# A problem to the FFT Block

hello all,

When I use the fft blcok, it seems not right.

I build the test system like this: Square source -> IFFT -> FFT ->
File
Sink, when the input is 1, the output in the file sink is 1, and when
the
intput is 0, the output is random, sometimes 0, sometimes between -1 and
1.

Like this:

input:

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0
output:

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0

0

0

0

0

0
0

0

0

0

0

0

0 0

0

0

0

0

0

0

0

0
0 0

0
0
0

0

0
0

0

0

0
0
0

0 0

0 0 0

0 0
0 0 0
0

0

0

0

0

0

0 0

0

How should I do, so that the source data is the same as the sink data?

Thanks.

Heyutu

On Tue, Nov 13, 2012 at 10:43 AM, Zhiwen He [email protected] wrote:

0 0 0 0 0 0 0

0
0
0 0

0

0 0 0

0 0

Thanks.

Heyutu

Hi Heyutu,

I just tried this right now and don’t have a problem. This is my full
graph:

sig_source (square wave) → throttle → stream_to_vector(fftsize) →
fft(fftsize, reverse) → multiply_const(fftsize*[1/fftsize], fftsize) →
fft(fftsize, forward) → vector_to_stream → time sink

Tom

Thank you very much. I have solved the problem, I find that, I didn’t
use
the multiply_const block. But there is another question, when I see the
data in Matlab with plot function, the figure before fft and after ifft
is
not same, there are something wrong on the position, where the valueis 0
in
the square source. why?

Thanks

Zhiwen He

On Wed, Nov 14, 2012 at 5:08 AM, Zhiwen He [email protected] wrote:

Thank you very much. I have solved the problem, I find that, I didn’t use
the multiply_const block. But there is another question, when I see the
data in Matlab with plot function, the figure before fft and after ifft is
not same, there are something wrong on the position, where the valueis 0 in
the square source. why?

Thanks

Zhiwen He

I feel like there’s a representational issue here. Why is your scale
x10^9
(before the FFT) when the amplitude of the signal generator is 1? I’m
also
not clear what I’m looking at in the plotted figures. Can you label
them?

Tom