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%傅立叶变换,时域卷积等效于频域乘积
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%滤波操作在时域表现为输入信号余滤波器脉冲响应的卷积
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%从频域上看滤波器操作表现为,输入信号的傅立叶变换和脉冲响应的傅立叶变换做乘积
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%对于FIR滤波器,滤波器系数即为脉冲响应
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%因此,对于FIR滤波器,系数的FFT变换即为滤波器的频率响应曲线
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close all
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clear
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Fpass = 0.45; % Passband Frequency
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Fstop = 0.55; % Stopband Frequency
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Apass = 1; % Passband Ripple (dB)
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Astop = 60; % Stopband Attenuation (dB)
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h = fdesign.lowpass('fp,fst,ap,ast', Fpass, Fstop, Apass, Astop);
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Hd = design(h, 'equiripple', ...
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'MinOrder', 'any', ...
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'StopbandShape', 'flat');
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Hlp = Hd.Numerator;
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fvtool(Hlp);
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HLP=fft(Hlp);
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HLP=abs(HLP);
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F=linspace(0,2,length(Hlp));
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N=ceil(length(F)/2);
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plot(F(1:N),20*log10(HLP(1:N)));
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grid on;