The problem was that the step size was too small for the increments in the t vector. When Ts is changed to .0001 the code works as expected.
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)
