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Digital Distortion Compensation for Measurement Setup
Digital Distortion Compensation for Measurement Setup
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Old 7th June 2019, 07:55 AM   #221
phofman is offline phofman  Czech Republic
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Quote:
Originally Posted by PMA View Post
I hope that the phofman's method does not try to eliminate/cancel any harmonic frequencies it finds.
I would have thought the method had been explained thoroughly in detail a few times here.

The compensation does not look for any harmonics. It keeps compensating the harmonics (adjusted for current fundamental amplitude and phase) measured during the calibration phase.

Several times have I shown a spectrum where artificially added distortions on playback side were properly identified. Yesterday I tried -130dB for 2nd and -130dB for 4th, Arta measured -135dB for 2nd and -130dB for 4th, while the signal went through a voltage divider of -1.6dB and the input stage has -0.3dB attenuation compared to the output. That means the distortions of -130dBFS on output were actually -132dBFS on input.

Last edited by phofman; 7th June 2019 at 08:06 AM.
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Old 7th June 2019, 11:13 AM   #222
JohnPM is offline JohnPM  United Kingdom
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Originally Posted by KSTR View Post
which might be REW as well once it has an offline feature added, in order to use the analyzer on files -- 64bit floats --rather than on the 24bit input stream
The ability to drag a WAV file onto the RTA window for processing was added in REW V5.20 beta 6.
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Old 7th June 2019, 11:46 AM   #223
phofman is offline phofman  Czech Republic
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As of files - the tool supports playback and recording of audio files on any side. This gives many new options:

* A spectrum analyser without file support (e.g. Arta, jaaa) can read from the tool loopback "soundcard", while the file is being played by the tool.

* An audio file can be recorded in loopback on one machine, copied over to a machine running the tool (no soundcard required), and a distortion-compensated (joint-sides only) version can be stored by the tool for cleaner playback on the original machine.

* A testing signal produced by some analyser can be bit-perfectly recorded by the playback side.

* DUT output signal can be recorded at any time, while being displayed in the analyser

* etc.

Last edited by phofman; 7th June 2019 at 11:48 AM.
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Old 7th June 2019, 02:15 PM   #224
keantoken is offline keantoken  United States
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Since the noise gain resistor also magnifies offset, you might want to put a large capacitor in series with it to prevent that from happening at DC. This may help the harmonic profile match if the offset is significant enough to change the crossover point of the output stage. it is of course only valid above the crossover frequency of the noise gain resistor and the capacitor.

I would not expect an electrolytic cap to cause error in this position since the AC across it is extremely small, and it's distortion only applies to the error signal, not the full fundamental voltage swing.

Everything about the circuit should be the same except for the addition of the noise gain resistor and cap, if what you want is a valid comparison. So there is no need to change the load resistor.
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Old 7th June 2019, 08:05 PM   #225
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by JohnPM View Post
The ability to drag a WAV file onto the RTA window for processing was added in REW V5.20 beta 6.
Excellent, I didn't know this was possible. Thanks!

Seems I've found a small bug, will report in the official REW forum, plus a request for 64 raw double input data.
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Old 12th June 2019, 09:55 AM   #226
keantoken is offline keantoken  United States
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For more details on the distortion gain method, Samuel Groner used it to test several opamps:

http://www.nanovolt.ch/resources/ic_...distortion.pdf
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Old 12th June 2019, 11:03 AM   #227
phofman is offline phofman  Czech Republic
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Thanks a lot, very interesting.

We have been talking about the circuit for amplifying/measuring common-mode distortion (Fig. 2.1b, attached). But the distortion measured by the compensated loop for plain non-inverting circuit is the sum of all the distortion types discussed in the paper. Therefore, a direct multiple will not hold.

Nevertheless, I will be happy if the precision is within correct order of magnitude :-)

I have to rework my calibration adapter first, to shield the multiturn pot properly and to improve overall shielding and signal separation.

Also, I can compare opamp results measured with the existing PCI soundcard and E-MU 0404 USB E-MU 0404 and Linux (also issues with REW at 96 kHz)
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File Type: png opamp-measurement.png (6.7 KB, 96 views)
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Old 18th June 2019, 10:05 PM   #228
phofman is offline phofman  Czech Republic
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Still playing with E-MU 0404 USB. This time REW analyzer.

Joint-sides calibration at 48kHz, balanced loopback.

First screenshot no distortion added.

Second screenshot added distortions -140dB at 2nd, 3rd, 4th harmonics to playback side.

I think it will be a decent measurement card.
Attached Images
File Type: png rew-emu0404.png (72.9 KB, 59 views)
File Type: png rew+distortions.png (74.1 KB, 62 views)
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Old Yesterday, 06:16 PM   #229
phofman is offline phofman  Czech Republic
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Slowly learning REW - disabled peaks view.

1) Loopback corresponds to specs DAC THD+N 0.001% + ADC THD+N 0.0009% => approx 0.002%

2) Joint-sides compensation

3) Added playback-side distortions 2nd + 3rd + 4th @ -145dB
Attached Images
File Type: png emu-pass.png (66.2 KB, 38 views)
File Type: png rew-emu0404.png (64.4 KB, 36 views)
File Type: png rew+distortions.png (64.1 KB, 19 views)
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Old Yesterday, 09:44 PM   #230
phofman is offline phofman  Czech Republic
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E-MU 0404 noise floor at no input and 1211Hz@-1dB output. The input TRS sockets ground both inputs when no connector is inserted.

Right input channel shows no signs of the output signal, the left input channel has the signal at -150dB.

Unfortunately the compensation cannot compensate hot and cold lines of a single balanced input/output independently.

Balanced LPF would be possible, but balanced voltage divider would either require two precisely matched multiturn pots, or a single-pot balanced VD would present a variable load for the output, changing its distortion profile at each position.
Attached Images
File Type: png right-no-signal.png (48.8 KB, 15 views)
File Type: png left-no-signal.png (49.5 KB, 15 views)

Last edited by phofman; Yesterday at 09:47 PM.
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