What is 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 has been in use since the very earliest days of radio technology. The first recorded instance of its use was in 1901 when a signal was transmitted by a Canadian engineer named Reginald Fessenden. To achieve this, he used a continuous spark transmission and placed a carbon microphone in the antenna lead. The sound waves impacting on the microphone varied its resistance and in turn this varied the intensity of the transmission. Although very crude, signals were audible over a distance of a few hundred metres. The quality of the audio was not good particularly as a result of the continuous rasping sound caused by the spark used for the transmission.

Later, continuous sine wave signals could be generated and the audio quality was greatly improved. As a result, amplitude modulation, AM became the standard for voice transmissions.

Amplitude modulation applications

Amplitude modulation is used in a variety of applications. Even though it is not as widely used as it was in previous years in its basic format it can nevertheless still be found.

  • Broadcast transmissions:   AM is still widely used for broadcasting on the long, medium and short wave bands. It is simple to demodulate and this means that radio receivers capable of demodulating amplitude modulation are cheap and simple to manufacture. Nevertheless many people are moving to high quality forms of transmission like frequency modulation, FM or digital transmissions.
  • Air band radio:   VHF transmissions for many airborne applications still use AM. . It is used for ground to air radio communications as well as two way radio links for ground staff as well.
  • Single sideband:   Amplitude modulation in the form of single sideband is still used for HF radio links. Using a lower bandwidth and providing more effective use of the transmitted power this form of modulation is still used for many point to point HF links.
  • Quadrature amplitude modulation:   AM is widely used for the transmission of data in everything from short range wireless links such as Wi-Fi to cellular telecommunications and much more. Effectively it is formed by having two carriers 90° out of phase.

These form some of the main uses of amplitude modulation. However in its basic form, this form of modulation is being sued less as a result of its inefficient use of both spectrum and power.

Amplitude modulation basics

When an 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.

Diagram showing amplitude modulation and how an RF waveform is modulated by a low frequency audio signal to provide the combined AM signal
Amplitude Modulation

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. An amplitude modulation signal 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 advantages & disadvantages

Like any other system of modulation, amplitude modulation has several advantages and disadvantages. These mean that it is used in particular circumstances where its advantages can be used to good effect..

Advantages Disadvantages
  • It is simple to implement
  • It can be demodulated using a circuit consisting of very few components
  • AM receivers are very cheap as no specialised components are needed.
  • An amplitude modulation signal is not efficient in terms of its power usage
  • It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency
  • An amplitude modulation signal is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.

In view of its characteristics advantages and disadvantages, amplitude modulation is being used less frequently. However it is still in widespread use for broadcasting on the long, medium and short wave bands as well as for a number of mobile or portable communications systems including some aircraft communications.

Modulating and demodulating AM signals

Two key elements of any AM system are the circuits where the signal is modulated and demodulated.

Modulating an AM signal can be achieved in a number of ways. Essentially the simplest is to pass the RF carrier and the modulating signal into a mixer. The resulting output will be the required amplitude modulated signal.

Demodulation of AM can similarly be undertaken in a number of ways. The simplest is the simple diode detector.


Note on AM Demodulation:

In order to be able to extract any information being carried by an AM signal, it is necessary to pass it through a demodulator. The output from this stage provides the information that was carried by the AM signal. There are many methods of achieving this, using circuits that employ a variety of different techniques.

Read more about AM demodulation


Amplitude modulation related signals

Amplitude modulation forms the basis of a number of forms of signal apart from the basic mode. These signal formats are typically generated by removing or suppressing the carrier, and then utilising the sidebands. These formats are defined in greater detail in further pages of this tutorial.


Description Abbreviation ITU Designation
Double sideband, full carrier
i.e. basic AM
DSB full carrier A3E
Single sideband reduced carrier SSB reduced carrier R3E
Single sideband full carrier SSB full carrier H3E
Single sideband suppressed carrier SSBSC J3E
Independent sideband ISB B8E
Vestigial sideband VSB C3F

Highlights

While amplitude modulation is one of the simplest and easiest forms of signal 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, and many amplitude modulation signals can still be heard on the various long, medium and short wavebands where they will undoubtedly be heard for many years to come.

By Ian Poole


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