Amplitude Modulation Efficiency

- notes and details about AM, amplitude modulation efficiency, particularly in terms of power and spectrum.

Amplitude modulation, AM has advantages of simplicity, but it is not the most efficient mode to use.

Amplitude efficiency both in terms of spectrum usage and power utilisation is poor when compared to the many other modes of transmission hat are available.

It is for this reason that it only has limited applications for broadcast and two way radio communications systems.

AM power efficiency basics

In terms of power, the amplitude modulation efficiency is very low. To see why this occurs, it is necessary to look at the composition of the radio signal.

When a radio frequency signal is modulated by an audio signal the envelope will vary. The level of modulation can be increased to a level where the envelope falls to zero and then rises to twice the un-modulated level. Any increase on this will cause distortion because the envelope cannot fall below zero. As this is the maximum amount of modulation possible it is called 100% modulation.

Amplitude modulated index of 1.0
A fully modulated carrier, i.e. 100% modulation

Even with 100% modulation, i.e. modulation index of 1.0, the power utilisation is very poor.

When the carrier is modulated sidebands appear at either side of the carrier in its frequency spectrum. Each sideband contains the information about the audio modulation. To look at how the signal is made up and the relative powers take the simplified case where the 1 kHz tone is modulating the carrier. In this case two signals will be found 1 kHz either side of the main carrier.

Level of sidebands of an amplitude modulated signal
Level of sidebands of amplitude modulated signal

When the carrier is fully modulated i.e. 100% the amplitude of the modulation is equal to half that of the main carrier, i.e. the sum of the powers of the sidebands is equal to half that of the carrier. This means that each sideband is just a quarter of the total power. In other words for a transmitter with a 100 watt carrier, the total sideband power would be 50 watts and each individual sideband would be 25 watts. During the modulation process the carrier power remains constant. It is only needed as a reference during the demodulation process. This means that the sideband power is the useful section of the signal, and this corresponds to (50 / 150) x 100%, or only 33% of the total power transmitted.

Amplitude modulation spectrum efficiency

Not only is AM wasteful in terms of power, it is also not very efficient in its use of spectrum. If the 1 kHz tone is replaced by a typical audio signal made up of a variety of sounds with different frequencies then each frequency will be present in each sideband.

Bandwidth of an amplitude modulated signal
Bandwidth of an amplitude modulated signal

Accordingly the sidebands spread out either side of the carrier as shown and the total bandwidth used is equal to twice the top frequency that is transmitted. With pressure on the radio spectrum, and other forms of modulation able to achieve far higher levels of spectrum efficiency, amplitude modulation is one of the lowest in this respect.

By Ian Poole

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