RealFFT problem

[Je6rry]

Hi guys:

            I used measurement studio  2015. Sampling from ADC to get 1024  data(singals: 30~100HZ 1~5Vpp sin wave) ,saved to excel. imported  from excel to make FFT. according to the filter examples .

codes as following:

void CalculateFFTFunction(double []waveform)
{
//waveform data size
int datasize = waveform.Length;

//number of samples for FFT data
int fftnumofSamples = datasize/2;
xwaveform = new double[fftnumofSamples];
magnitudes = new double[datasize];
subsetOfMagnitudes = new double[fftnumofSamples];
phases = new double[datasize];
subsetOfPhases = new double[fftnumofSamples];
logMagnitudes = new double[fftnumofSamples];
FFTValue = new ComplexDouble[datasize];
int i;

try
{
// Calculate the FFT of waveform array.
FFTValue = NationalInstruments.Analysis.Dsp.Transforms.RealFft(waveform);

// Get the magnitudes and phases of FFT array..
NationalInstruments.ComplexDouble.DecomposeArrayPolar(FFTValue, out magnitudes, out phases);

double scalingFactor = 1.0/(double)datasize;

double deltaFreq = samplingRateNumericEdit.Value * scalingFactor;
textEdit1.EditValue = samplingRateNumericEdit.Value;
double maxvalue= new double();
double imaxvalue=new double();


subsetOfMagnitudes[0] = magnitudes[0] * scalingFactor;

// It's sufficient to plot just the half of numberOfSamples points to show the FFT.
// Because the other half will be just the mirror image of the first half.
for(i=1; i<fftnumofSamples; i++)
{
// Generating xwaveform with respect to which magnitude and phase will be plotted.
xwaveform[i] = deltaFreq * i;
subsetOfMagnitudes[i] = magnitudes[i]*scalingFactor*Math.Sqrt(2.0); // Storing only half the magnitudes array.
maxvalue = ArrayOperation.GetMax(subsetOfMagnitudes);
imaxvalue = ArrayOperation.GetIndexOfMax(subsetOfMagnitudes);

subsetOfPhases[i] = phases[i]; // Storing only half of the phases array.
}

// Display mode: linear or exponential
switch(displayModeComboBox.SelectedIndex)
{
// Plot the magnitudes and the phases.
default:
case 0: // Linear mode.
magnitudePlot.YAxis.Caption = "Magnitude VRMS";
magnitudePlot.PlotXY(xwaveform, subsetOfMagnitudes);
phasePlot.PlotXY(xwaveform, subsetOfPhases);
break;
case 1: // Exponential mode.
for(i=0; i<fftnumofSamples; i++)
{
logMagnitudes[i] = 20.0*System.Math.Log10(magnitudes[i]);
}
magnitudePlot.YAxis.Caption = "Magnitude in dB";
magnitudePlot.PlotXY(xwaveform, logMagnitudes);
phasePlot.PlotXY(xwaveform, subsetOfPhases);
break;
}
}
catch (Exception exp)
{
MessageBox.Show(exp.Message);
}
}

Attached the wave, signal is 80hz 1vpp Sinwave  

but i found the magnitude and frequcney valule  are incorrect.  Is there anyone who can give me some advice? Thank you for your advice.

[Je6rry]

samplingRateNumericEdit.Value=500;