— 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(2nPI*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 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 to evt.data is the positive side of my signal while
evt.data to evt.data 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 is the center frequency (2.5G in your case) evt.data to
evt.data is the positive side and evt.data to evt.data is
the negative side.