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# the FFT problem,help!

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#### power_pwer

##### Junior Member level 1
dear all:
happy new year! and i need your help with the discrete fourier transform for my first order
sigle bit sigma delta modulator output.the problem is like this:
the input signal is sig=1.5+0.5*sin(2*pi*200K*t);then i FFT the signal:sig1=1.5+0.5*sin(2*pi*K/100)(K=1,2,3....400). the results of this FFT are F(1)=600;F(5)=F(397)=100;F(any other point)=0;
however,after the signal pass the first order sigle bit modulator(sampling frequency is 20M),400 points bitstream is got,then I FFT the bitstream,the results become:F(1)=600;F(177)=F(225)=102.5643;F(5)=F(397)=101.0066;F(169)=F(233)=89.7403;......
my question is: why are the two results different? and why the values on the harmonic frequency are not zero in the practical system output?

hello,anyone help?

Hi,the calculation is ideal.Do you practical system is simulation or real circuit?

hello,snail888,my practical system is a real circuit

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