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Help me understand FFT analyzes

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Y

yen

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I have a matlab question.
My input is a 1MHz sine wave and my clk is 80MHz.
When I do fft about SFDR,my peak value is not in 1MHz in fft picture.
Can you tell me detail if you know.
Thank you.


My input is out1.txt

0010101010
0010101010
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1100000001
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1100000001
0111111111
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and my matlab code is
clc;
clear;
fclk=80E+6;
numpt=1200;
numbit=10;
load out1.txt; % Load data from disk;
a=out1';
N=length(a);
[M,N]=size(a); % Number of data;
for i=1:1:N; %
c=int2str(a(i)); % Change integer type data to string type;
temp=0; %
Nlength=length(c); % Length of the string;
for j=1:1:Nlength; %
d=str2num(c(j))*2^(Nlength-j); % Binary to decimalization
temp=temp+d; %
end; %
code(i)=temp;%/3988*2.5; %
end;
N=length(code);
%Display a warning, when the input generates a code greater than full-scale
if (max(code)==2^numbit-1) | (min(code)==0)
disp('Warning: ADC may be clipping!!!');
end
%Plot results in the time domain

zoom xon;
%Recenter the digital sine wave
Dout=code-(2^numbit-1)/2;
subplot(3,1,1);
plot([1:N],Dout);
title('TIME DOMAIN');
xlabel('SAMPLES');
ylabel('DIGITAL OUTPUT CODE');
%If no window function is used, the input tone must be chosen to be unique and with
%regard to the sampling frequency. To achieve this prime numbers are introduced and the
%input tone is determined by fIN = fSAMPLE * (Prime Number / Data Record Size).
%To relax this requirement, window functions such as HANNING and HAMING (see below) can
%be introduced, however the fundamental in the resulting FFT spectrum appears 'sharper'
%without the use of window functions.
%Doutw=Dout;
window=hanning(numpt);
subplot(3,1,2);
plot(window);
Doutw=Dout.*window';
%Doutw=Dout.*hamming(numpt);
%Performing the Fast Fourier Transform
Dout_spect=fft(Doutw,numpt);
%Recalculate to dB
Dout_dB=20*log10(abs(Dout_spect));

subplot(3,1,3);

plot([1:N/2],Dout_dB(1:N/2));
%Display the results in the frequency domain with an FFT plot

maxdB=max(Dout_dB(2:numpt/2));
%For TTIMD, use the following short routine, normalized to —6.5dB full-scale.
plot([0:numpt/2-1].*fclk/numpt,Dout_dB(1:numpt/2)-maxdB-6.5);
%plot([0:numpt/2-1].*fclk/numpt,Dout_dB(1:numpt/2)-maxdB);

title('FFT PLOT');
xlabel('ANALOG INPUT FREQUENCY (MHz)');
ylabel('AMPLITUDE (dB)');
%a1=axis; axis([a1(1) a1(2) -120 a1(4)]);
 

1011011001

First, you should do fft analysis on 1024 points not 1200 points as in you code.
Second, you shouldn't sample only one period of sin wave.
For example: Fclk=80M, sample points=1024, circles=499(should be a prime number), so fsin='80*499/1024'M
 

1010101110

flushrat said:
First, you should do fft analysis on 1024 points not 1200 points as in you code.
Second, you shouldn't sample only one period of sin wave.
For example: Fclk=80M, sample points=1024, circles=499(should be a prime number), so fsin='80*499/1024'M
Click to expand...

Dear flushrat,
Why do I do fft must be 1024 or 512 points?
Is your fsin a input sine wave?
Why circles must be a prime number?
Thank you.
 

0110100100

Dear Yen,

The FFT is FAST algorithm to obtain the Discrete Fourier Transform (DFT) of a signal, where the number of points MUST be a power of two.

The DFT assumes that the input vector (i.e. the samples) repeats itself until infinity. Theferore, when making the FFT/DFT of a sinewase one must guarantee that the first and last samples are ADJACENT. If this does not happen there is SPECTRAL LEAKAGE. To find out more go to:

https://zone.ni.com/devzone/conceptd.nsf/webmain/4D2F2B3C170CC02E862568C1006857EB

or search google. This is ensured by carefully choosing either the input or the clock frequency:
M.Ti = N.Ts
Where
Ti-input signal period
Ts- clock period
N- number of points in the DFT/FFT
M- Prime number.

In principle, M could be any number, but there would be the risk of sampling repeatedly the the same values of the input sinewave. If M is prime this does not happen.

Rgds
 

1000111010

Hi yen,

Your question in the forum sparked my interest. How do you generate the file out1.txt? Is it from SPICE simulation output? Is it resulted from transient analysis?If so what is the syntax to generate the file? Appreciate if you can share.

thanks in advance
-snoop835-
 

0001001001

Dear Yen,
this page can help you
**broken link removed**
Rgds
 

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