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.
It can also recieve the signal level on the tuned frequency. This enables transmitter application to scan all frequencies, checking for vacant ones, and suggesting empty frequencies to the user for selection as transmission frequencies.
The n900 controls the FM transmitter through the I2C bus, and supplies analog audio to the transmitter.
The FM signal is transmitted from a loop antenna built into the case, and connected to the PCB with clips.
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