I think The bug is Ts is too big. so I decreased Ts(divided by 100000) value to get accurate graph.
F0 =13;
T0 =1/F0;
Ts = 0.07/100000;
t = 0:Ts:13*T0;
x = real(exp(j*(2*pi*F0*t-pi/2)));
plot(t,x)
