How to generate a signal which frequency are obtained?
Your code is wrong because when you want to get a signal whose frequency changes with time, then the argument of the sine you generate is given by the integral of the function you want to generate in frequency (see https://en.wikipedia.org/wiki/Chirp). That's why, in your case, your f function should be:

$f(t)=\int_0^t800+500\sin{2\pi t}=\phi_0 + 800t -\frac{500}{2\pi} \cos(2\pi t)$

If you set the zero phase $\phi_0=0$, then you would obtain the signal you are looking for. Apart from that, take into account that if you just want that signal in the positive part of the spectrum of your double sided spectrum, then you should do $e^{if(t)}$, because if you just take the sine of $f(t)$ then the signal would be replicated in your negative side of the spectrum.