# Amplitude Modulation Index & Depth It explains basics of modulation index in AM and FM. In terms of a definition: In view of the differences between the two forms of modulation, the FM modulation index is measured in a different way. Amplitude modulated index of 0.

## Amplitude modulation depth The amplitude modulation, AM, modulation index can be defined as the measure of extent of amplitude variation about an un-modulated carrier. As with other modulation indices, the modulation index for amplitude modulation, AM, indicates the amount by which the modulated carrier varies around its static un-modulated level.

When expressed as a percentage it is the same as the depth of modulation. In other words it can be expressed as:. A is the carrier amplitude. M is the modulation amplitude and is the peak change in the RF amplitude from its unmodulated value. From this it can be seen that for an AM modulation index of 0. A complementary figure to modulation index is also used for amplitude modulation signals.

Known as the modulation depth, it is typically the modulation index expressed as a percentage. Typically the modulation index of a signal will vary as the modulating signal intensity varies. However some static values enable the various levels to visualised more easily. Amplitude modulated index of 0.

When the modulation index reaches 1. Amplitude modulated index of 1. In FM, carrier frequency is varied in accordance to modulating signal frequency. The modulation index is ratio of modulating signal voltage Vm to the carrier voltage Vc.

The modulation index equation is as follows. When m is greater than 1, severe distortion results into the modulated waveform. This condition results when Vm is greater than Vc and it is also known as over modulation.

The ideal condition is when Vm is equal to Vc and m is equal to 1. In this situation, greater output is generated at the receiver with no or minimal distortion. Modulation index can be calculated by knowing modulating voltage and carrier voltage. But it is very common to measure the modulation index from the modulated waveform. The same can be viewed in the CRO i.

After the modulated envelope is displayed in the Oscilloscope, Vmax and Vmin is noted down. Using this Vm and Vc is derived using following formulas or equations. Now modulation index is calculated either taking ratio of Vm by Vc as mentioned in equation-3 below or directly using equation When modulation index is multiplied by , the degree of modulation is expressed as a percentage.