHDL language to realize the function of each module of DDS system, and the simulation is performed in Quartus II. The software design adopts the design idea of "top-down", design by modular, makes the design process simple, highlight the function of each module. Hardware simulation link to verify the design is correct, to ensure the reliability and safety of the system. The VHDL circuit simulation by program, downloaded to the PLD chip of Altera company carries on the simulation and hardware debugging.Finally, after the test, reached the design requirements.