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problem with filtering a signal

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hossein
hossein 2013-7-8
I wanna filter a signal by band-pass filter and the problem is when I exert the filter on my signal the output is so wrong.I try to exert the filter on a easy signal to test it: for example when I exert a band pass filter with 8-12 frequency range to the x=sin(2*pi*10*t)+sin(2*pi*20*t) the output must similar to sin(2*pi*10*t).but its not. I dont know where I am wrong?

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回答（2 个）

Wayne King
Wayne King 2013-7-8
Fs = 250;
t = 0:1/Fs:10-1/Fs;
x = sin(2*pi*10*t)+sin(2*pi*20*t)+0.25*randn(size(t));
y = sosfilt(SOS,x);
I used the following parameters in fdatool:
Fs = 250 fstop1 = 7 fpass1 = 9 fpass2 = 12 fstop2 = 14
60 dB of attenuation in both stopbands and 1 db of passband ripple.
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hossein
hossein 2013-7-9
so what about the gain of filter. the y has the amplitude by 10^4 times bigger than x here. you didn't use the filter's gain

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Wayne King
Wayne King 2013-7-8
编辑：Wayne King 2013-7-8
You need to post the code you have used to design your filter. Also, include the sampling rate for your data.
For example:
Fs = 100;
t = 0:1/Fs:10-1/Fs;
x = sin(2*pi*10*t)+sin(2*pi*20*t)+0.25*randn(size(t));
d = fdesign.bandpass('Fst1,Fp1,Fp2,Fst2,Ast1,Ap,Ast2',4,8,12,16,50,1,50,100);
Hd = design(d);
y = filter(Hd,x);
periodogram(y,[],length(y),100)
  1 个评论
hossein
hossein 2013-7-8
编辑：hossein 2013-7-8
I design the filter by fdatool by these specification:
chebyshef type 2; fs=250; fstop1=7;fpass1=9;fpass2=12;fstop2=13;
then I get the SOS and G matrixes then
[a,b]= sos2tf(SOS,G)
x= sin(2*pi*10*t)+sin(2*pi*20*t)
y=filtfilt(a,b,x) or y=filter(a,b,x)

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