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## Re: [Discuss-gnuradio] question regarding usrp_fft.py

 From: Firas Abbas Subject: Re: [Discuss-gnuradio] question regarding usrp_fft.py Date: Sat, 11 Jul 2009 20:15:11 -0700 (PDT)

```Hi,

> so if the bin of fft is treated as individual, what is the relationship >
> between fft_size and the db calculation? what is the usage of fft_size
> in the calculation?

Ok let me explain. Let x(n) = cos(2*n*PI*f/fs)  , n=0,1,2,......,N

Now take the FFT 3 times with N = 256, N = 1024 and N = 2048.

You will see that the output of the FFT X(n) will be different in magnitude for
the same input sequence because of different FFT size. The spectrum shape is
similar in the 3 cases, but the the amplitude of each spectrum bin is different.

>
> if i choose to monitor the 2.5G center frequency, so the evt.data[0] is
>  the place where the 2.5G is right? if the fusb is 4Ms/s and our
> fft bin is 256, so our bandwidth of monitoring is only 2MHz right? and
> evt.data[1] to evt.data[128] is the positive side of my signal while
> evt.data[129] to evt.data[256] is my negative side.. am i right?
>

No some numbers are wrong. Assuming complex samples, when Fusb is 4MB/s the
bandwidth is 1 MHz (2 bytes for I and 2 for Q). For FFT 256, evt.dat[0] is the
center frequency (2.5G in your case) evt.data[1] to evt.data[127] is the
positive side and evt.data[128] to evt.data[255] is the negative side.

BR

Firas

```