Hi,
The issue with your code is that you are not actually modulating the frequency of the square wave correctly. The way you are implementing it, you are adding a phase shift to the cosine terms, which does not directly modulate the frequency.
To modulate the frequency of a square wave correctly using a DC value, you need to generate the square wave first, and then modulate its frequency based on the DC value.
% Define parameters
t = linspace(0, 1, 1000); % Time vector
fc = 5; % Fundamental frequency (Hz)
kf = 0.1; % Frequency deviation constant
Vc = 1; % Amplitude of the carrier signal
Vdc_values = [0, 5, 10]; % Values of Vdc for the DC signals
% Generate carrier signal (square wave)
vc = square(2*pi*fc*t);
% Plot carrier signal
figure;
plot(t, vc, 'LineWidth', 2);
xlabel('Time');
ylabel('Amplitude');
title('Carrier Signal (Square Wave)');
ylim([-1.2, 1.2]); % Adjust ylim as needed
grid on;
% Frequency domain parameters
fs = 1000; % Sampling frequency
f = linspace(0, fs, 1000); % Frequency vector
% Generate and plot frequency-modulated signals separately for each Vdc
for i = 1:length(Vdc_values)
Vdc = Vdc_values(i);
% Generate frequency-modulated signal
fm_freq = fc + kf * Vdc; % Modulated frequency
v_fm = square(2*pi*fm_freq*t); % Frequency-modulated square wave
% Compute Fourier transform of frequency-modulated signal
V_fm = abs(fft(v_fm));
% Plot frequency-modulated signal in the time domain
figure;
subplot(2, 1, 1);
plot(t, v_fm, 'LineWidth', 2);
xlabel('Time');
ylabel('Amplitude');
title(['Frequency-Modulated Signal with Vdc = ', num2str(Vdc), ' (Time Domain)']);
ylim([-1.2, 1.2]); % Adjust ylim as needed
grid on;
% Plot frequency-modulated signal in the frequency domain
subplot(2, 1, 2);
plot(f, V_fm, 'LineWidth', 2);
xlabel('Frequency (Hz)');
ylabel('Magnitude');
title(['Frequency-Modulated Signal with Vdc = ', num2str(Vdc), ' (Frequency Domain)']);
xlim([0, 200]); % Adjust xlim as needed
grid on;
end
Thanks, hope this helps.
