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)
Bani, Arviclyn C. June 29, 2011
Communications 1 / BSECE 41A1 Score:
Eng'r. Grace Ramones
Instructor

2. AMPLITUDE MODULATION
The process or result of the process whereby the amplitude of a carrier wave is changed in
accordance with a modulating wave. This broad definition includes applications using sinusoidal
carriers, pulse carriers, or any other form of carrier, the amplitude factor of which changes in
accordance with the modulating wave in any unique manner.
Practical examples of amplitude modulation (AM) include AM radio broadcasting, single-
sideband transmission systems, vestigial-sideband systems, frequency-division multiplexing,
time-division multiplexing, phase-discrimination multiplexing, and reduced-carrier systems.
Amplitude modulation is also defined in a more restrictive sense to mean modulation in which
the amplitude factor of a sine-wave carrier is linearly proportional to the modulating wave. AM
radio broadcasting is a familiar example. At the radio transmitter the modulating wave is the
audio-frequency program signal to be communicated; the modulated wave that is broadcast is a
radio-frequency, amplitude-modulated sinusoid.
In AM the modulated wave is composed of the transmitted carrier, which conveys no
information, plus the upper and lower sidebands, which (assuming the carrier frequency exceeds
twice the top audio frequency) convey identical and therefore mutually redundant information. J.
R. Carson in 1915 was the first to recognize that, under these conditions and assuming adequate
knowledge of the carrier, either sideband alone would uniquely define the message. This
eventually led to the development of single-sideband (SSB) and vestigial-sideband (VSB)
modulation. Apart from a scale factor, the spectrum of the upper sideband and lower sideband is
the spectrum of the modulating wave displaced, respectively, without and with inversion by an
amount equal to the carrier frequency.

3. TYPES OF AMPLITUDE MODULATION
Double-Sideband Amplitude Modulation Amplitude- modulation that results in two sidebands
and a carrier is often called double-sideband amplitude modulation (DSB-AM). Amplitude
modulation is inefficient in terms of power usage. At least two-thirds of the power is concentrated
in the carrier signal, which carries no useful information (beyond the fact that a signal is present).
Double-Sideband Suppressed-Carrier- To increase transmitter efficiency, the carrier can be
removed (suppressed) from the AM signal. This produces a reduced-carrier transmission or
double-sideband suppressed-carrier (DSBSC) signal. A suppressed-carrier amplitude modulation
scheme is three times more power-efficient than traditional DSB-AM.
Double-Sideband Reduced-Carrier - If the carrier is only partially suppressed, a double-
sideband reduced-carrier (DSBRC) signal results. DSBSC and DSBRC signals need their carrier
to be regenerated (by a beat frequency oscillator, for instance) to be demodulated using
conventional techniques.
Single sideband Full Carrier- This could be used as compatible AM broadcasting system with
DSB-FC receivers.
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.
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.
Vestigial Sideband- Here a vestige or trace of the unwanted sideband is transmitted, usually
with the full carrier. This is used in video transmission.
Lincompex- This is an acronym that stands for 'linked compressor and expander'. it is used
commercial HF radio telephony.

4. MODULATION INDEX
Amplitude modulation is one of the earliest radio modulation techniques. The receivers used to
list to AM-DSB-C are perhaps the simplest receivers of any radio modulation technique; which may be
why that version of amplitude modulation is still widely used today.
Amplitude modulation (AM) occurs when the amplitude of a carrier wave is modulated, to
correspond to a source signal. In AM, we have an equation that looks like this:
Fsignal(t) = A(t)sin(ωt)
We can also see that the phase of this wave is irrelevant, and does not change (so we don't even
include it in the equation).

5. POWER IN AMPLITUDE MODULATION
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.