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Old 2nd July 2012, 07:54 AM   #1
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Default Intermodulation thd measurements using pc usb scopes or usb sound cards


I do electronics as a hobby but my budget are very limited, so I can't afford a real spectrum analyser.

I need to measure distortions down to -95 db in FFT mode.

And I've made a 16 signals low distortions sinewave generator for complex signals intermodulation thd measurements using the FFT, here's the frequencies: 22, 30, 52, 83, 123, 199, 316, 499, 802, 1248, 1998, 3152, 4997, 7997, 12498, 19997.

Wich affordables pc usb scopes or usb sound cards, in FFT mode, can do those measurements and without having intermodulations ?

Thank you



Last edited by gaetan8888; 2nd July 2012 at 07:57 AM.
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Old 2nd July 2012, 09:10 AM   #2
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You must be carefull with USB scopes because most (except the expensive ones) are only 8 bit vertical resolution. That means you can measure at most to -48dB...

I have only a little experience with sound cards but there are cheap 24bit/96 or 192kHz sound cards which would be much better suited.
Software like ARTA will also function as a scope with a sound card.
With the 24bit/96kHz you can measure to 48kHz at better than -100dB.

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Old 2nd July 2012, 09:27 AM   #3
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I can have a M-Audio Transit usb sound card for a very good price, any opinions about this usb sound card ?

What bug me are the level of intermodulations thd of a usb sound card with a multi-signals sinewaves of 16 frequencies.




Last edited by gaetan8888; 2nd July 2012 at 09:35 AM.
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Old 2nd July 2012, 03:35 PM   #4
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I think some USB scopes can do signal averaging to improve resolution. You can also build a notch filter for the fundamental - a passive twin-T will help.
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Old 2nd July 2012, 11:18 PM   #5
richiem is offline richiem  United States
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I'm using an E-MU 0204 USB sound card -- 192kHz ADC & DAC, 24-bits -- (mine was about $105 US) to do high resolution THD analysis in conjunction with ARTA. This combination, at least for mid-band audio signals, offers accurate measurements well below -110dBu. The EMU and ARTA together resolve a 1kHz low-distortion sine down to about 0.0002%. Then I add an Active Twi-T notch filter ahead of the EMU to remove the fundamental by 40dB or more, and the resolution increases to better than 20dB lower. See the results given by these tools on my webpage about a rebuild of the Heath IG-18 oscillator into Bob Cordell's state-variable design here (there is a page on my site also for details of the active Twin-T):
IG-18 #2, the BIG-18

For looking at signals above 15kHz, there is no great solution. I can see the 4th harmonic of 20kHz, but not higher, since the antialiasing filtering cuts them off, and even so, the noise floor has a pretty steep rise above 30kHz, due to noise-shaping in the sigma-delta converter. ARTA will do nice twin-tone IMD too, but as you mention, cross-modulation garbage sets a real limit to resolution.

I've been frustrated by the low resolution of available USB scopes -- there's one that's 12 bits (Stingray) and is moderately priced, but it's only good for 200kHz max. Any with more BW are at most 10-bits. Given that there are plenty of ADCs now available for more resolution and BW, I don't know why we aren't seeing them in the market....
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