说明：  fpga实现dds和下变频。DDS 技术具有频率切换时间短，频率分辨率高，频率稳定度高，输出信号的频率和相位可以快速切换，输出相位可连续，并且在改变时能够保持相位的连续，很容易实现频率、相位和幅度的数字控制。它在相对带宽、频率转换时间、相位连续性、高分辨率以及集成化等一系列性能指标方面远远超过了传统频率合成技术。因此在现代电子系统及设备的频率源设计中，尤其在通信领域，直接数字频率合成器的应用越来越广泛。
(fpga implementation dds and downconversion. DDS technology with frequency switching time is short, high frequency resolution, the frequency stability is high, the output signal frequency and phase can be quickly switched, the output phase can be continuous, and changes to maintain a continuous phase, it is easy to achieve frequency, phase, and amplitude digital control. It is the relative bandwidth, frequency conversion time, phase continuity, and a series of high-resolution and integrated performance indicators far beyond the traditional frequency synthesis. Therefore, in modern electronic systems and equipment design frequency source, especially in the field of communication, direct digital frequency synthesizer is used more widely.)