N900 FM radio transmitter

The SI4713 chip is used to transmit FM radio in the range 76-108MHz from a digital or analog audio input.

(which is used on the n900?)

https://www.silabs.com/products/audiovideo/fmtransmitters/Pages/Si471213.aspx

The documentation for the driver, along with some information on the chip is at http://www.mjmwired.net/kernel/Documentation/video4linux/si470x.txt