Transfer function of a filter
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I want to plot the transfer function of a butterworth filter and the frequency should be in hertz.
I tried this code
[n wn]=buttord(2*pi*3/2000,2*pi*300/2000,3,20);
[b a]=butter(n,wn,'low');
[h w]=freqz(b,a);
stem(w,20*log10(abs(h)));
But what is 'w'? I want to represent the frequency in Hz. What should I do now so that I get -3 db attenuation at 3Hz and -20 db attenuation at 300 Hz in the graph.
回答（3 个）
Wayne King
2011-10-11
[h w]=freqz(b,a,[],Fs);
where Fs is your sampling frequency.
2 个评论
Wayne King
2011-10-11
Of course you might want to call the output, f.
[h,f] = freqz(b,a,[],Fs);
i Venky
2011-10-11
Wayne King
2011-10-11
I recommend that you use fdesign.lowpass
d = fdesign.lowpass('Fp,Fst,Ap,Ast',290,300,0.5,20,2000);
Hd = design(d,'butter');
fvtool(Hd);
zoom in on 300 Hz and see what you think.
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