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THD Value vs HP FFT Analyzer

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THD Value vs HP FFT Analyzer

Postby ServoTech » Fri Mar 28, 2008 8:48 pm

Hi PicoTech,
Comparing the 3224 vs a HP 35665A FFT Analyzer, the PicoScope reads approximately 20% lower values. Any thoughts why?
Thank you!
Mike
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Postby picojohn » Mon Mar 31, 2008 8:26 am

Hello Mike,

What are the characteristics of the test signal used for your comparison, and what do HP have to say?

Regards
John
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THD Value vs HP FFT Analyzer

Postby ServoTech » Mon Mar 31, 2008 6:58 pm

Hi John,

Thanks for the reply. The waveform is a sine wave; the Back EMF of a Servo Motor. The HP FFT Analyzer reads .24%THD, and a Lab View application agrees exactly with the HP. The PicoScope, however, reads .14% when sampling at the exact fundamental frequency. When the sample frequency is a notch off of the fundamental, the PicoScope reads .19% THD (siginal is 19Hz. PicoScope frequency divisions go in steps of 19, and 21Hz). So, the actual error is 42%.

Also, the program produces an error when I attempt to save the FFT waveform as a .csv file, and crashes. The .psdata file saves OK, and the .csv file in scope function saves OK. I am using the new 6_r6_0_13 software.

Thanks for your help!

Regards,
Mike
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THD Value vs HP FFT Analyzer

Postby ServoTech » Tue Apr 01, 2008 2:18 pm

Hi John,

On another motor, this one with a poor quality Back EMF waveform, the PicoScope again reads 41% lower value compared to both the HP Analyzer and the Lab View program. The HP and Lab View program both read 5.3% THD, and the PicoScope reads 3.1%. When sampling at a frequency inbetween sample points, the PicoScope reads 5.2% THD.

Best regards,
Mike
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Postby picojohn » Wed Apr 02, 2008 10:10 am

Hello Mike,

Could you attach a copy of the .psdata files for the measurements in question please, so that we may take a look?

Regards
John
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THD Value vs HP FFT Analyzer

Postby ServoTech » Wed Apr 02, 2008 8:25 pm

Hi John,
When I try to add an attachment, I get an error saying that the .psdata file extension is not allowed. Do you have personal e-mail that I can send to?
Thank you for your help!
Best regards,
Mike
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Postby picojohn » Thu Apr 03, 2008 11:14 am

Hi Mike,

Please send the file to support@picotech.com for my attention.

Regards
John
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