代码：
b = [0.0181, 0.0543, 0.0543, 0.0181]; % filter coefficient array b
a = [1.0000, -1.7600, 1.1829, -0.2781]; % filter coefficient array a
m = 0:length(b)-1; l = 0:length(a)-1; % index arrays m and l
K = 500; k = 0:1:K; % index array k for frequencies
w = k*pi/500; % freqency between 0 and +pi, [0,pi] axis divided into 501 points. num = b * exp(-j*m'*w); % Numerator calculations
den = a * exp(-j*l'*w); % Denominator claculations
H = num ./ den; % Frequency response magH = abs(H); angH = angle(H); realH = real(H); imagH = imag(H); %% --------------------------------------------------------------------
%% START H's mag ang real imag
%% --------------------------------------------------------------------
figure('NumberTitle', 'off', 'Name', 'Example3.16');
set(gcf,'Color','white');
subplot(2,2,1); plot(w/pi,magH); grid on; axis([0,1,0,1.5]);
title('Magnitude Response');
xlabel('frequency in \pi units'); ylabel('Magnitude |H|');
subplot(2,2,3); plot(w/pi, angH/pi); grid on; % axis([-1,1,-1,1]);
title('Phase Response');
xlabel('frequency in \pi units'); ylabel('Radians/\pi'); subplot('2,2,2'); plot(w/pi, realH); grid on;
title('Real Part');
xlabel('frequency in \pi units'); ylabel('Real');
subplot('2,2,4'); plot(w/pi, imagH); grid on;
title('Imaginary Part');
xlabel('frequency in \pi units'); ylabel('Imaginary');
%% -------------------------------------------------------------------
%% END X's mag ang real imag
%% -------------------------------------------------------------------
运行结果：
上图看出，是个低通滤波器。