Why the FFT spectrum gives values of RMS?

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• Why the division operator give me different results in different screws?

  Hi all

  I'm quite the LabVIEW newbie, so please forgive me if it's elementary.  I have a signal that I'm acquisition.  The signal is fairly low amplitude (.01-0,1 V).  During each acquisition, I share the values of the sample defined by another few (0.047) using the division operator.  I see inconsistent results and I am puzzled as to why.

  In ex1, which I built from scratch for this post, everything works as expected.  I can confirm with the debugger that every reading of the sample overall is well divided and the average, max and waveform graph look like I expect.

  When I use a similar construction in my real application, however, the division often results in the INF file, and when I look with the debugger I see after fracture of the values of the signal... just don't make sense to me (x 2).

  I thought I must be missing something in the real application associated with split and merge multiple signals, so I ripped out everything except the Division (ex3).  Although this code looks a lot like the ex1 works well, the product always division infs and strange results.

  These three examples receive the same signal.  My best guess is at this point that the built-from-scratch example is force double passes to the operator of division "the right way" and that the examples of dysfunction are force double in a different way, incorrect.  I tried to delete and re-create the signal path in the examples of dysfunction without result.  My problem is that I don't know enough about the LabVIEW data types to find out how to impose the appropriate constraint.

  Can someone point me in the right direction for a solution?  I would be very happy to help.

  Your first image divides your signal of 0.047. Your second and third images are 0.047 dividing your signal. Reverse your son in fracture on image 2 and 3 knots, and I think you should be ok.

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• RMS of the FFT or tip?

  Hello

  I have a sinusoidal signal that oscillates around zero the axis in the negitave and positive. It is to measure the height of the waves on the static water level in a wave tank. I am calculating the FFT but the amplitude is less than the height of the waves in the time domain. Does this mean that I'm the wrong calculation? I don't know wheather I should use the FFT Peak or RMS? I assumed that the amplitude of the FFT should give me the same amplitude as the time domain.

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  Thank you

  Donners,

  It can get complicated, but I suspect that what you see is the result of the frequency of the waves is not not exactly at the frequency of the FFT bins.  For example, suppose that df = 1 Hz. Then each element of the array FFT represents the energy in a "sandbox" a hertz wide centred on the frequencies of the integer.  For a wave with most of his energy to 7.3 Hz TFF likely values not null for trays Hz 7 and 8.  In real systems with noise and light frequency variations, it is not uncomon for data can be spread over 10 or 20 locations. To get the fundamental signal amplitude, you would need to combine the energy in all stores.  Since the FFT produces complex output, the phase component must also be considered. Non-sinusoidal Waveshapes means that part of the energy is in harmonics. A pulse square can have a large enough amplitude, but there's still no broad spectral components.

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• Why the acquired data are not from min to max (DAQmxCreateAIVoltageChan, PXI-4472) values

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  I don't understand why the data stored in the wav file are not-10V,-10V but-5V, 5V... (the value of the gain is 0dB). (Perhaps a scale of 0.5?...)

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  You're right, in my example, I considered as unsigned data.

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• Output power of the frequency spectrum

  Hello

  I have a pretty simple question. I m using the power spectrum of FFT and PSD vi. and have trouble understand output frequencies. If I understand the first line is DC-value and delta frequency then describe the following frequencies. But when I put for example in 8 points say sampling frequency of 4 Hz = 2 seconds of data to analyze, I'll be back 4 lines of data (including 0). If the first is DC, then the 2nd should be 1 / 2 Hz 1 Hz = 3rd place and 4th place = 3 / 2 Hz, but what about the frequency of 2 Hz Nyquist, why Don t I get this one? or is there something I don't understand?

  I also tried the example of Units.vi of FFT spectrum and can understand, but I still Don t get it.

  Help, please!

  Best

  Jesper

  FFT returns to power in frequency bins. The frequency of the bins is determined by the Fs (sampling frequency) divided by N (number of points), from DC (0Hz). For your case, the table returned would be:

  Bin 0: 0-0.5 Hz

  Bin 1: 0, 5 - 1 Hz

  Tray 2: 1-1.5 Hz

  Bin 3: 1.5 - 2 Hz

  So effectively Bin 3 holds the power of 1.5 to 2 Hz (which contains your Nyquist frequency).