Amplitude Modulation Efficiency: AM efficiency

Amplitude modulation, AM offers simplicity but it is not the most efficient form of modulation in terms of power or spectrum usage.

Amplitude Modulation, AM Tutorial Includes:
Amplitude modulation, AM     AM basic theory & formulas     AM bandwidth & sidebands     Modulation index & depth     AM efficiency     AM demodulation / detection     Diode detector     Synchronous detector     AM modulators     Single sideband, SSB     SSB demodulation    

Modulation formats:     Modulation types & techniques     Frequency modulation     Phase modulation     Quadrature amplitude modulation    

Amplitude modulation, AM was the first form of modulation to be used for voice and audio broadcasting. It has advantages of simplicity, but it does not offer the levels of efficiency provided by many other forms of modulation.

Amplitude efficiency levels fall behind other types of modulation in terms of both spectrum usage and power utilisation.

It was its relative simplicity that enabled it to be used in the early days of wireless communications, but nowadays, other forms of modulation offer much greater levels of efficiency and effectiveness.

AM power efficiency

The power efficiency of amplitude modulation is very low. To see why this is the case, it is necessary to look at the composition of the radio signal and the relative power levels of its constituents: the carrier and the two sidebands.

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 rises to twice the un-modulated level and then falls to zero. This is called 100% modulation as it is the maximum amount that can be applied.

If the level of modulation is increased beyond the 100% level, then distortion will arise because the envelope cannot fall below zero.

Amplitude modulation signal with 100% modulation
Amplitude modulated signal with 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.

Sideband levels for 100% modulated AM signal
Sideband levels for 100% modulated AM 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

The spectral efficiency of amplitude modulation is also poor. With pressure on radio spectrum increasing, spectral efficiency is a factor that is of growing importance in areas where the spectrum is required.

To see why the spectral efficiency of amplitude is poor, take the example where an audio signal is applied to the carrier. Under these conditions the audio spreads out either side of the carrier - the audio spectrum being reflected either side of the carrier as shown below.

bandwidth of an amplitude modulated signal showing poor spectrum efficiency
Bandwidth of 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 of the audio or other signal that is modulated onto the transmitted signal. 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.

As a result of its poor efficiency levels, the use of amplitude modulation is dwindling. Its current main uses are for broadcasting on the long, medium and short wave bands as well as some short range use for aircraft communications.

AM broadcasting usage is falling as better forms of broadcasting re available, both with frequency modulation and with digital formats. AM offers low levels of efficiency as well as a low tolerance to interference. Accordingly its use will continue to fall.

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