说明：  DDS的字长决定了正弦/余弦基准信号样点的个数和所产生频率信号的量化精度。 最小频率间隔决定了DDS所能产生信号的最小频率。DDS所产生信号的频率为最小频率间隔的整数倍。
(The word length of the DDS determines the number of sample points of the sine / cosine reference signal and the quantization accuracy of the generated frequency signals. The minimum frequency interval determines the minimum frequency that the DDS can generate. The frequency of the signal generated by the DDS is an integer multiple of the minimum frequency interval.)