# A problem to the FFT Block **URL:** \ **Category:** GNU Radio\ **Created:** [November 13, 2012, 4:44pm UTC](https://www.ruby-forum.com/t/a-problem-to-the-fft-block/224480 "2012-11-13T16:44:20Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\ **Post date:** [November 13, 2012, 4:44pm UTC](https://www.ruby-forum.com/t/a-problem-to-the-fft-block/224480/1 "2012-11-13T16:44:20Z")
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 ---
**Author:** ![Zhiwen\_He](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/2bfe46/32.png) [@Zhiwen\_He](https://www.ruby-forum.com/u/Zhiwen_He)\ **Post date:** [November 14, 2012, 4:40am UTC](https://www.ruby-forum.com/t/a-problem-to-the-fft-block/224480/2 "2012-11-14T04:40:11Z")
On Tue, Nov 13, 2012 at 10:43 AM, Zhiwen He [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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 ---
**Author:** ![Zhiwen\_He](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/2bfe46/32.png) [@Zhiwen\_He](https://www.ruby-forum.com/u/Zhiwen_He)\ **Post date:** [November 15, 2012, 4:11am UTC](https://www.ruby-forum.com/t/a-problem-to-the-fft-block/224480/3 "2012-11-15T04:11:59Z")
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 ---
**Author:** ![Zhiwen\_He](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/2bfe46/32.png) [@Zhiwen\_He](https://www.ruby-forum.com/u/Zhiwen_He)\ **Post date:** [November 20, 2012, 4:48pm UTC](https://www.ruby-forum.com/t/a-problem-to-the-fft-block/224480/4 "2012-11-20T16:48:21Z")
On Wed, Nov 14, 2012 at 5:08 AM, Zhiwen He [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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