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

2. AMPLITUDE MODULATION
Amplitude modulation is a type of modulation where the amplitude of the carrier signal is
varied in accordance with the information bearing signal.
The envelope, or boundary, of the amplitude modulated signal embeds the information
bearing signal. A nonlinear device is used to combine the carrier and the modulating
signal to generate an amplitude modulated signal. The output of the nonlinear device
consists of discrete upper and lower sidebands.The output of a nonlinear device does not
vary in direct proportion with the input.
Amplitude Modulation is abbreviated AM.
Amplitude modulation (AM) is a method of impressing data onto an alternating-
current (AC) carrier waveform. The highest frequency of the modulating data is
normally less than 10 percent of the carrier frequency. The instantaneous amplitude
(overall signal power) varies depending on the instantaneous amplitude of the
modulating data.

3. POWER IN AMPLITUDE MODULATION
The total power of the transmitted signal varies with the modulating signal, whereas the
carrier power remains constant.
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.

4. TYPES OF AMPLITUDE MODULATION
In radio communication, a continuous wave radio-frequency signal (a sinusoidal carrier wave)
has its amplitude modulated by an audio waveform before being transmitted.
In the frequency domain, amplitude modulation produces a signal with power concentrated at the
carrier frequency and in two adjacent sidebands. Each sideband is equal in bandwidth to that of the
modulating signal and is a mirror image of the other. 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).
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. 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.
Improved bandwidth efficiency is achieved—at the expense of increased transmitter and receiver
complexity—by completely suppressing both the carrier and one of the sidebands. This is single-sideband
modulation, widely used in amateur radio due to its efficient use of both power and bandwidth.
A simple form of AM often used for digital communications is on-off keying, a type of
amplitude-shift keying by which binary data is represented as the presence or absence of a carrier wave.
This is commonly used at radio frequencies to transmit Morse code, referred to as continuous wave (CW)
operation.
ITU designations
In 1982, the International Telecommunication Union (ITU) designated the various types of
amplitude modulation as follows:
Designation Description
A3E double-sideband full-carrier - the basic AM modulation scheme
R3E single-sideband reduced-carrier
H3E single-sideband full-carrier
J3E single-sideband suppressed-carrier
B8E independent-sideband emission
C3F vestigial-sideband
Lincompex linked compressor and expander

5. 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).