直接数字式合成信号的杂散性能分析

Analysis of spur property of direct digital synthesis signal

为精确计算直接数字式合成 (DDS)技术输出频谱中杂散谱线的幅度和位置 ,建立了 DDS的杂散产生模型 ,确定了杂散的 3个主要来源 :相位累加器的截断 ,正弦函数表的有限精度和数模转换器的非线性。由此产生的杂散谱可根据 DDS的频率控制字和 2 N 的最大公约数进行分类 ,N是DDS相位累加器的字长。归于同类的杂散谱相互间存在对应关系。这一结果可用于精确计算杂散谱线的幅度和位置。将应用此方法预测的杂散谱线与数值仿真的结果进行了对比 ,二者完全符合。此方法只要做 N组离散傅立叶变换(DFT)就可以得到全部的杂散谱 。

DDS (direct digital synthesizers) error model has been dcvcloped to calculate the amplitude and position of spurious noise lines in the output spectrum of DDS. The model shows that these spurious lines come from three sources: the truncation of the phase accumulator, the finite precision of the sine lookup table, and the nonlinearity of the digital to analog converter. The spurious lines caused by those sources can be classified into different sets according to the greatest common divisor of the DDS frequency control word (FCW) and 2 N, where N is the word length of the DDS phase accumulator. The spurious lines in each set of spectra are all related. A new method based on this relation is introduced to calculate the amplitude and position of these spurious lines. The spurious lines predicted by this method compare well with the results of number simulations. This efficient method can resolve all the spurious spectra after only N discrete Fourier transforms.