UPGRADE YOUR BROWSER

We have detected your current browser version is not the latest one. Xilinx.com uses the latest web technologies to bring you the best online experience possible. Please upgrade to a Xilinx.com supported browser:Chrome, Firefox, Internet Explorer 11, Safari. Thank you!

cancel
Showing results for 
Search instead for 
Did you mean: 
Observer nagender_patel
Observer
2,773 Views
Registered: ‎03-21-2017

fft output v9.0

hello

      i have used fft ip core in vivado . i have generated a sine wave from dds compiler. the generated sine wave i have feed to fft core. so i got the result shown in the pic.

i have generated 10mhz sine wave

fft transform length i have choosen is 1024, radix-4 burst i/o,output ordering bit reversal..ect

so my dought is im getting the output but how to predict it frequency domail.

if i apply a signal with frequency F and amplitude A then fft output will be at F the amplitude will be A in frequency domain

plz help how to know the frequency of my fft output

fft.png
0 Kudos
8 Replies
Xilinx Employee
Xilinx Employee
2,771 Views
Registered: ‎08-01-2008

Re: fft output v9.0

you required to calculate if there is a spike of the amplitude of the FFT output at index 5, that means your input has a component in a frequency range of 5*Fs/N +/- Fs/2N.
Thanks and Regards
Balkrishan
--------------------------------------------------------------------------------------------
Please mark the post as an answer "Accept as solution" in case it helped resolve your query.
Give kudos in case a post in case it guided to the solution.
0 Kudos
Observer nagender_patel
Observer
2,736 Views
Registered: ‎03-21-2017

Re: fft output v9.0

thanks for reply sir

 

i got it ,but can u please say somewhat clearly

with following data

fft spike amplitude is 574412538467282136 (signed decimal)

at the spike my index value is 853

at point 23507.4nsec i got that spike

my input sine wave frequency 20mhz

 

how to find output freqency, for my input frequency, please help

 

fft.png
0 Kudos
Xilinx Employee
Xilinx Employee
2,727 Views
Registered: ‎08-01-2008

Re: fft output v9.0

amplitude not matter here
what is sampling frequency . you will get two frequency , N= 853 and put Fs in this formula

5*Fs/N - Fs/2N., 5*Fs/N + Fs/2N.
Thanks and Regards
Balkrishan
--------------------------------------------------------------------------------------------
Please mark the post as an answer "Accept as solution" in case it helped resolve your query.
Give kudos in case a post in case it guided to the solution.
0 Kudos
Observer nagender_patel
Observer
2,698 Views
Registered: ‎03-21-2017

Re: fft output v9.0

hello sir

    

sir im applying my input as sine wave to fft

sine wave frequency = 20Mhz; sampling frequency = 200Mhz;

 

with the same sine values in matlab i got a spike at 20mhz.

 

in xilinx i got spike at at index 745

form your formula  N= 745; fz =200Mhz

so, 5*Fs/N - Fs/2N., 5*Fs/N + Fs/2N.

 

((5*200M /745) - (200M / (2*745)) =1208053

 

((5*200M /745) -+(200M / (2*745))= 1476510

 

this are the calculations 

at spike it should be 20Mhz na.

 

please help

 

 

0 Kudos
Observer nagender_patel
Observer
2,694 Views
Registered: ‎03-21-2017

Re: fft output v9.0

my main dought is,

    how to find  the output frequecy of FFT at my spike in xilinx simulation

 

 

 

 

 

 

0 Kudos
Observer eiwave
Observer
1,733 Views
Registered: ‎12-28-2017

Re: fft output v9.0

hiii, 

 

I am working on FFT core 9.0 , facing the problem of input signal  of FFT , ...I have read your comment you did this with dds compiler .... Can you help me  how to generate a sine wave signal from dds compiler .......

thanks in advance,

mail id ---- shubhamgwl04@gmail.com

0 Kudos
Newbie tamer_samy
Newbie
505 Views
Registered: ‎02-10-2019

Re: fft output v9.0

Am facing same problem hope you solved it

0 Kudos
Scholar dgisselq
Scholar
474 Views
Registered: ‎05-21-2015

Re: fft output v9.0

@nagender_patel,

Yes, staring at traces can be quite confusing.  Manipulating the same traces within something like MATLAB or Octave can be simpler--and might really help you understand what's going on.

When I tested my own FFT implementation, I recorded the input to the FFT, Fourier transformed it using FFTW, and then compared it against the output.  The comparison wasn't simple: all kinds of truncation errors accumulate, and the scaling through the system might surprise you.  Internal overflow can really be annoying, and even difficult to track down.  Further, you have to be careful to properly synchronize to the FFT coming out of your logic.  My approach was to record the data going into the FFT, FFT it via FFTW, and then compare that to the data coming out of the FFT.

You can read about it here.  You can also find the test bench code I used to verify it here.  You might also find this approach to DSP-based debugging valuable.

Dan

0 Kudos