FM Index and Carson Bandwidth Tool

Frequency Modulation Calculator

Calculate FM modulation index, Carson bandwidth, frequency deviation, and approximate occupied range from deviation and modulating frequency.

FM Index, Deviation, and Carson Bandwidth

Calculate FM Modulation Index

Enter frequency deviation and modulating frequency to calculate FM modulation index and estimate bandwidth with Carson's Rule.

Frequency Deviation

kHz

0250 kHz

Modulating Frequency

kHz

0.150 kHz

Carrier Frequency

MHz

0500 MHz

Formula: FM modulation index beta = frequency deviation / modulating frequency. Carson bandwidth = 2 x (deviation + modulating frequency).

FM Modulation Index Formula

Frequency modulation index beta equals frequency deviation divided by the modulating frequency. Higher beta values indicate wider deviation relative to the message frequency.

Carson Rule Bandwidth Formula

Carson's Rule estimates FM bandwidth as 2 x (frequency deviation + highest modulating frequency). It is useful for quick channel planning and classroom calculations.

Narrowband vs Wideband FM

Index Range	FM Type	Meaning
Below 1	Narrowband FM	Small frequency deviation relative to the modulating frequency.
1 to 5	Medium deviation	Moderate deviation and wider occupied bandwidth.
Above 5	Wideband FM	Large deviation with many significant sidebands.

Frequently Asked Questions

How do you calculate FM modulation index?+

FM modulation index equals frequency deviation divided by modulating frequency. It is often written as beta = delta f / fm.

What is Carson Rule for FM bandwidth?+

Carson's Rule estimates FM bandwidth as 2 x (frequency deviation + highest modulating frequency).

What is the difference between AM and FM modulation?+

AM changes carrier amplitude. FM changes carrier frequency. Both carry information by varying a carrier wave in different ways.

Is Carson bandwidth exact?+

No. Carson bandwidth is a practical estimate for significant FM sidebands, not an exact limit for every signal component.