I have a couple of questions about what total harmonic distortion is, and what the measurement means. The definition I've read most places is:(adsbygoogle = window.adsbygoogle || []).push({});

[itex]\frac{D}{S}[/itex] × 100% , where S is the amplitude of the fundamental frequency, and D is the amplitude of the sum of all of the harmonics.

A common method I've read to measure THD is as follows.

1) Insert a sine wave (the fundamental frequency) into the input of the device under test.

2) Using an RMS AC voltmeter, measure the output signal, and call it V_{1}.

3) Use a notch filter to notch out the fundamental frequency at the output.

4) Use the RMS AC voltmeter to measure the output with the fundamental frequency suppress This measures only the harmonic distortion, V_{2}+ V_{3}+ ... + V_{n}

5) Divide the second reading by the first reading giving you:

THD = ( V_{2}+ V_{3}+ ... + V_{n})/( V_{1}) × 100% = [itex]\frac{D}{S}[/itex]

The part that I question isdoes this give you [itex]\frac{D}{S}[/itex]? It would seem to me that the first measurement V_{1}would be the total amplitude, not just the amplitude of the fundamental frequency, and so I would think that this method would actually give you [itex]\frac{D}{S+D}[/itex]. Am I looking at this the wrong way?

Also I was wondering if the phase of the harmonics relative to the fundamental frequency makes a difference. If the fundamental and its harmonics are all in phase then it makes sense that they can be added arithmetically. If however there are phase disparities then measuring total amplitude with the voltmeter doesn't seem valid? Through a non linear device will the harmonics be in phase?

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# Total Harmonic Distortion, THN measurement

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