Timothy asked . 2021-04-02

Problem with Square() for high frequenzies

xsmpling=2
T = 1; % simulation time in seconds
fs =44100*xsmpling; Ts = 1/fs; % sampling frequency in Hz and sampling period
Nmax = T*fs; n = 0:Nmax-1; % simulation time (norm.)

f=20000%fsignal

x = sin(2*pi*f*Ts*n);
d = square(2*pi*f*n*Ts);

%% Spectrum
X = abs(fft(x)); X = X/max(X);
%figure('Name','frequenzy domain','NumberTitle','off');
figure()

plot((fs/Nmax)*(0:Nmax-1),20*log10(X),'LineWidth',2), grid
hold on 
line([fs/2 fs/2],[-100 10],'LineWidth',1,'LineStyle','--')
line([20000 20000],[-100 10],'color','green','LineWidth',1,'LineStyle','--')
title('Analysis of Aliasing','FontSize',20)
hold on
axis([0 fs -60 0]);
ax = gca;
ax.FontSize = 20;
xlabel('Frequency in Hz \rightarrow','FontSize',20)
ylabel('Spectrum Magnitude in dB \rightarrow','FontSize',20)
Y = abs(fft(d)); Y = Y/max(Y);
%figure('Name','Chirp signal','NumberTitle','off');
plot((fs/Nmax)*(0:Nmax-1),20*log10(Y),'LineWidth',2), grid
legend('Signal','Nyquist','20kHz','Distortion','FontSize',20)
hold off

matlab , signal , signal processing