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mohamed shffat

FFT problems doesn't finish never

Question

Hello 

After fixing some problems in the FFT IP core v 9.0, i got another trouble here . 

My problems are in the natural order output signal , and the reversed order with harmonics ! When i set my output core to reversed bit order i get a results but not only two peaks , i get another peaks or harmonics as it called ! In the same time when i change it to the natural order i get nothing from the core ! just an empty window with no peaks such as shown in the pics below . 

Details of my FFT core : 

512 points scaled with SCALE_SCH = [01 10 10 10 11] 

forward FFT and real time mode . 

Target CLK 100 MHZ and 50 MSPS out .

Fixed point 

Pipelined streaming I/O . 

Input 16 bit width comes from XADC 1MSPS . 

1st pic show the results comes in reversed order , 2nd pic shows the natural order results . 

And the input of the core is a sinusoidal wave sampled with XADC core . 

If someone have any idea of what is going on here I would really appreciate the help!

wrong !.jpg

wrong 2.jpg

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Yes, your problem makes perfect sense: you aren't gating the values coming into the core.  I mentioned this in the discussion associated with your other post here:

The problem is that you are running the FFT at a clock near 100MHz, but only placing values in at about 1MHz.  This in itself isn't a problem, but if you don't adjust the CE line appropriately or gate your analysis of the output with (it's either the CE or DV line), you will have this problem.  What you need to do is to raise the CE line for one clock, and one clock only, for each valid sample going into the FFT.  Then, on the output, you will get one sample out at the same rate: roughly one valid FFT output sample every 100 input samples.  That's just ... how a pipelined FFT works.

Dan

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