summation of 2 Sine waves

Hi all,

I was hoping someone can help me wrap my head around why matlab does not add sine waves together.

Im trying to plot 2 added sine waves that are 180 degrees out of phase with the same amplitude. The added plot should show a stright line at 0 but im getting a strange array of signals.

If I plot the sine waves and sum wave on the some plot they seem to work which is confusing me even more.

here is my code.

A = 1 % Amplitude is 1 V

w = 2*pi*2; % w = 2Hz (frequency)

b = 2*pi/.5 % calculating wave length gives 0.5m

x = 0 :.005:1; % x axis from 0 to 1 with sampling every .005

L = 1; % length of transmission line

t = 0.0; % time = 0 (starting time)

%--------------------------------------------------------------------------

% signal generation

y = A*sin(b*x + (w*t)); % Wave equation for a transmission line

subplot(3,1,1);

plot (x, y);

axis ([0 1 -1.25 1.25]); % graph limits

grid on;

grid minor;

title ('Waveform Along A Transmission Line')

xlabel ('Metres');

ylabel ('Amplitude (V)');

set(gca, 'fontsize', 12); % change font size

%--------------------------------------------------------------------------

% Reflected wave

r = A*sin(b*(L-x)+ (w*t)); % reflected waveform, 180 out of phase

subplot(3,1,2);

plot (x, r);

axis ([0 1 -1.25 1.25]); % graph limits

grid on;

grid minor;

title ('Reflected Waveform Along A Transmission Line');

xlabel ('Metres');

ylabel ('Amplitude (V)');

set(gca, 'fontsize', 12); % change font size

%-------------------------------------------------------------------------

% Waveform sum

t = r + y;

subplot(3,1,3);

%plot (x,y);

%hold on;

%plot (x,r);

%hold on;

plot(x,t);

hold on;

I hope someone can clear up my mistake because it's driving me mad haha.

Jay

Answer 1

[Star Strider]

It does show a straight line at zero. Look at the magnitude of the y-axis (±2E-15), and you will see that it is essentially plotting floating-point approximation error.

Add:

ylim([-1 1]) 

to subplot(3,1,3) (so it’s the same as the others) and you get the result you expect.

The full code for it now being:

t = r + y;
subplot(3,1,3);
%plot (x,y);
%hold on;
%plot (x,r);
%hold on;
plot(x,t);
hold on;
ylim([-1 1])

.

[Jay Perks]

You my friend, have no idea how much time I have spent on this :P

Thankyou so much for your answer!

[Star Strider]

As always, my pleasure!