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Altera_Forum
Honored Contributor I
879 Views

dft problem

greetings, 

 

hi i have typed followin code to verify samplng theorem but the answer is coming something different.may i know the reason. 

clc; 

fs=800; 

t=0:1/fs:1 

x=cos(2*pi*400*t); 

xm=(fft(x)); 

k=0:length(xm)-1; 

subplot(2,2,1); 

stem(k,xm) 

xlabel('hertz') 

ylabel('magnitude') 

title('sampling') 

 

 

the result should come at only one frequency(i.e one line at frequency of cosine) as the signal is cosine but spectrum is continious. why ? 

the spectrum (o/p)is attached here 

thanks in advance
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4 Replies
Altera_Forum
Honored Contributor I
21 Views

You're doing: 

t=0:1/fs:1 

x=cos(2*pi*400*t); 

 

The step is 1/fs, which is 1/800 = 0.00125 

This means, that from range 0 to 1, every 0.00125 step You'll get a new frequency (because argument 't' in cosine function is changing), that is: 800 results. So what do You expect in frequency domain to see?
Altera_Forum
Honored Contributor I
21 Views

your frequency is half sampling rate and is one line mirrored 

plot(k,abs(xm));
Altera_Forum
Honored Contributor I
21 Views

 

--- Quote Start ---  

You're doing: 

t=0:1/fs:1 

x=cos(2*pi*400*t); 

 

The step is 1/fs, which is 1/800 = 0.00125 

This means, that from range 0 to 1, every 0.00125 step You'll get a new frequency (because argument 't' in cosine function is changing), that is: 800 results. So what do You expect in frequency domain to see? 

--- Quote End ---  

 

 

but my signal is cosine and the sampling rate is also twice of highest frequency ,so corresponding i must gate only two frequency component.
Altera_Forum
Honored Contributor I
21 Views

But I told you you have got it. Just your plot is wrong.

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