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Observer
9,577 Views
Registered: ‎08-26-2010

Axes for FFT

( SysGen) FFT o/p (xk_re) plotting on Wavescope. Need to extract points & plot so have taken o/p of FFT Core to WORKSPACE.

The x-axis shows the simulation time specified in the Simulink Model. How can I change it to frequency domain?

I want the exact values of the frequency.

1 Solution

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Xilinx Employee
7,559 Views
Registered: ‎11-28-2007

Re: Axes for FFT

RIght, System Generator doesn't have a log10 block readily available. If you do some research you will probably find some algorithms for log10 calculation that you could implement in other building blocks in System Generator.

Cheers,
Jim
16 Replies
Xilinx Employee
9,567 Views
Registered: ‎08-16-2007

Re: Axes for FFT

I don't think WaveScope has the ability to display in the frequency domain as it is intended to allow you to observe the time-changing values of any wires in the design.

Highlighted
Explorer
9,557 Views
Registered: ‎08-14-2007

Re: Axes for FFT

try spectrum scope

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Xilinx Employee
9,555 Views
Registered: ‎11-28-2007

Re: Axes for FFT

First of all I would suggest you grab a textbook or google FFT to get basic understanding of how FFT works.

A few points:

* FFT results are complex, so you will need to use both xk_re and xk_im outputs.

* You can never get the exact frequency of your input frequency using FFT. FFT puts the input signal into different "frequency bins". The width of each bin is the sampling frequency divided by the transform size (width=Fs/N). This is where xk_index output comes into play. xk_index=0 represents the first frequency bin (0 to Fs/N), xk_index=1 represents the 2nd frquency bin (Fs/N to 2*Fs/N), and so on. So you will need to use xk_index output to figure out the input frequency. E.g. if you see a big spike at xk_index = 5, then the input has a frequency component in the bin [5*Fs/N 6*Fs/N).

* If your input is a real signal, the FFT output is symmetric. So you can discard the output from N/2 to N.

@ravics wrote:

( SysGen) FFT o/p (xk_re) plotting on Wavescope. Need to extract points & plot so have taken o/p of FFT Core to WORKSPACE.

The x-axis shows the simulation time specified in the Simulink Model. How can I change it to frequency domain?

I want the exact values of the frequency.

Cheers,
Jim
Tags (1)
Highlighted
Observer
9,546 Views
Registered: ‎08-26-2010

Re: Axes for FFT

Thanks Jim.

My xk_index values are NOT consecutive values (like +001, +002)..I'm however able to find the approximate frequency using xk_index value.

Since FFT results are complex should both real & imaginary components be considered for obtaining the FFT points? Is any manipulation of xk_re & xk_im (like adding the two) necessary to extract FFT points?

Highlighted
Xilinx Employee
9,526 Views
Registered: ‎11-28-2007

Re: Axes for FFT

By default, the xk_index is in bit reverse order. You can change it to "natural order" (check the properties window of the FFT block) and then xk_index will come out sequentially.

You will need to check the magnitude of xk_re and xk_im (the real and imaginary part of the complex output). As I suggested, it'd be beneficial to go back to a textbook and go over the FFT stuff.

@ravics wrote:

Thanks Jim.

My xk_index values are NOT consecutive values (like +001, +002)..I'm however able to find the approximate frequency using xk_index value.

Since FFT results are complex should both real & imaginary components be considered for obtaining the FFT points? Is any manipulation of xk_re & xk_im (like adding the two) necessary to extract FFT points?

Cheers,
Jim
Highlighted
Observer
9,506 Views
Registered: ‎08-26-2010

Re: Axes for FFT

Thanks Jim.

I would like to know how the fft o/p can be scaled to obtain dB scale on y-axis.

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Xilinx Employee
9,466 Views
Registered: ‎11-28-2007

Re: Axes for FFT

In hardware or software?

@ravics wrote:

Thanks Jim.

I would like to know how the fft o/p can be scaled to obtain dB scale on y-axis.

Cheers,
Jim
Highlighted
Observer
9,463 Views
Registered: ‎08-26-2010

Re: Axes for FFT

Software.

Highlighted
Xilinx Employee
9,453 Views
Registered: ‎11-28-2007

Re: Axes for FFT

It's easy in software, but I will leave it to you to figure out what "dB" is and go from there.

@ravics wrote:

Software.

Cheers,
Jim
Highlighted
Observer
6,382 Views
Registered: ‎08-26-2010

Re: Axes for FFT

Do i need to use Cordic Log block to scale?

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Xilinx Employee
6,351 Views
Registered: ‎11-28-2007

Re: Axes for FFT

If you're only instersted in log scale in SW, you can simply use the log function in Matlab.

If you need to implement it in HW, you could use the Cordic Log block. Keep in mind though the Cordic core calculates natural logarithm (base e) not the log10 (base 10).

Cheers,
Jim
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Observer
6,337 Views
Registered: ‎08-26-2010

Re: Axes for FFT

Thanks Jim.

So there's no other way to find log base 10 using System Generator which could be implemented in H/W?

Highlighted
Xilinx Employee
7,560 Views
Registered: ‎11-28-2007

Re: Axes for FFT

RIght, System Generator doesn't have a log10 block readily available. If you do some research you will probably find some algorithms for log10 calculation that you could implement in other building blocks in System Generator.

Cheers,
Jim
Highlighted
6,103 Views
Registered: ‎09-10-2008

Re: Axes for FFT

Jim,

I just ran my 256 pts fft and I am getting out 256 bins.  Shouldn't I expect only 128 bins from 256 pts fft?  what's the other 128 bins?

b

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Xilinx Employee
6,094 Views
Registered: ‎11-28-2007

Re: Axes for FFT

If your input is real, you only need to look at 128 bins. The other half is just the image of the first half, which you can ignore.

Cheers,
Jim
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Observer
1,435 Views
Registered: ‎10-16-2016