Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Equipment & Tools From test equipment to hand tools

Distortion analysis - a thought experiment
Distortion analysis - a thought experiment
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 10th August 2019, 12:31 PM   #1
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Westende Resort, BE coast
Default 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 extend 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
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio articles - SilentSwitcher - Treg high voltage regulator - High-voltage delay unit
  Reply With Quote
Old 10th August 2019, 12:50 PM   #2
phofman is offline phofman  Czech Republic
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...
  Reply With Quote
Old 10th August 2019, 01:02 PM   #3
mbrennwa is offline mbrennwa  Switzerland
diyAudio Member
 
mbrennwa's Avatar
 
Join Date: Mar 2005
Location: Zurich, Switzerland
Distortion analysis - a thought experiment
Quote:
Originally Posted by jan.didden View Post
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?
__________________
------
Group buy for augerpro waveguides for Scan Speak tweeters -- elliptical version -- circular version
  Reply With Quote
Old 10th August 2019, 01:12 PM   #4
phofman is offline phofman  Czech Republic
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
  Reply With Quote
Old 10th August 2019, 02:12 PM   #5
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Westende Resort, BE coast
Quote:
Originally Posted by phofman View Post
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
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio articles - SilentSwitcher - Treg high voltage regulator - High-voltage delay unit
  Reply With Quote
Old 10th August 2019, 02:41 PM   #6
trobbins is offline trobbins  Australia
diyAudio Member
 
trobbins's Avatar
 
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.
  Reply With Quote
Old 10th August 2019, 02:46 PM   #7
phofman is offline phofman  Czech Republic
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.
  Reply With Quote
Old 10th August 2019, 02:55 PM   #8
phofman is offline phofman  Czech Republic
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...
  Reply With Quote
Old 10th August 2019, 03:00 PM   #9
JohnPM is offline JohnPM  United Kingdom
diyAudio Member
 
Join Date: Oct 2008
Quote:
Originally Posted by phofman View Post
REW's distortions end at -120dB. Perhaps John could lower the limit, it may be a simple change.
Sure, what would you like?
  Reply With Quote
Old 10th August 2019, 03:03 PM   #10
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Westende Resort, BE coast
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.
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
File Type: png twintone.PNG (5.7 KB, 315 views)
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio articles - SilentSwitcher - Treg high voltage regulator - High-voltage delay unit

Last edited by jan.didden; 10th August 2019 at 03:07 PM.
  Reply With Quote

Reply


Distortion analysis - a thought experimentHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Multi-amp thought experiment Moondog55 Multi-Way 31 13th June 2019 03:47 AM
The Chinese Room Thought Experiment thadman The Lounge 43 20th December 2012 12:53 AM
Just a thought experimentů no1herenow Multi-Way 29 23rd January 2012 07:35 PM
Who Wants To Be A Millionaire? (a thought experiment) thadman The Lounge 10 11th May 2011 10:54 PM
BIB thought experiment Brisso57 Full Range 3 27th January 2009 04:13 PM


New To Site? Need Help?

All times are GMT. The time now is 01:03 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki