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Howto - Distortion Measurements with REW
Howto - Distortion Measurements with REW
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Old 22nd June 2019, 12:52 PM   #101
Mark Johnson is offline Mark Johnson  United States
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Howto - Distortion Measurements with REW
Or excellent PSRR at 20 kHz
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Old 22nd June 2019, 04:22 PM   #102
xrk971 is online now xrk971  United States
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Howto - Distortion Measurements with REW
Quote:
Originally Posted by poseidonsvoice View Post
X,

Interesting. So are you saying the DUT should have a low noise supply or the Focusrite unit or both?

Best,
Anand.
The Focusrite power supply is internal and powered via its usb so nothing to do there except having a pc with a low noise motherboard.

For the DUT, the measurements shows you a combination of the performance of the PSU and the amp. For the above amp, it’s a SE Class A design with a CCS but having a clean PSU is important.
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Old 23rd June 2019, 06:26 PM   #103
delange is offline delange  Belgium
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Interesting stuff, great initiative!

So I installed REW followed the instructions and came up with the attached plot. This is using the internal signal generator.
I found the results greatly depends on the input amplifier and output settings. Which is probably quite logic (less gain less distortion, and also less noise). The input gain of my Focusrite 2i2 is set at min while the output is at max.

One question: when I click the camera icon in the top left I'm able to save the current plot. But that doesn't show the distortion panel. Is here a setting somewhere I need to modify? (the attached picture is a screenshot generated by the prtscrn button).
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File Type: png Focusrite 2i2.png (83.6 KB, 391 views)
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Old 24th June 2019, 01:57 AM   #104
xrk971 is online now xrk971  United States
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Howto - Distortion Measurements with REW
There is a checkbox when you save the image of the screenshot to show distortion panel. It works. The only thing that doesn’t show is the settings panel - which is useful for showing what the averaging is, number of points, window used, etc. for that you need screen capture of window with print screen button.

Your result looks very good.
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Old 25th June 2019, 02:36 PM   #105
Panelhead is offline Panelhead
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Default I am cranked to get started

Got several”projects” I want to test. Need to reread this. Played with REW a little, just updated the software. If it can measure a noise floor at -150 dB it will work for my use.
Purchased the 2 ppm 1 kHz generator kit. Need to build it. Want to start with internal signal generator, like to see noise floor fall when changing from 16 bit to 24 bit signal. Good indicator of interface resolution.
Will use a Focusrite Clarett. But the cheap models test excellent. Not sure an expensive one can be deliver better measurements.
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Old 25th June 2019, 02:50 PM   #106
xrk971 is online now xrk971  United States
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Howto - Distortion Measurements with REW
The noise floor depends on your audio interface - the Focusrite Solo/2i2/2i4 seems to have circa -125dB to -135dB noise floor. Depends on input gain and noise of your PC’s USB power supply too. I noticed newer laptops with SSD are quieter and can make measurements plugged into wall power even.
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Old 25th June 2019, 05:24 PM   #107
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Default Thru the floor - I hope

Quote:
Originally Posted by xrk971 View Post
The noise floor depends on your audio interface - the Focusrite Solo/2i2/2i4 seems to have circa -125dB to -135dB noise floor. Depends on input gain and noise of your PC’s USB power supply too. I noticed newer laptops with SSD are quieter and can make measurements plugged into wall power even.
I use an optical Thunderbolt cable for galvanic isolation, a galvanic isolated iFi iPower supply for Clarett, and some magic in the interface and computer.
Hope to see better than normal noise floor. But will not know until tested.
There is one test I want to run, tricked out interface and a completely out of the box unit with stock supply and Apple TB cable to see if the changes make a measurable difference.
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Old 25th June 2019, 06:09 PM   #108
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Howto - Distortion Measurements with REW
Good idea on galvanic isolation and iFi PSU. Good luck and I hope you get the -150dB floor - that's AP territory for almost peanuts.
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Old 25th June 2019, 07:22 PM   #109
Mark Tillotson is offline Mark Tillotson
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You realize noise-floor is given by a power-spectral-density plot, not a power spectrum plot?
You have to correct for noise bandwidth, number of FFT bins and the window function shape to convert between the two.

If you don't do this you'll see your noise floor level changes with different FFT sizes and sampling rates and changing window functions, rather than being a measurement of anything real!

Not sure many people understand these points. Here's a great paper that goes into full detail on the subtleties of spectral measurement using the FFT: http://holometer.fnal.gov/GH_FFT.pdf
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Old 25th June 2019, 09:47 PM   #110
chede is offline chede  Germany
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Howto - Distortion Measurements with REW
These are the folks from my old institute.

When I was still working in Physics, I did most of my research at the predecessor institute; the current groups are still in the old buildings.


Best regards,
Claas
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