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ServoTech
Active User
Joined: 24 Mar 2008
Posts: 7
Location: Radford, Virginia, USA
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 Posted: Fri Mar 28, 2008 8:48 pm Post subject: THD Value vs HP FFT Analyzer |
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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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picojohn
Site Admin
Joined: 11 Jun 2007
Posts: 333
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| Posted: Mon Mar 31, 2008 8:26 am Post subject: |
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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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ServoTech
Active User
Joined: 24 Mar 2008
Posts: 7
Location: Radford, Virginia, USA
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| Posted: Mon Mar 31, 2008 6:58 pm Post subject: THD Value vs HP FFT Analyzer |
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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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ServoTech
Active User
Joined: 24 Mar 2008
Posts: 7
Location: Radford, Virginia, USA
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| Posted: Tue Apr 01, 2008 2:18 pm Post subject: THD Value vs HP FFT Analyzer |
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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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picojohn
Site Admin
Joined: 11 Jun 2007
Posts: 333
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| Posted: Wed Apr 02, 2008 10:10 am Post subject: |
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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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ServoTech
Active User
Joined: 24 Mar 2008
Posts: 7
Location: Radford, Virginia, USA
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| Posted: Wed Apr 02, 2008 8:25 pm Post subject: THD Value vs HP FFT Analyzer |
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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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picojohn
Site Admin
Joined: 11 Jun 2007
Posts: 333
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| Posted: Thu Apr 03, 2008 11:14 am Post subject: |
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Hi Mike,
Please send the file to support@picotech.com for my attention.
Regards
John |
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