In labs we had to measure the oscillations of a pendulum using an electric sensor. So basically, my data consists of time and voltage (which represents amplitude) pairs. The task was then to perform FFT analysis on this data to obtain the frequency of the pendulum.
After going through the motions using QtiPlot (removing mistakes, fixing offset, smoothing and interpolating), I get the major peak at 2.61Hz, however, if I perform a sinusoidal fit (using SigmaPlot), I obtain the period to be 1.94s. Both of this values have a very high precision and are completely irreconcilable.
I am at my wits end, I've been at this for days. Please help me.
$T=1.9312\pm2.6044E-005$ according to this fit
My procedure with the data regarding the fft is to first find the offset by calculating the average value. Then I smooth data. Then I cut data so that I have 2^n point, and finally I interpolate points with the same number of starting points. After that, I ask the QtiPlot to perform FFT. The algoritm used to smooth is FFT filter, but I honestly haven't noticed a major difference when using some different algorythm.