FFT signal output waveform FPGA

[SharpWeapon]

Hello,

A design of FFT processor on FPGA has a bit strange output, at first It was a smooth curve with two picks at the frequency of the input signal and image frequency. But once I added a few more modules to the design, all went wrong. The matlab plot looks like this.

What could potentially cause this oscillation kinda signal?

Thanks!

 

[KlausST]

Hi,

Did you add a window function module?

Klaus

 

[SharpWeapon]

The FFT processor takes the input from the FPGA memory which is a sin wave. There is no window function module, I just gave the sin wave saved in the memory to the FFT processor.

 

[KlausST]

Hi,

it seems to me like a 64 point FFT.
Usually an FFT has two input arrays:
* one for the ADCsamples (real part. Your sine table)
* one for an imaginary part. Either your FFT processor fills it with zeroes or you have to do this. Check this.

As far as i can see you have 7 full waves of sine in your table. Be sure it is an integer value (not: 7 fullwaves plus a little bit of the 8th)

... but somehow the graph reminds me on a window function.


Klaus

 

[SharpWeapon]

Thank you. That is right, I have real and Imaginary parts. And I put the Imaginary part to zero. But the thing you mentioned about full sin waves, I think that will just make the signal to spread rather than having a pick. I don't think that will make the signal oscillate like the one I have. Don't you think?

The spreading is not as such a problem for me right now, I want the reason why I have a signal oscillating at its ends.

 

[KlausST]

Hi,

your signal is not oscillating. The FFT result show the frequencies.

But you must have an integer number of sinewaves in your array to get sharp peak in FFT result (frequency) and low flat noise floor.


Klaus