Hi,
Does this sine wave look like about 7% THD?
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I am wondering cause it looks pretty noisy, but the method of subtracting out the sine wave from the signal gives 7% THD.
cheers, Jamie
With all that noise, it's hard to see how much actual harmonic distortion there is. It looks as if it could be 7% THD plus noise, though.
-- John
John is right. Subtraction of the fundamental results in distortion plus noise. To measure THD, you must extract (via sharp bandpass filters) each harmonic component. You then take the square root of the sum of squares of the harmonics to get the THD. Audio analyzers automate this process, but they are very expensiv Regards, Jon
This might help
havent used it in years, but it might prove useful
martin
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Looks like it wants to be lowpass-filtered :-)
-- Regards, Joerg http://www.analogconsultants.com/
Hello Jamie,
Assuming that the frequency spectrum of the noisy deviation from a sine wave is in the band of interest, this one can have (N+D)/S = 0.07 (7%), but I expected less then 7%, because it looks like a reasonable sine wave. Did you square the components before adding, as THD is defined as a power ratio.
If it was a pure sinewave with additive noise (within band of interest), I expected N/S to be better (< 0.004, power ratio).
Best regards,
Wim PA3DJS
Hello Jamie,
Assuming that the frequency spectrum of the noise deviation from a sine wave is in the band of interest, this one can have (N+D)/S = 0.07 (7%), but I expected less then 7%, because it looks like a reasonable sine wave. Did you square the components before adding, as THD is defined as a power ratio.
If it was a pure sine wave with additive noise (within band of interest), I expected N/S to be better (< 0.004, power ratio).
Best regards,
Wim PA3DJS
Given that you have captured the waveform as a .jpg, and that you've subtracted the fundamental, I assume you have the data in digital form, or it would be reasonably easy to get it into that form. Then just run an FFT on it, and see where the energy is spectrally (versus frequency). It would be best if the block you FFT contains an integral number of cycles of the fundamental; else be careful to window it properly. All the tools you need to do the windowing, FFT, converting to dB, and displaying graphically, are contained in the free-to-download Scilab. As others have pointed out, you will almost certainly find that most of the energy is in what looks like noise on the waveform--it appears to not repeat from cycle to cycle, so it's apparently not harmonically related to the sinewave.
Cheers, Tom
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