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Total Harmonic Distortion
Real world devices introduce unwanted side effects called distortion in addition to the desired response. Total Harmonic Distortion (THD) and (THD+Noise) are two widely used measurements of this quantity. Both measurements are expressed as a percentage. The lower the THD value, the "cleaner" the response.
Total Harmonic Distortion (THD) - the ratio of the harmonic power to the fundamental power. It is computed by searching the entire spectrum to find the peak frequency (fundamental) and then calculating the total power in the harmonic frequencies. The THD level is then computed as the ratio of the total harmonic power to the fundamental power. Residual noise is not included in this calculation.
Total Harmonic Distortion + Noise (THD+N) - the ratio of the harmonic power plus noise to the fundamental power. It is computed by searching the entire spectrum to find the peak frequency (fundamental) and then calculating the total power in the remaining spectrum (harmonics plus noise). The THD level is then computed as the ratio of the total harmonic+noise power to the fundamental power.
THD+N values will almost always be greater than the THD values for the same signal.
A typical THD measurement will use a 1kHz reference test tone. This reference signal is passed through the device under test and its spectrum computed. It is often useful to employ long term averaging to cancel the random noise components and enhance the measurement accuracy.
It should be noted that the purity of both the D/A channel must exceed the device being measured if meaningful results are to be expected.
This utility window can be resized as required by the user and its contents will update anytime a new spectrum is computed. The window will contain the channel index and name; the channel name can be modified in the Calibration Settings.
Right clicking on the utility window will allow you to copy the contents to the clipboard.
Notes:
Hanning or Blackman smoothing window is recommended because of its low noise quality.
An FFT size of 2048 or greater is recommended in order to to provide adequate spectral resolution.
See Also: Measurement Accuracy , THD+N vs Frequency