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First FFT Bin Empty?

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Visitor
Posts: 19
Registered: ‎07-28-2017
Accepted Solution

First FFT Bin Empty?

Hi all,

I am working on a simple design in System Generator to become familiar with the Simulink. I am using matlab 2016b and Vivado 2017.2, and I seem to be running into a strange problem when using the Fast Fourier Transform 9.0 IP.  For some reason my first bin is always empty, but the bins after are correct. The data valid signal is asserted during the first bin so I am confused.

 

Here is my design (You can ignore the lines to the right, they're unrelated.)

 

Design

And the signals:

Waveforms

 


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Teacher
Posts: 5,148
Registered: ‎03-31-2012

Re: First FFT Bin Empty?

@jacob.boline all that's needed is the sum of one full cycle to add up to zero (which is what the DFT does). You can calculate it yourself from the data you but your waveform looks pretty symmetric so I'd say it has no DC.

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Teacher
Posts: 5,148
Registered: ‎03-31-2012

Re: First FFT Bin Empty?

@jacob.boline if you are generating a perfect sinusoid, first bin should be empty because your signal has no dc component.

- Please mark the Answer as "Accept as solution" if information provided is helpful.
Give Kudos to a post which you think is helpful and reply oriented.
Visitor
Posts: 19
Registered: ‎07-28-2017

Re: First FFT Bin Empty?

How perfect of a sinusoid are you saying? Mine looks like this:

 

Capture.PNG

Highlighted
Teacher
Posts: 5,148
Registered: ‎03-31-2012

Re: First FFT Bin Empty?

@jacob.boline all that's needed is the sum of one full cycle to add up to zero (which is what the DFT does). You can calculate it yourself from the data you but your waveform looks pretty symmetric so I'd say it has no DC.

- Please mark the Answer as "Accept as solution" if information provided is helpful.
Give Kudos to a post which you think is helpful and reply oriented.
Visitor
Posts: 19
Registered: ‎07-28-2017

Re: First FFT Bin Empty?

Awesome, thanks.
Visitor
Posts: 5
Registered: ‎12-28-2017

Re: First FFT Bin Empty?

Hiii,

 I am trying to Implement FFT using 9.0 Core , Through DDS by generating a sine wave . but I am getting incorrect output.  I Have attached the image...can you help  me out.

I also want to know that you are using some  (Z block at the output of DDS ) what is this .and why are you using this.

I am using 100MHz  clock as input 

generating 20 KHz sine wave ..... and I have checked that was correct......

The problem is only with FFT core configuration.

12.png