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THD of sinewave

Discussion in 'Electronic Design' started by Jamie Morken, Feb 26, 2008.

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  1. Jamie Morken

    Jamie Morken Guest

  2. 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.
     
  3. Guest

    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
     
  4. On Tue, 26 Feb 2008 08:23:49 -0800 (PST), in sci.electronics.design
    This might help
    http://audio.rightmark.org/products/rmaa.shtml

    havent used it in years, but it might prove useful


    martin
     
  5. Eeyore

    Eeyore Guest

  6. Eeyore

    Eeyore Guest

    There a number of free programs that will calculate audio THD using a sound
    card.

    Graham
     
  7. Eeyore

    Eeyore Guest

    Remove the noise and I doubt it'll be over a couple of percent.

    Graham
     
  8. Joerg

    Joerg Guest

  9. john jardine

    john jardine Guest

    Yep. Same thought here.
     
  10. Wimpie

    Wimpie Guest

    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
    www.tetech.nl (Dutch)
     
  11. Wimpie

    Wimpie Guest

    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
    www.tetech.nl (Dutch)
     
  12. Tom Bruhns

    Tom Bruhns Guest

    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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