# Measuring THD in REW



## gperkins_1973 (Aug 25, 2008)

Hi there,

I thought I would start a new thread about this. I have opened REW and clicked on the Spectrum tab, fired up the pink tone and clicked on measure distortion and then hit the red button. The graph moves quite a bit and occasionally I get the distortion numbers up but they go off again.

I left it running for a while but it didn't stop. Can anyone tell me where I am going wrong please.

cheers
graham


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## laser188139 (Sep 19, 2009)

We just had a thread a couple days ago that began with how to measure THD. It looks like you have the same confusion: REW's THD calculation is based on a single sine wave, not pink noise. 

Bill


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## gperkins_1973 (Aug 25, 2008)

Thanks bill for that. Sorry if this is a silly question but on the window in REW you have the THD and THD-N and then there are 2nd to 9th. What do these represent and is it just the reading where is says THD that I need to note down?

cheers

Graham


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## laser188139 (Sep 19, 2009)

It probably depends on what you want. 

Here's the text from the helpfile:


> When the Calculate distortion check box is selected the analyser calculates distortion figures for the input, including THD and THD+N and the relative levels of the 2nd to 9th harmonics. The highest peak is used to determine the fundamental frequency of the input, this is displayed with the level of the fundamental. The THD figure is based on the number of harmonics whose levels are displayed and is calculated from the sum of those harmonic powers relative to the power of the fundamental. The THD+N figure is calculated from the ratio of the input power minus the fundamental power to the total input power (note that it is possible for THD+N to be lower than THD using these definitions). The example below shows data for a 1kHz sine input that is being driven 0.1dB into clipping, generating odd harmonics. The positions of the harmonics are shown on the spectrum or RTA plot.


So it looks to me as if both THD and THD+N are interesting measures of distortion. The Wikipedia article probably starts taking you into more detail than you want to know. It does highlight the question of what are you comparing against, and how was that measured. 

Bill


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## gperkins_1973 (Aug 25, 2008)

Bill,

Thanks for that I will have a read up tomorrow.

cheers

Graham


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