FREQUENCY MODULATION (FM) BROADCASTING BASIC TUTORIALS

WHAT IS FREQUENCY MODULATION (FM) BROADCASTING?

The monophonic system of FM broadcasting was developed to allow sound transmission of voice and music for reception by the general public for audio frequencies from 50 to 15,000 Hz, all to be contained within a +/−75-kHz RF bandwidth.

This technique provided higher fidelity reception than was available with standard broadcast AM along with less received noise and interference. FM broadcasting in the U.S. is allocated the 88–108 MHz frequency band.

Pre-emphasis is employed in anFMbroadcast transmitter to improve the received signal-to-noise ratio. The pre-emphasis upper-frequency limit is based on a time constant of 75 μs as required by the FCC for FM broadcast transmitters.

Audio frequencies from 50 to 2120 Hz are transmitted with normal FM; whereas audio frequencies from 2120 Hz to 15 kHz are emphasized with a larger modulation index. There is significant signal-to-noise improvement at the receiver, which is equipped with a matching de-emphasis circuit.