You can define total harmonic distortion plus noise (THD+N) with the following equation:
Alternatively, other sources define THD+N with the following equation:
When the fundamental tone dominates the signal, the previous two definitions are equivalent. The NI Sound and Vibration Measurement Suite uses the definition in the first equation. In the first equation, the power in the harmonic components and noise is divided by the total signal power to compute the THD+N. The THD+N is the inverse of the SINAD. When expressed in decibels, the THD+N is the negative of the SINAD.
The SVT THD+N VI identifies the fundamental tone and removes the fundamental tone from the input signal to measure the power in the harmonics and noise. The SVT THD+N VI identifies the tone with the highest amplitude as the fundamental tone unless you wire a value to the expected fundamental frequency [Hz] terminal.
Specify a frequency range to refine the measurement. The measured power in the total signal is distributed across the entire measurement bandwidth as a result of the noise in the measured signal. You can limit the measurement to a particular frequency range, exclude low frequency noise, or exclude high frequency noise by selecting an appropriate frequency range. For example, if you want to specifically exclude DC energy from the measurement, you must set the start frequency to 8(df). If there are frequency components of interest below 8(df) that are not DC, reduce df by increasing the measurement duration.