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Distortion analysis - a thought experiment
Distortion analysis - a thought experiment
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Old 10th August 2019, 02:04 PM   #11
phofman is offline phofman  Czech Republic
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Fantastic. How about -150dB :-) ? Thanks a lot for your incredible support.
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Old 10th August 2019, 02:09 PM   #12
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by trobbins View Post
REW allows insertion of selected harmonics (2 to 9), with settable magnitude and phase, for its tone generator, along with bandwidth limiting and distortion component measurements of the loopback spectrum.

Using my EMU0404 USB as DUT with loopback, I have 0.0006% 2nd and THD at 389.6Hz test tone when limiting bandwidth to just include 2nd harmonic. I can then null that out (0.0001%) with -104dB of added 2nd H with 90 deg phase shift.
So how do you determine the phase of the harmonic wrt the fundamental? Take it from the complex FFT?

Jan
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Old 10th August 2019, 02:11 PM   #13
phofman is offline phofman  Czech Republic
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Originally Posted by jan.didden View Post
Powerful!
Honestly, considering the price tag - generating a multitone in software with any initial phase is trivial :-)
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Old 10th August 2019, 02:11 PM   #14
phredrick is offline phredrick
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Maybe keep in mind that a specific component of distortion, namely a particular harmonic, has been considered. It may not be possible for a simple summation to yield the results you describe. Can there be a destructive result? Inspection of computed results suggests no and it’s been about 30 years since I (briefly) looked at the math.
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Old 10th August 2019, 02:14 PM   #15
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by mbrennwa View Post
I would agree that C = L + D (with C, L and D complex numbers). This should hold at least approximately (considering things like noise, imbalances in the DUT and loopback channels of test hardware, etc.). Therefore D = C - L.

If there is an easy test setup, I could try and do some tests with my RTX6001 + MATAA. Since the RTX is rather low distortion, the DUT would need to have low distortion, too. And the whole setup would need to be low noise and stuff. Can anyone think of an easy-peasy setup to test this?
Can you use one channel of the RTX as measurement channel and the other as the test tone generator/DUT? What would happen if both channels had exactly the same distortion, would the result from this 'experiment' come out zero?

Jan
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Old 10th August 2019, 03:03 PM   #16
BamboszeK is offline BamboszeK  Poland
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If I remember correctly, it is required to measuring equipment be atleast 10dB better than device under test in order to know exact parameters of DUT.
Citing AES17 standard:
Quote:
4.3.3 Unless otherwise specified, the equipment used for measurements in this standard shall have an accuracy
in the parameter being measured of at least 3 times better than the specification being verified.
And 10dB is pretty close to 3 in ratio. However I'm not 100% sure where I heard that and it may be not a case with THD measurments.


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Old 10th August 2019, 03:28 PM   #17
trobbins is offline trobbins  Australia
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Jan, i just incrementally adjusted the added 2H magnitude (1dB steps) and phase (1deg steps) to null the DUT's 2H level. John made a very simple to use settings window for this.
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Old 10th August 2019, 03:29 PM   #18
jan.didden is offline jan.didden  Europe
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Yes, that's the accepted wisdom, and I intend to break it ;-)

And I am just last in a long row, with people like phoffmann being at the forefront.

Jan
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Old 10th August 2019, 05:29 PM   #19
phofman is offline phofman  Czech Republic
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Quote:
Originally Posted by jan.didden View Post
And I am just last in a long row, with people like phoffmann being at the forefront.
There is a very big difference between the wide and deep knowledge/experience you have acquired throughout the many years vs. the very narrow specific focus I learned in the single year of my distortion project.
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Old 10th August 2019, 06:18 PM   #20
mbrennwa is offline mbrennwa  Switzerland
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Quote:
Originally Posted by jan.didden View Post
Can you use one channel of the RTX as measurement channel and the other as the test tone generator/DUT?
Not sure I understand this. Is "measurement channel" not the same as "test tone / DUT"? Did you mean "loopback" instead?
Anyways, the RTX works like a USB soundcard. It has two outputs and two inputs, and you can output whatever you like on each channel (ok, that depends on the software features, too). Same for the inputs.

Quote:
Originally Posted by jan.didden View Post
What would happen if both channels had exactly the same distortion, would the result from this 'experiment' come out zero?
That depends on what the software does with the data. Most software will deconvolve the data recorded from the DUT and from the loopback channels. This will fix the timing of the measurement (like latency of the sound I/O of the computer) or the "linear distortion" (like non-flat frequency response of the test equipment). I don't think the deconvolution will fix non-linear distortion.
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