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I have a circuit with rectifier and this cause the non-linearity and consequently harmonic. I show it on the oscilloscope and then collect the signal from the oscilloscope to the computer by using the Labview. The procedure is to find the Total Harmonic Distortion. Could anyone please guide me what I have to do in order to find the THD? I know that the f(input),f(sampling), N(number of samples)and M(number of cycles) are very important. Thanks in advance!

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  • $\begingroup$ So, this is a bit too broad; THD is defined in literature, so I'm sure you've come up with an approach so far. Discuss that! It's far easier to help you when it's clear where you're stuck. $\endgroup$ – Marcus Müller Jan 16 '20 at 22:14
  • $\begingroup$ Also, this feels like you want to calculate what you're looking for in a purely mathematical consideration before you do any circuit analysis with an oscilloscope. Did you do that? How can you calculate the THD of that mathematical representation of perfect rectifier? That gives you info on where to look (hint: spectrum). $\endgroup$ – Marcus Müller Jan 16 '20 at 22:22
  • $\begingroup$ my input signal is a (sin) signal to the rectifier and the task is to make an automatic calculation to calculate the THD.I collected 4096 samples of my rectified signal that I already collected from the oscilloscope through Labview. I went from the Discrete to FFT then I stuck there ... $\endgroup$ – SnowMan Jan 16 '20 at 22:30
  • $\begingroup$ yeah, as said, with pen and paper, calculate where the harmonics of a perfect rectifier would lie – and then just look at the same places in your labview software. $\endgroup$ – Marcus Müller Jan 16 '20 at 22:37
  • $\begingroup$ Your question is a bit too broad. However, there are standards in the power industry on how to measure the THD. For example, the IEC 61000-4-30 specifies a method. webstore.iec.ch/preview/info_iec61000-4-30%7Bed3.0%7Db.pdf However it's not free $\endgroup$ – Ben Jan 17 '20 at 3:07
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this depends a lot how "good" this estimate needs to be. Here is a "rough & dirty" method.

  1. Put your scope in spectrum analyzer mode. Make sure you see all harmonics.
  2. Look where the highest harmonic is and that's still visibly sticking out over the noise floor.
  3. Quadruple that frequency and use this as your sample rate.
  4. Sample your signal for a sizable portion. You want at least a 1000 cycles
  5. Filter the signal with a steep notch filter at the line frequency
  6. Throw away the first 50 cycles or so.
  7. Divide the power of the filtered signal by the power of the unfiltered signal. That's your THD estimate
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