The time step is too big, so each time it samples at the time step it is pulling a point from a different cycle. It's doing a discrete time sample on a continuous function and making it look like a continuous function by connecting the points. To fix this, make the time step smaller.
F0 =13; T0 =1/F0; Ts = 0.0005; t = 0:Ts:13*T0; x = real(exp(j*(2*pi*F0*t-pi/2))); plot(t,x)
