Axes for FFT

ravics · ‎08-26-2010

( 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.

criley · ‎08-16-2007

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.

ticktack · ‎08-14-2007

try spectrum scope

ywu · ‎11-28-2007

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

ravics · ‎08-26-2010

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?

ywu · ‎11-28-2007

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?