somethings wrong with LPF filter?

Question

Ngoc Lam 2014-11-30

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/164828-somethings-wrong-with-lpf-filter

回答： Ngoc Lam 2014-12-2

Hi all, I want to simulate a LPF Filter with parameters: fc=5Khz,Fs=20.5Khz, N =30, Hamming window.

so I initialized a sine wave with fcarrrier = 20Khz, each sine wave was initialized with 200 sample. then it was multiplied by itself! following this code below:

      % code
M=200; % the number of sample of sine periodic
fcarrier=1e4;
T=128/fcarrier; % time to transmit sine wave
P=128; % the number of sine wave initialized!
 t=0: T/M/P: T-T/M/P;
 fs=length(t)/(T-T/M/P);
 df=1/(T-T/M/P);
 f=-fs/2:df:fs/2-df;
 s=sin(2*pi*fcarrier*t);
 s1=s.*s;
 % fc=5Khz,fs= 20.5Khz, N=30,Hamming
 Num=[-0.00142569822564206,0.00104684023675550,0.00257766825164500,-0.00197666192599148,-0.00613453687959036,0.00369061404220200,0.0132498601493775,-0.00590859914987426,-0.0257994044307132,0.00825012956971075,0.0481483043656084,-0.0103040044663240,-0.0961937471263727,0.0117048121269473,0.315044085886135,0.488060675152253,0.315044085886135,0.0117048121269473,-0.0961937471263727,-0.0103040044663240,0.0481483043656084,0.00825012956971075,-0.0257994044307132,-0.00590859914987426,0.0132498601493775,0.00369061404220200,-0.00613453687959036,-0.00197666192599148,0.00257766825164500,0.00104684023675550,-0.00142569822564206];
 s2=filter(Num,1,s1);
 figure(1)
 subplot(2,1,1)
 plot(s)
  title('sine initialized');
 subplot(2,1,2)                                                                                                                                                                              
 plot(f,abs(fftshift(fft(s))))
 figure(2)
 subplot(2,1,1)
 plot(s1)
 title('sine after mul');
 subplot(2,1,2)
 plot(f,abs(fftshift(fft(s1))))
  figure(3)
 subplot(2,1,1)
 plot(s2)
 title('sine after filtering')
 subplot(2,1,2)
 plot(f,abs(fftshift(fft(s2))))

the filter seem does not effect the sine wave, the spectrum of sine wave after filtering seem similar with sine wave after mul! Can anyone help me out! thanks