SO - REW Harmonic Distortion issue

pschnob

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Sep 24, 2019
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3
Hello,
I have looked through the manual and forum but not found anything addressing the following.

The issue is in RTA mode if you look at the Harmonic Distortion levels in dB they do not correlate at all with the levels of those harmonics shown in the graph. We are currently using 5.30.9 but have also tried 5.20 and found the same issue.

The data in the table below comes from the attached RTA screen grab with a 1kHz sine wave. As you can see, a) the differences between primary and harmonic levels shown in the RTA and THD panel are completely different and b) the differences shown between the RTA harmonics are very small but all over the place in the THD panel data.
Best Regards,
Pschnob


RTA graph SPL dB dataDifference between primary and harmonic levelsTHD panel dB data
primary50
2nd37.5-12.5-18.5
3rd36.5-13.5-42.1
4th36.7-13.3-27.9
5th36.9-13.1-44.8
6th37-13-32.5
7th37.5-12.5-45.6
8th37.5-12.5-32.2
9th37.9-12.1-44.9
 

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  • REW THD test curve.jpg
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Last edited:

John Mulcahy

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Apr 3, 2017
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7,403
RTA bins show the total energy in the whole span of the bin. The distortion data table shows levels at the harmonic frequencies. Try switching to the spectrum view to see the actual harmonic levels.
 

pschnob

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Sep 24, 2019
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Thank you John. FWIW, we were using 1/3rd octave warble tones so we were thinking since the primary signal spans the full 1/3rd octave band the distortion figures in RTA mode would match the graph but I guess that's not the case.. :(
 

John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
7,403
Not when the harmonics are so near the noise floor. The spectrum view will show more clearly the distribution of energy in the fundamental and the harmonics.
 
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