I want to change my code to a code like the one in the picture below, but how can I modify it?

clc
close all
dt = 1e-4;
Fs = 1/dt;
t=0:dt:0.5;
f=5;
y=sin(2*pi*f*t);
subplot(3,1,1)
plot(t,y)
hold on
yline(0)
xline(0.2)
ylim([-2 2])
hold off
subplot(3,1,2)
y1=sin(2*pi*100*t);
y3=y1.*y;
plot(t,y3)
hold on
yline(0)
hold off
subplot(3,1,3)
y4=y3.*sin(2*pi*100*t);
% y5=lowpass(y4,1,1e4)
[BlpFilt,AlpFilt] = butter(4,0.01);
% y5 =filter(BlpFilt,AlpFilt,y4);
y5 =filtfilt(BlpFilt,AlpFilt,y4);

plot(t,2*y5,'-*r')
hold on
ylim([-2 2])
plot(t,y);
hold off

[Rik]

I suspect it will be much easier to plot the sinusoid and the envelope as separate line objects.

[EonYak Kang]

thanks for the advice :)

Answer 1

[Jonas]

are you searching for something like that:

close all;
fs=4000;


t=0:1/fs:0.8;

myData=[0 3;
        0.2 -2;
        0.4 -1;
        0.6 -2;
        0.8 3]; % x y

myData=interp1(myData(:,1),myData(:,2),t,'pchip');

subplot(3,1,1);
plot(t,myData)
yline(0);

modulated = ammod(myData,100,fs);
% or modulated=sin(2*pi*100*t).*myData; % if you don't have the toolbox
subplot(3,1,2);
plot(t,modulated)

subplot(3,1,3)
plot(t,0.5*modulated+0.5*myData)

[EonYak Kang]

Thank you so much! This code was what I wanted to solve :)

Answer 2

[Sam Chak]

Hi @EonYak Kang

Looks like a signal modulation problem in Communications Engineering.

If my guess is correct, then is the Modulator and is the Carrier signal.

You must have some knowledge in the mathematics of signal processing in order to "guess" the shape of the modulator.

Try adjusting the angular frequency yourself. The basic shape is shown below:

t = linspace(0, 2*pi, 2001);
m1 = 0.35*cos(2*pi*t/2);
m2 = cos(2*pi*t/6);
m = m1 + m2;
plot(t, m1, t, m2)
hold on
plot(t, m, 'linewidth', 2)
hold off

If there is no change in the carrier signal, I think you can complete the rest just like @Jonas has demonstrated.

[EonYak Kang]

Thank you! I'll try it a try as you said.