Distortion analysis - a thought experiment
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 10th August 2019, 12:31 PM #1 jan.didden   diyAudio Member     Join Date: May 2002 Location: Westende Resort, BE coast Distortion analysis - a thought experiment Modern FFT distortion analyzers can reliably measure down to -120dB or so harmonics. They are limited to their own distortion (and to a lesser extent self-noise) in the measurement and A-D circuitry. For instance, my AP SYS2722 has self-distortion of about -134dB 2nd harmonic. Suppose I measure my super-duper stuff and I measure -134dB 2nd. What does that mean? Surely my super duper stuff does not have zero distortion, but it may be so low that it doesn't change the result. Another possibility is that it is not very low at all but that the 2nd is in opposite phase to the analyzer own distortion, in such a way that the vector sum of the result comes out to -134dB. Is there a way to figure that out? Let us say I do complex FFT in loopback and the result is L=a+bi. Then I do a complex FFT measurement of the DUT and the result is C=p+qi. Can I then say that the DUT distortion product is D=C-L=(p+qi)-(a+bi)=(p-a)+(q-b)i? Jan Last edited by Variac; Yesterday at 06:00 AM.
 10th August 2019, 12:50 PM #2 phofman   diyAudio Member   Join Date: Apr 2005 Location: Pilsen Put a LPF behind your DUT. If your DUT does not introduce any distortion, only the fundamental will be rotated and the overall measured distortion will change only with respect to the attenuated fundamental. If your DUT (together with the generator) produces a distortion, it will be rotated/attenuated and the vector sum with ADC distortion (created by the fundamental) will be different. That is what I use for splitting the overall measured distortions into DAC and ADC components in the digital distortion compensation - replacing a voltage divider with LP filter producing exactly same level of fundamental must not bring up any (extra) distortions (to the extent allowed by physics and circuits quality). The next step - compensate both sides to keep distortions of both sides below your DUT distortion. Unavailable for AP though...
mbrennwa
diyAudio Member

Join Date: Mar 2005
Location: Zurich, Switzerland
Quote:
 Originally Posted by jan.didden Let us say I do complex FFT in loopback and the result is L=a+bi. Then I do a complex FFT measurement of the DUT and the result is C=p+qi. Can I then say that the DUT distortion product is D=C-L=(p+qi)-(a+bi)=(p-a)+(q-b)i?
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?

 10th August 2019, 01:12 PM #4 phofman   diyAudio Member   Join Date: Apr 2005 Location: Pilsen You can always add any level of distortion on the output (generator) side if you have signal generation under control. Either using chebyshev polynomials (only level under control), or adding pregenerated higher harmonics sine (both level and phase under control). E.g. chebyshev method Digital Distortion Compensation for Measurement Setup
jan.didden
diyAudio Member

Join Date: May 2002
Location: Westende Resort, BE coast
Quote:
 Originally Posted by phofman Put a LPF behind your DUT. If your DUT does not introduce any distortion, only the fundamental will be rotated and the overall measured distortion will change only with respect to the attenuated fundamental. If your DUT (together with the generator) produces a distortion, it will be rotated/attenuated and the vector sum with ADC distortion (created by the fundamental) will be different. That is what I use for splitting the overall measured distortions into DAC and ADC components in the digital distortion compensation - replacing a voltage divider with LP filter producing exactly same level of fundamental must not bring up any (extra) distortions (to the extent allowed by physics and circuits quality). The next step - compensate both sides to keep distortions of both sides below your DUT distortion. Unavailable for AP though...
Thanks - I will read it a few more times to be sure I grok it fully ;-)

The precomp stuff is not available on the AP, I know. But if I have a complex FFT from both situations I should be able to write a simple macro that extracts that difference and plot it, no?

Jan

 10th August 2019, 02:41 PM #6 trobbins   diyAudio Member     Join Date: Feb 2009 Location: Melbourne, Oz 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.
 10th August 2019, 02:46 PM #7 phofman   diyAudio Member   Join Date: Apr 2005 Location: Pilsen REW's distortions end at -120dB. Perhaps John could lower the limit, it may be a simple change.
 10th August 2019, 02:55 PM #8 phofman   diyAudio Member   Join Date: Apr 2005 Location: Pilsen For playing with distortion measurements, effects of compensation on each side, comparing channel performance etc., you can use the distortion compensation tool. No hardware calibration adapter is required for joint-sides compensation (on either side), just simple loopback (w&w/o DUT). Single-tone, dual-tone distortion measurement/compensation. I know I promised the pre-installed image, will do so in the coming weeks...
JohnPM
diyAudio Member

Join Date: Oct 2008
Quote:
 Originally Posted by phofman REW's distortions end at -120dB. Perhaps John could lower the limit, it may be a simple change.
Sure, what would you like?

jan.didden
diyAudio Member

Join Date: May 2002
Location: Westende Resort, BE coast
Quote:
 Originally Posted by trobbins 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.
I think the AP can do that too in the multitone generator. You can specify individual tones for amplitude and phase.
Need to check.

Edit: yes it can, see attached. Powerful!

Jan
Attached Images
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Last edited by jan.didden; 10th August 2019 at 03:07 PM.

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