What is deviation ratio?

In frequency modulation (FM), the deviation ratio is defined as the maximum carrier frequency deviation divided by the highest audio modulating frequency. It establishes a standard measure of the FM system's performance constraints and bandwidth requirements. It is important to distinguish deviation ratio from modulation index. While modulation index uses the actual frequency deviation and audio frequencies present at a specific moment, the deviation ratio strictly uses the absolute maximum limits designed into the communications system. Distractors mentioning the center carrier frequency or average audio modulating frequency are incorrect. The deviation ratio only deals with the maximum frequency shift (deviation) and the maximum pitch (highest audio frequency) introduced by the audio signal.