因為脈衝訊號的頻譜是全頻段的，如下所示。

所以濾波器脈衝響應的傅立葉變換即為濾波其的頻率響應，下圖為fvtool得到的頻率濾波器頻率響應。

濾波器的脈衝響應如下圖所示。

脈衝響應的FFT變換如下。

本例源程式如下：

clear
clc


%frequency of impluse
Impluse = zeros(1,1024);
F=linspace(0,2,1024);
Impluse(1)=1;
IMP = fft(Impluse);
IMP = abs(IMP);
figure
plot(F(1:512),IMP(1:512));
grid on

% generate filter
Fpass = 0.15;  % Passband Frequency
Fstop = 0.2;   % Stopband Frequency
Apass = 1;     % Passband Ripple (dB)
Astop = 60;    % Stopband Attenuation (dB)

h = fdesign.lowpass('fp,fst,ap,ast', Fpass, Fstop, Apass, Astop);

Hd = design(h, 'equiripple', ...
    'MinOrder', 'any', ...
    'StopbandShape', 'flat');
fvtool(Hd)

% plot filter's impluse response
HdImpluse = Hd.Numerator;
figure
plot(HdImpluse)
grid on

HdImp=zeros(1,1024);
HdImp(1:length(HdImpluse))=HdImpluse;
HDIMP = fft(HdImp);
HDIMP = abs(HDIMP);
figure
plot(F(1:512),20*log10(HDIMP(1:512)));
grid on