Hi,

— On Sat, 7/11/09, adib_sairi [email protected] wrote:

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