Hi, Thanks for your reply Sebastiaan, I am taking some data samples with a piece of code in Windows that takes the raw data samples. Now, I want to analyze the samples, whether my data is correct or not. For this, I tune to the FM frequency of 106.2 MHz (a radio station) and pick some data samples.
I arbitaraily picked 956 samples and took their FFT, kept my x-axis label between 105 - 108 MHz and made a plot in MATLAB: http://i695.photobucket.com/albums/vv318/ujalaqasim/fftplotWin.jpg The decimation rate is 250, so my sampling frequency in this case is 64Msps/250 = 256KHz Now, shouldn't I be seeing a peak at 106.2 MHz? Instead at 105MHz? There is no station at 105MHz. For comparison I also took some data samples from GNURadio for the same frequency with Octave and plotted them in MATLAB: http://i695.photobucket.com/albums/vv318/ujalaqasim/GNURadioSamples.jpg Even here I don't see a peak at 106.2MHz I also used the values that you told me: -samp_freq/2:samp_freq/no_samples:(samp_freq/2)-1 as my decimation rate is 250, my sampling frequency is: 256kHz and the no. of samples I am using is 956 so my range is: -128kHz:256kHz/956:127.999kHz Here is the plot, again taken from Windows: http://i695.photobucket.com/albums/vv318/ujalaqasim/samp_freq.jpg Now I don't see any peak at 106.2MHz in any of these plots. What could be the problem? On Sun, May 31, 2009 at 4:10 AM, Sebastiaan Heunis <sheu...@gmail.com>wrote: > Ujala > > I don't know what you mean by the range of frequencies? I'm guessing > you're just referring to the x-axis labels in Matlab. That would > depend on your sampling frequency after the decimation and the number > of points that you use for the FFT. Let's say your sampling frequency > is 16Msps and you're taking 1s of data for the FFT. Your x-axis will > look as follows: > > -16000000/2:1:(16000000/2)-1 or > -samp_freq/2:samp_freq/no_samples:(samp_freq/2)-1 > > If you get a peak in the middle of your data, then you have a DC > offset somewhere. > > Sebastiaan >
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