The Complete Handbook of FM Transmitters

Chapter 1: Understanding FM Transmitters

An FM transmitter is not just a single piece of equipment. It is a system, and understanding that system is the difference between a station that sounds reliable and one that spends its life in a state of intermittent failure. At its core, an FM transmitter takes an audio signal, uses it to modulate a carrier wave, amplifies the result, and then delivers it to an antenna for radiation into the surrounding area. This is the basic, unglamorous truth. However, the details of how that process is executed, and the interactions between the different stages, are where the practical problems begin.

Let's strip the process down. The audio source is generally a program feed - a mixing console output, an automation system, a remote codec, or even a simple microphone preamplifier. This audio is applied to the transmitter's exciter. The exciter is the heart of the system. It generates the base radio frequency carrier, typically a crystal-controlled oscillator in modern units. The audio signal is then used to modulate that carrier. In frequency modulation, the instantaneous frequency of the carrier is varied in sympathy with the audio signal. Louder audio pushes the frequency further from the resting center frequency; softer audio brings it closer. The amount of this deviation is the core of FM broadcasting.

For the broadcast standard used around the world, the maximum deviation is typically +/-75 kHz. That is a critical number, and you'll see it in every serious transmitter spec sheet.