www.gusucode.com > signal 案例源码程序 matlab代码 > signal/FilterASinusoidExample.m

    %% Lowpass Filtering of Sinusoids
% Lowpass filter a discrete-time signal consisting of two sine waves.

%%
% Create a lowpass filter specification object. Specify the passband
% frequency to be $0.15\pi$ rad/sample and the stopband frequency to be
% $0.25\pi$ rad/sample. Specify 1 dB of allowable passband ripple and a
% stopband attenuation of 60 dB.

d = fdesign.lowpass('Fp,Fst,Ap,Ast',0.15,0.25,1,60);

%%
% Query the valid design methods for your filter specification object, |d|.

designmethods(d)

%%
% Create an FIR equiripple filter and view the filter magnitude response
% with |fvtool|.

Hd = design(d,'equiripple');
fvtool(Hd)

%%
% Create a signal consisting of the sum of two discrete-time sinusoids with
% frequencies of $\pi/8$ and $\pi/4$ rad/sample and amplitudes of 1 and
% 0.25, respectively. Filter the discrete-time signal with the FIR
% equiripple filter object, |Hd|.

n = 0:159;
x = 0.25*cos((pi/8)*n)+sin((pi/4)*n);
y = filter(Hd,x);

%%
% Compute the Fourier transform of the original signal and the filtered
% signal. Verify that the high-frequency component has been filtered out.

freq = 0:(2*pi)/160:pi;
xdft = fft(x);
ydft = fft(y);

figure
plot(freq/pi,abs(xdft(1:length(x)/2+1)))
hold on
plot(freq/pi,abs(ydft(1:length(y)/2+1)))
hold off

legend('Original Signal','Filtered Signal')
ylabel('Magnitude')
xlabel('Normalized Frequency (\times\pi rad/sample)')