This document discusses types and properties of amplitude modulation (AM). It describes seven main types of AM: double sideband full carrier, double sideband suppressed carrier, single sideband suppressed carrier, single sideband full carrier, single sideband reduced carrier, independent sideband emission, and vestigial sideband. It also discusses modulation index and explains that AM has advantages of simplicity but is inefficient in terms of power and spectrum usage compared to other modulation techniques.

1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY
Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite
Assignment # 2
AMPLITUDE MODULATION
(Types of Amplitude Modulation)
(Power in Amplitude Modulation)
(Modulation Index)
Lingad, Valentino June 29,2011
Communications 1 / BSECE 41A1 Score:
Eng'r. Grace Ramones
Instructor

2. WHAT IS AM, AMPLITUDE MODULATION
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.
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.

3. POWER IN AMPLITUDE MODULATION
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.

4. 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.

5. TYPES OF AMPLITUDE MODULATION
1. Double Sideband Full Carrier (DSB- LC)
This type of Amplitude modulation is also known as 'Full AM' or 'Standard AM'. Here the frequency
spectrum of th AM will have the carrier frequency, Upper sideband and the Lower Sideband.
Therefore the DSB-LC signal may be written as
2. Double Sideband- Suppressed Carrier (DSB-SC)
In this type of amplitude modulation, both the sidebands namely Lower sideband and Upper
sideband are present in the frequency spectrum but the carrier component is suppressed, hence
the name Double Sideband suppressed Carrier. The Carrier does not contain any information, so it
is suppressed during modulation to obtain a better Power Efficiency.
3. Single sideband- Suppressed Carrier (SSB-SC)
In this type of amplitude modulation, the carrier is suppressed and it is either the Upper sideband
(USB) or the Lower Sideband ( LSB) that gets transmitted. In DSC-SC the basic information is
transmitted twice, once in each sideband. This is not required and so SSB-SC has an upper hand.
4. Single sideband Full Carrier. This could be used as compatible AM broadcasting system with
DSB-FC receivers.
5. Single Sideband - Reduced Carrier: Here an attenuated carrier is reinserted into the SSB
signal, to facilitate receiver tuning and demodulation. This method is steadily replaced by SSB-SC.
6. Independent Sideband Emission: Two independent sidebands, with a carrier that is most
commonly suppressed or attenuated is used here. It is used in HF point-to -point radiotelephony,
in which more than one channel is required.
7. Vestigial Sideband: Here a vestige or trace of the unwanted sideband is transmitted, usually
with the full carrier. This is used in video transmission.
8. Lincompex: This is an acronym that stands for 'linked compressor and expander'. it is used
commercial HF radio telephony.