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Class D Switching Power Amplifiers and Power D/A conversion

Spectrum of Class D 1kHz Signal Before/After Filter
Spectrum of Class D 1kHz Signal Before/After Filter
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Old 13th May 2019, 12:04 PM   #1
shadders is offline shadders  United Kingdom
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Join Date: Feb 2010
Location: London
Default Spectrum of Class D 1kHz Signal Before/After Filter

Hi,
I have searched the forum, and multiple threads, but was unable to locate any pictures of a class D amplifier outputting a 1kHz signal, with the spectrum before and after the output filter.

I did search Google too - and none were easily found.

What has spurred this that when a class D amplifier is tested in regards to THD, they use a 22kHz filter, yet class A, A/B or other, they usually use a 80kHz filter. The claim is that nothing can be heard above 20kHz, so the use of 22kHz filter for class D is appropriate. I am ambivalent in this aspect, but i would like to examine any class D output above 20kHz with a 1kHz test tone.

As such, does anyone have links to existing spectrum plots of class D amplifiers with a 1 kHz test tone, before and after the output filter ?.

Thanks and regards,
Shadders.

Last edited by shadders; 13th May 2019 at 12:04 PM. Reason: Formatting
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Old 13th May 2019, 06:07 PM   #2
Mark Tillotson is offline Mark Tillotson
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Join Date: May 2018
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You can't see the spectrum properly on an audio analyzer, it goes up to GHz. (And RF spectrum analyzer's seldom go below 10kHz)

Last edited by Mark Tillotson; 13th May 2019 at 06:15 PM.
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Old 13th May 2019, 06:54 PM   #3
shadders is offline shadders  United Kingdom
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Join Date: Feb 2010
Location: London
Hi,
I am looking for an oscilloscope output of the class D amplifier before and after the filter, where someone has calculated FFT of the signal.

AP refer to this as per the following website : Class d and linear power amplifier testing Slide 11 and 12 details, but i would prefer a graphic that i can post elsewhere and one with more detail than the AP slides.

Regards,
Shadders.
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Old 13th May 2019, 07:32 PM   #4
MorbidFractal is offline MorbidFractal
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Join Date: Nov 2017
Model it. May not be the best example but...


Three Level PWM amplifier
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Old 13th May 2019, 08:53 PM   #5
shadders is offline shadders  United Kingdom
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Join Date: Feb 2010
Location: London
Hi,
Thanks. I did want the plots for an actual amplifier so that there were no arguments that it is not representative.

I am surprised that such plots were not on DIY Audio - it would have been the first thing i did should i have purchased or built a class D amplifier.

Regards,
Shadders.
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Old 13th May 2019, 11:32 PM   #6
kokoriantz is offline kokoriantz  Lebanon
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Join Date: Dec 2015
Location: south east asia
Since a month I am learning about class d amplifiers, and as you I am looking for real measurements . If I found much more about PWM for motor drive than audio , but It gives good understanding about distortion due to dead time . Document AN1071 from IR shows scope pictures of DT distortion for 40ns and 15ns , but TI patent REDUCTION OF DEAD-TIME DISTORTION IN CLASS D AMPLIFIERS - Patent application (Pure Path) explains things differently . The distortion is very similar to cross over type with 5th harmonics .
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