[Discuss-gnuradio] Spectrum Analysis with FFT plotting

[Ujala Qasim]

hi,

I have some raw data samples of FM from USRP. I now want to take their FFT and analyze the spectrum to see whether the data that I have captured is valid or not. My question is that when plotting FFT in MATLAB should I take the range of frequencies from 0 - 10 MHz (as the signal has been shifted to IF of 5.75 MHz) or the range of frequencies should be 88 - 100 Mhz. 

Though, No matter what range I take I always see a peak right in the middle of my frequency range:

e.g. if I keep the range at 0 -10 MHz, I'll get a peak at 5.0 MHz

when I keep it at 88MHz - 108Mhz I get the peak at 98 MHz

Why is this so?