The problem was that Ts (1/Sampling Frequency) was not smaller enough which means the Sampling frequency was not big enough. The Sampling Frequency should have been at least bigger than 2*pi*F0 and Ts should have been smaller than $ {1/(2*pi*F0)} $,
F0 =13; T0 =(1/F0); Ts = 0.0001; t = 0:Ts:13*T0; x = real(exp(j*(2*pi*F0*t-pi/2))); plot(t,x) %F0 =13; %T0 =1/F0; %Ts = 0.07; %t = 0:Ts:13*T0; %x = real(exp(j*(2*pi*F0*t-pi/2)));
