From what I understand of your post, you've got an IR remote control type signal and you want to demodulate it to obtain the digital information so that you can re-transmit it via RF.
In order to demodulate the signal you need to dispose of the 40Khz carrier, but before you do this you need to design an AGC stage so that the signal provided to the the demod circuit is of the correct level. Most IR Remote protocols have information contained within the transmission (the leading pulse) which set the level for the AGC. Once the level of the incomming signal has been set properly you can strip away the 40Khz signal which leaves you with the original digital information. To do this you need to design a suitable filter circuit to take away the 40Khz carrier.
If all that sounds like way too much work, you could always use one of the "ready made" components out there which will do all of the work for you. You can get IR Remote ICs which allow you to feed in the 40Khz carrier signal and they will feed out the digital information, and you can even get packages which make up the receiving IR Diode, the AGC stage and demod circuit all in one go! The Sharp GP1U52X is an example of this. With this device you can receive the IR Remote signal and the device will feed out the digital information for you, so there's no need to be designing anything yourself in that respect.
Hope this is of use.
By the way, hi everyone - this is my first post.
Brian