RTA harmonic distortion frequency range

Discussion in 'Official REW (Room EQ Wizard) Support Forum' started by trobbins, Apr 16, 2018 at 7:08 AM.

  1. trobbins

    trobbins New Member
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    I've used the signal generator and RTA with distortion panel to assess harmonic distortions of equipment, and really appreciate the frequency analysis down to 2Hz for assessment of certain circuits.

    I just noticed that a valid analysis and presentation of harmonic distortion went down to a fundamental frequency of 8Hz for a simple external sine oscillator circuit, but responded with N/A below that frequency, for the max 131072 FFT length. Would that be consistent with the programming of REW and FFT limits when there is no internal generator fundamental frequency to lock to?
     
  2. John Mulcahy

    John Mulcahy REW Author

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    If REW's generator is not being used the peak must be at least 5 FFT bins from the band edges. At a 48k sample rate that would allow operation down to 2 Hz with a 128k FFT, unless the spectrum of the input is such that the peak occurs lower than the centre frequency. The window choice also has an effect as higher order windows (such as BH-7) have a greater spreading effect. If using the REW generator the best option is probably to select the option to lock the generator frequency to the FFT length and use a rectangular window.
     

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