In this post we present modulation index formula and calculators for three different modulation formats: **AM**, **FM** and **FSK**.

Table of Contents

**Amplitude Modulation Index**

Modulation index is a measure of the variation of the carrier signal by the modulating signal.

**Formula**

*m = A _{m} / A_{c}*

Where,

**m**is the modulation index**A**is the modulation amplitude_{m}**A**is the carrier amplitude_{c}

**Calculator**

Enter **A _{m}** and

**A**in volt.

_{c}**Example**

The picture below shows the message or modulating waveform and the carrier.

The three different pictures represent m=0.5, 1.0 and 1.5.

## Frequency Modulation

The modulation index of a FM signal is a measure of how much the carrier frequency deviates as a result of the modulating frequency.

**Formula**

**h = f_{d} / f_{m}**

Where,

**f**is the highest frequency component present in the modulating signal_{m}is the maximum deviation of the instantaneous frequency from the carrier frequency**f**_{d}

**Calculator**

**Examples**

An audio signal has a maximum frequency **f _{m}** of 20 kHz. If the FM system used to broadcast this information has a maximum deviation

**f**of ±75 kHz, then the modulation index

_{d}**h**is 3.75.

## Frequency Shift Keying

FSK is the digital form of FM. As such the modulation index of an FSK signal is defined as the ratio of the frequency deviation of the modulated signal to the bandwidth of the modulating signal (or the message).

**Formula**

*h* = 2 * *f _{d}T* / (

*M*-1)

Where,

**h**is the modulation index**T**is the symbol period**f**is the peak frequency deviation_{d}**M**is the order of the modulation

**Calculator**

**Example**

For a 4-FSK signal with a peak frequency deviation of 1 MHz and symbol period of 2 micro-seconds, the modulation index is 1.33.