level sweep for THD

O

oliver

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• Dec 23, 2024

• #1

Hi John,

I need to measure the THD as the voltage increases on some microdrivers,
would it be possible to add in REW the ability to measure the THD with a level sweep?
In the generator you could select the frequency at which to perform the measurement, the starting level and the final level ( being able to select different units of measurement as it is now), and then on the graph you could have a display of this kind:

Thank you.
Oliver

 

John Mulcahy

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• Dec 23, 2024

• #2

For that you can use stepped sine, THD vs level.

 

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oliver

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• Dec 23, 2024

• #3

Ok great, thanks.
Would be possible to select a different unit for the level?

 

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oliver

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• Dec 23, 2024

• #4

Hi John,
I did some measurements.

These are two different level, 100mV and 300mV

This is the stepped level THD:

This way I am not able to display at the cursor the THD percentage and I can't see any correlation with the THD values in the first image.

These are the setting I have used:

 

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oliver

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• Dec 23, 2024

• #5

Sorry,
forgot to tell you I am on OS X Sequoia 15.2, REW V. 5.31.3 .

 

John Mulcahy

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• Dec 24, 2024

• #6

Why use such a huge step, 20 dB? You end up with a graph that has only two points. Why limit the RTA THD to just 2nd and 3rd harmonic?

 

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oliver

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• Dec 25, 2024

• #7

Hi John,
I am trying to understand what settings to use, I have reduced the steps to 2 now and incresed the number of harmonics to show.
I have performed two measurements, one at 100mV and the other at 300mV:

I am not able to get the same THD percentage at 1KHz when performing the Sweep level,
what settings would you suggest in the "Stepped Measurement" window in order to get the same results?

Thank you.

 

John Mulcahy

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• Dec 26, 2024

• #8

Usually best to enable the distortion LP and distortion HP. A 64k FFT is long enough, no need for 512k.

 

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oliver

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• Dec 27, 2024

• #9

Hi John,
I am on OS X 15.2 with REW 5.40 beta 61.
I was experimenting with different THD meaurement methods and had a strange behavior.
this is an overview of the distorsion panel with some measurements:

All is good.
Then I deselected some of the curves in order to compare just two of them, then I selected a different panel and went back to the distorsion panel and this happened:

The two curves where displayed shifted and at this point the cursor movement was showing only the distortion value for curve No. 1, the others did not show any value.
So I tried to save the graph as image and hit the relative button:

but as soon as I chosed the folder to save the curves went aligned again:

I tried the same on REW 5.31.3 and doesn't happen, all is fine.

Oliver.

 

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oliver

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• Dec 27, 2024

• #10

another report,
when in distortion panel
is not possible to enter manually any value in the set graph axis limits, the values can be changed only by the arrows.
This happen on REW 5.31.3 and 5.40 beta 61.

would it be possible to choose the unit of measurement in Voltage as well as dBFS in the Stepped measurement panel?
Thank you,
Oliver

 

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John Mulcahy

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• Dec 29, 2024

• #11

oliver said:

I was experimenting with different THD meaurement methods and had a strange behavior.

Click to expand...

I have fixed that for the next build.

oliver said:

when in distortion panel
is not possible to enter manually any value in the set graph axis limits, the values can be changed only by the arrows.

Click to expand...

That is the case when the axis only permits certain values.

oliver said:

would it be possible to choose the unit of measurement in Voltage as well as dBFS in the Stepped measurement panel?

Click to expand...

That is awkward to handle as the steps usually need to be logarithmically spaced, so don't correspond to a fixed voltage. That means the panel would need to offer options for either the number of points along the interval or a dB step size, as well as offering a choice of units for the start and end levels. I can add it to the todo list, but it won't be done for some time.