Circuit Transfer Function Measurement with REW

Discussion in 'Official REW (Room EQ Wizard) Support Forum' started by Rob Campbell, Mar 3, 2019.

  1. Rob Campbell

    Rob Campbell New Member
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    Is it possible to measure the phase and amplitude response of a signal level circuit such as an active (op amp) filter using REW and a sound card like the Behringer UCA202?

    Thanks, much!
     
  2. John Mulcahy

    John Mulcahy REW Author

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  3. Rob Campbell

    Rob Campbell New Member
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    Thanks, John! Will creating a calibration file affect the phase response of measurements at all? I'll need to get reasonably accurate phase from about 10 Hz to perhaps 3 kHz as I will be designing (attempting to design?) a feedback loop.
     
  4. John Mulcahy

    John Mulcahy REW Author

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    Yes, it will correct it to flat so you get the response of the device you are measuring and not the soundcard.
     
  5. Rob Campbell

    Rob Campbell New Member
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    Cheers.
     
  6. Sorepinky

    Sorepinky New Member

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    I have tried this for several DIY LR4 active crossovers and am confused by the phase traces. If I measure a simple loop-back through the sound card I get a flat phase line. (Focusrite 2i2 with cal file loaded). If I sweep through the LR4 high pass section I get what I'd expect to see, but the phase response through the LP section has unexpected shape. The LR4 is working perfectly and the amplitude traces look great. I see this on all the active crossovers that I've built and they all work fine. Here is an example of a 530Hz crossover that displays an "odd" REW LP phase curve:

    [trouble with OneDrive link]

    Have I missed a setting?

    Cheers,
    Ian
     

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  7. John Mulcahy

    John Mulcahy REW Author

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    Probably missing a timing reference, so the t=0 point is being set as the peak of the response. There are a couple of things you can do:
    - Use the Estimate IR Delay feature in the graph controls to have REW estimate a correct t=0 position
    - Use a loopback connection on the other soundcard channel as a timing reference, telling REW that with the setting on the Measure dialog just below the SPL/Impedance buttons
     
  8. Sorepinky

    Sorepinky New Member

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    Thank you so much Mr Mulcahy for this answer and in general for your most brilliant software. I did what you said and now see a proper-looking LP phase trace per attachment. Probably telling me that I could have matched the capacitors a bit better. :)
     

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  9. John Mulcahy

    John Mulcahy REW Author

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    Best check the HP as well, or measure both with a loopback connection in place to ensure they have a common reference.
     
  10. Sorepinky

    Sorepinky New Member

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    That was HP and LP overlaid. Not with loopback but with "Estimate IR Delay" and shift. Looks good enough. Thanks.
     

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