Total Harmonic Distortion (THD) Sometimes Greater Than THD + Noise

Updated Jan 25, 2018

Reported In

Software

  • LabVIEW Full
  • LabVIEW Professional

Issue Details

I am using the SINAD Analyzer VI and Harmonic Distortion Analyzer VI to calculate Total Harmonic Distortion (THD) and THD + Noise (THD + N). For some signals, the THD is greater than THD + N. Is this an issue with the calculations, or is this expected behavior?

Solution

The following equations are used to calculate THD and THD + N:
  • THD = ∑Power (harmonics) / Power (fundamental)
  • THD + N = ∑Power (harmonics) + ∑Power (noise)) / Power (fundamental)
In some cases, the Power (noise) can be a negative value. In these cases, it is expected that THD will be greater than THD + N. 

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