如何正确使用 Multi-Instrument (万用仪)
测量总谐波失真（THD）及其相关参数
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目录
一、前言 ................................................................................................................................................................ 4
1.1 失真的分类 ................................................................................................................................................. 4
1.1.1 线性失真 ............................................................................................................................................. 4
1.1.2 非线性失真 .......................................................................................................................................... 4
1.1.2.1 谐波失真 ....................................................................................................................................................... 4
1.1.2.2 非谐波失真 ................................................................................................................................................... 4
1.2 概述 ............................................................................................................................................................. 4
二、THD、THD+N、SINAD、SNR、ENOB、NL、SFDR 的定义 ............................................................. 5
2.1 总谐波失真 THD (TOTAL HARMONIC DISTORTION) ................................................................................... 5
2.2 总谐波失真及噪声 THD+N (TOTAL HARMONIC DISTORTION PLUS NOISE) .............................................. 5
2.3 信纳比 SINAD (SIGNAL TO NOSIE AND DISTORTION RATIO) ...................................................................... 6
2.4 信噪比 SNR (SIGNAL TO NOSIE RATIO) ...................................................................................................... 6
2.5 有效位数 ENOB (EFFECTIVE NUMBER OF BITS) ......................................................................................... 6
2.6 噪声电平 NL (NOISE LEVEL) ...................................................................................................................... 6
2.7 无寄生动态范围 SFDR (SPURIOUS FREE DYNAMIC RANGE) ..................................................................... 7
三、 如何避免或减小频谱泄漏 ........................................................................................................................... 8
3.1 什么是频谱泄漏.......................................................................................................................................... 8
3.2 频谱泄漏的解决方法 .................................................................................................................................. 9
3.2.1 整周期采样(Full-Cycle Sampling) / 相干采样(Coherent Sampling)法 .............................................. 9
3.2.2 加窗采样法 ........................................................................................................................................ 10
3.2.3 选择整周期采样还是加窗采样？ .................................................................................................... 11
四、如何避免将量化噪声误测为谐波失真 ...................................................................................................... 13
4.1 避免采样频率与信号频率之比为整数或可相约 ................................................................................... 13
4.2 为测试信号添加抖动（DITHERING） ...................................................................................................... 15
五、软件自环验证和硬件自环验证 .................................................................................................................. 18
5.1 为什么要进行软件自环和硬件自环验证？ ............................................................................................ 18
5.2 例子：RTX6001 音频分析仪的硬件自环测试 ....................................................................................... 18
5.3 例子：更多的声卡硬件自环测试 ............................................................................................................ 21
5.4 用仿真失真信号估计软件测量准确度 .................................................................................................... 24
六、谐波失真残余波形 ...................................................................................................................................... 27
6.1 FIR 数字滤波法 ........................................................................................................................................ 27
6.1.1 谐波失真中仅含偶次谐波 ................................................................................................................ 29
6.1.2 谐波失真中仅含奇次谐波 ................................................................................................................ 30
6.1.3 交越失真 ........................................................................................................................................... 31
6.1.4 削顶失真 ........................................................................................................................................... 33
6.2 谐波分解重构法........................................................................................................................................ 35
6.2.1 第一步：通过谐波分解生成相应的多音合成配置文件 ................................................................ 35
6.2.2 第二步：由多音合成配置文件合成谐波失真残余波形 ................................................................ 35
七、THD 和 THD+N 随频率或幅度（功率）变化曲线 ................................................................................. 37
八、谐波失真频谱图常见的识图误区 .............................................................................................................. 39
8.1 噪声电平 ................................................................................................................................................... 39
8.2 基波和谐波的幅度.................................................................................................................................... 43
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