What is AM, Amplitude Modulation

- overview or tutorial about the basics of what is amplitude modulation, AM used for modulating a radio signal to carry sound or other information.

In order that a steady radio signal or "radio carrier" can carry information it must be changed or modulated in one way so that the information can be conveyed from one place to another. There are a number of ways in which a carrier can be modulated to carry a signal - often an audio signal and the most obvious way is to vary its amplitude.

Amplitude modulation, AM is the oldest form of analogue modulation. It was first used at the beginning the twentieth century, and it is still in use today. Currently amplitude modulation is primarily used for broadcasting, but it is still used for some forms of two way radio communications. Its main radio communications use is for local aviation related VHF two way radio links. It is sued for ground to air radio communications as well as two way radio links for ground staff as well.

Amplitude modulation basics

When amplitude modulated signal is created, the amplitude of the signal is varied in line with the variations in intensity of the sound wave. In this way the overall amplitude or envelope of the carrier is modulated to carry the audio signal. Here the envelope of the carrier can be seen to change in line with the modulating signal.

Amplitude modulation, AM is the most straightforward way of modulating a signal. Demodulation, or the process where the radio frequency signal is converted into an audio frequency signal is also very simple. It only requires a simple diode detector circuit. The circuit that is commonly used has a diode that rectifies the signal, only allowing the one half of the alternating radio frequency waveform through. A capacitor is used to remove the radio frequency parts of the signal, leaving the audio waveform. This can be fed into an amplifier after which it can be used to drive a loudspeaker. As the circuit used for demodulating AM is very cheap, it enables the cost of radio receivers for AM to be kept low.

Amplitude modulation efficiency

Amplitude modulation, AM has advantages of simplicity, but it is not the most efficient mode to use, both in terms of the amount of spectrum it takes up, and the usage of the power. It is for this reason that it only has limited applications for broadcast and two way radio communications systems.

the reason for its inefficiency occurs as a result of 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.

Even with 100% modulation the utilisation of power by an amplitude modulated signal 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. 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.

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. 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. In the crowded conditions found on many of the short wave bands today, this is a waste of space, and other modes of transmission which take up less space are often used.

Modulation index

It is often necessary to define the level of modulation that is applied to a signal. A factor or index known as the modulation index is used for this. When expressed as a percentage it is the same as the depth of modulation. In other words it can be expressed as:

M   =   (RMS value of modulating signal) / (RMS value of unmodulated signal )

The value of the modulation index must not be allowed to exceed one (i.e. 100 % in terms of the depth of modulation) otherwise the envelope becomes distorted and the signal will "splatter" either side of the wanted channel, causing interference and annoyance to other users.

Highlights

While amplitude modulation is one of the simplest and easiest forms of modulation to implement, it is not the most efficient in terms of spectrum efficiency and power usage. As a result, the use of amplitude modulation is falling in preference to other analogue modes such as frequency modulation, and a variety of digital modulation formats. Yet despite this decrease, amplitude modulation is in such widespread use, especially for broadcasting, that it will still be used for many years to come.