Design of a Cell phone Jammer

Hi guys,
It's time for a mini project in communications, and I've thought about a cell phone Jammer. Short range, of course.

Now, I want you guys to chip in some ideas about how to go about this.

Basically, I know that cell phones operate in the 800 - 1800 MHz band. And to jam the cell phones, we have to create a random noise at the same frequency with more power. This much I know.

Now, the doubt is, how to find on which particular frequency the phone is currently operating. I guess we cant do that, so we have to sweep the entire frequency range continuously. How fast should I do it? ie, how fast should the frequency sweep from the lower end to the higher end?

Ideas please.. Help in circuit level implementation is also welcome!! 😀😀