Given MATLAB Code
This MATLAB program attempts to plot 13 cycles of a 13 Hz sinusoid.
F0 =13;
T0 =1/F0;
Ts = 0.07;
t = 0:Ts:13*T0;
x = real(exp(j*(2*pi*F0*t-pi/2)));
plot(t,x)
Correct MATLAB Code
F0 =13;
T0 =1/F0;
Ts = 0.0007;
t = 0:Ts:13*T0;
x = real(exp(j*(2*pi*F0*t-pi/2)));
plot(t,x)
The problem was that the sampling frequency Ts was too high. I decreased it by a factor of 100 and it produced the correct graph.