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FFT windowing correction function

Hello,

 

I need some help with preprocessing my acquired data. I am measuring low frequencies with an PCI-5122. The signal is gathered and transformed with a FFT. In order to correct the errors from the splitting of the peak, I calculated the amplitude sum (A=sqrt(sum_i (A_i)^2 )) of the involved bars, as shown in the FFT tutorial from NI. (http://www.diadem-forum.de/attachment.php?aid=280 , sorry, only in German) This works well so far.

Now, I need a window function for my measurement. But, when I use a window function (von-Hann or Hamming, I tried both) and then calculate the amplitude sum the result is not correct even if I use the correction factors named in literature. The resulting amplitude I get with this method is larger (!!!) than the input signal (I tested this with a frequency generator).

 

Is there a problem in the use of both correction methods? Or do I make another mistake?

I would be glad about any suggestions.

 

Thanks.

Silja

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Message 1 of 4
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Well, thats quiet a specific problem. I am not sure whether I know enough about the topic but if you want I can check your code and try to figure out what is going wrong.

Just to get this straight. Your program is working right? So this is no question about how to use LabView but about the content.

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Hello,

 

I am not using Labview. I use a C# program which I assume to work right. The problem just occur in the evaluation steps, so I hope this is the right forum even if it is not Labview I am using. I transform the data with a FFT function. This steps works correct, too. Only if I try to improve the data with a window function, the results seem to be wrong.

 

Here is my FFT-code, including the window function and the correction term. Maybe, there is something wrong.

 

 unsafe private int DoFFT(string chan, int Samp, double[,] Data)
        {
            try
            {
                FFTDataK0 = new niscope_acq.Complex[PowerOfTwo];

                double h = 1.0;

                bool doTransform = (comboBoxFilter.SelectedIndex != 0);  //selects if I want to use awindowing function 

                for (int i = 0; i < PowerOfTwo; i++)
                {
                    if (doTransform)  
                    {                    
                                h = Math.Sqrt(8 / 3.0) * 0.5 * (1 + Math.Cos(2 * Math.PI * (i - (PowerOfTwo - 1) / 2) / (PowerOfTwo - 1)));  // 1/0.5 - correction value for  von-Hann-window (Hanning-window)

                    }

                    FFTDataK0[i] = new niscope_acq.Complex(Data[1, i] * h);

                }

                FFTDataK0 = niscope_acq.DiscreteTransforms.FastFourierTransform(FFTDataK0);//

            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
                Log("Error in DoFFT: " + ex.Message);
            }
            finally
            {

            }
            return 1;
        }

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Message 3 of 4
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Mmh sorry aber da bin ich wirklich überfragt. Das ist eine rein inhaltliche Frage und ich kann leider keinen wirklichen Bezug auf NI Produkte erkennen.

Am Besten versuchst du es in einem anderen Forum nochmal Smiley Happy

 

Viel Glück

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