Re: [Discuss-gnuradio] A problem to the FFT Block

[Zhiwen He]

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 4:49 AM, Marcus D. Leech <address@hidden> wrote:

On 11/13/2012 10:38 PM, Tom Rondeau wrote:

On Tue, Nov 13, 2012 at 10:43 AM, Zhiwen He <address@hidden> wrote:

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

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

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I did the attached.

Note that I've turned off the default window function in both the forward and reverse transform

-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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square_test.png
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square_test2.png
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