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Altera_Forum

Honored Contributor I

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07-13-2018
06:36 AM

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FFT output spectrum generation

Hi all,

I just have a simple question regarding to FFT output. I understand that the output of the real and imag signal have signed value, and these has to be divided by exponent value in order to get the real value. The question I have is how should I get the spectrum more specifically range and the magnitude of the specific frequency! I am new to FFT core. I am looking forward to your reply! Thank you very much! Best,Link Copied

7 Replies

Altera_Forum

Honored Contributor I

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07-13-2018
02:29 PM

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exponent scaling is for both real and imaginary output separately. Power is Re^2+Im^2 (without averaging) per bin of fft.

frequency of a bin = bin_index * sampling rate/fft resolution (for indices 0 - fft resolution/2-1), thereafter it is negative (mirror) = -(fft_resolution - bin_index) * sampling rate/fft resolution
Altera_Forum

Honored Contributor I

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07-16-2018
01:27 AM

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Altera_Forum

Honored Contributor I

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07-16-2018
07:17 AM

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Altera_Forum

Honored Contributor I

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07-16-2018
08:11 AM

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Hi,

I understand your point. But the when you actually coding, you can't just do a pack based on a fixed number, right? because the exponent value is different. So, are you suggesting to write a case statement to hard code the logic? If not, could you please give me some hints on this, because the only other way I figure out to do this is through a shifting register. And besides this, I still have one question. How to control the FFT output? By that, I mean way to control FFT to start streaming or stop streaming at our wish. Thank you very much!
Altera_Forum

Honored Contributor I

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07-16-2018
08:16 AM

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--- Quote Start --- Hi, I understand your point. But the when you actually coding, you can't just do a pack based on a fixed number, right? because the exponent value is different. So, are you suggesting to write a case statement to hard code the logic? If not, could you please give me some hints on this, because the only other way I figure out to do this is through a shifting register. Thank you very much! --- Quote End --- yes you put the logic but no need to wait, e.g.

```
case exponent is
when -3 =>
re_sc <= re(28 downto 0) & "000";
im_sc <= im(28 downto 0) & "000";
when -4 =>
re_sc <= re(27 downto 0) & "0000";
im_sc <= im(27 downto 0) & "0000";
```

Altera_Forum

Honored Contributor I

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07-16-2018
08:29 AM

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Hi,

Thank you very much for your reply! I made an edition on my original post. I have got one more question regarding to FFT output. is there a way to control the FFT output data at our wish? Thank you very much! best, Jingyuan Ma
Altera_Forum

Honored Contributor I

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07-16-2018
08:57 AM

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